Message ID | 20190408030804.5805-1-cmr@informatik.wtf (mailing list archive) |
---|---|
State | Superseded |
Headers | show |
Series | [v2] powerpc/xmon: add read-only mode | expand |
Context | Check | Description |
---|---|---|
snowpatch_ozlabs/apply_patch | success | Successfully applied on branch next (8c2ffd9174779014c3fe1f96d9dc3641d9175f00) |
snowpatch_ozlabs/build-ppc64le | success | Build succeeded |
snowpatch_ozlabs/build-ppc64be | success | Build succeeded |
snowpatch_ozlabs/build-ppc64e | success | Build succeeded |
snowpatch_ozlabs/build-pmac32 | success | Build succeeded |
snowpatch_ozlabs/checkpatch | success | total: 0 errors, 0 warnings, 0 checks, 148 lines checked |
On Mon, Apr 8, 2019 at 1:06 PM Christopher M. Riedl <cmr@informatik.wtf> wrote: > > Operations which write to memory and special purpose registers should be > restricted on systems with integrity guarantees (such as Secure Boot) > and, optionally, to avoid self-destructive behaviors. > > Add a config option, XMON_RW, to control default xmon behavior along > with kernel cmdline options xmon=ro and xmon=rw for explicit control. > Use XMON_RW instead of XMON in the condition to set PAGE_KERNEL_TEXT to > allow xmon in read-only mode alongside write-protected kernel text. > XMON_RW defaults to !STRICT_KERNEL_RWX. > > The following xmon operations are affected: > memops: > disable memmove > disable memset > disable memzcan > memex: > no-op'd mwrite > super_regs: > no-op'd write_spr > bpt_cmds: > disable > proc_call: > disable > > Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf> > --- > v1->v2: > Use bool type for xmon_is_ro flag > Replace XMON_RO with XMON_RW config option > Make XMON_RW dependent on STRICT_KERNEL_RWX Do you mean make it dependent on XMON? > Use XMON_RW to control PAGE_KERNEL_TEXT > Add printf in xmon read-only mode when dropping/skipping writes > Disable memzcan (zero-fill memop) in xmon read-only mode > > arch/powerpc/Kconfig.debug | 10 +++++ > arch/powerpc/include/asm/book3s/32/pgtable.h | 5 ++- > arch/powerpc/include/asm/book3s/64/pgtable.h | 5 ++- > arch/powerpc/include/asm/nohash/pgtable.h | 5 ++- > arch/powerpc/xmon/xmon.c | 42 ++++++++++++++++++++ > 5 files changed, 61 insertions(+), 6 deletions(-) > > diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug > index 4e00cb0a5464..0c7f21476018 100644 > --- a/arch/powerpc/Kconfig.debug > +++ b/arch/powerpc/Kconfig.debug > @@ -117,6 +117,16 @@ config XMON_DISASSEMBLY > to say Y here, unless you're building for a memory-constrained > system. > > +config XMON_RW > + bool "Allow xmon read and write operations" > + depends on XMON > + default !STRICT_KERNEL_RWX > + help > + Allow xmon to read and write to memory and special-purpose registers. > + Conversely, prevent xmon write access when set to N. Read and write > + access can also be explicitly controlled with 'xmon=rw' or 'xmon=ro' > + (read-only) cmdline options. Default is !STRICT_KERNEL_RWX. Maybe I am a dumb, but I found this *extremely* confusing. Conventionally Kconfig options will control what code is and is not included in the kernel (see XMON_DISASSEMBLY) rather than changing the default behaviour of code. It's not wrong to do so and I'm going to assume that you were following the pattern of XMON_DEFAULT, but I think you need to be a little more clear about what option actually does. Renaming it to XMON_DEFAULT_RO_MODE and re-wording the description to indicate it's a only a mode change would help a lot. Sorry if this comes across as pointless bikeshedding since it's the opposite of what Christophe said in the last patch, but this was a bit of a head scratcher. > config DEBUGGER > bool > depends on KGDB || XMON > diff --git a/arch/powerpc/include/asm/book3s/32/pgtable.h b/arch/powerpc/include/asm/book3s/32/pgtable.h > index aa8406b8f7ba..615144ad667d 100644 > --- a/arch/powerpc/include/asm/book3s/32/pgtable.h > +++ b/arch/powerpc/include/asm/book3s/32/pgtable.h > @@ -86,8 +86,9 @@ static inline bool pte_user(pte_t pte) > * set breakpoints anywhere, so don't write protect the kernel text > * on platforms where such control is possible. > */ > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\ > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > + defined(CONFIG_DYNAMIC_FTRACE) > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > #else > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h > index 581f91be9dd4..bc4655122f6b 100644 > --- a/arch/powerpc/include/asm/book3s/64/pgtable.h > +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h > @@ -168,8 +168,9 @@ > * set breakpoints anywhere, so don't write protect the kernel text > * on platforms where such control is possible. > */ > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) || \ > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > + defined(CONFIG_DYNAMIC_FTRACE) > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > #else > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > diff --git a/arch/powerpc/include/asm/nohash/pgtable.h b/arch/powerpc/include/asm/nohash/pgtable.h > index 1ca1c1864b32..c052931bd243 100644 > --- a/arch/powerpc/include/asm/nohash/pgtable.h > +++ b/arch/powerpc/include/asm/nohash/pgtable.h > @@ -22,8 +22,9 @@ > * set breakpoints anywhere, so don't write protect the kernel text > * on platforms where such control is possible. > */ > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\ > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > + defined(CONFIG_DYNAMIC_FTRACE) > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > #else > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c > index a0f44f992360..224ca0b3506b 100644 > --- a/arch/powerpc/xmon/xmon.c > +++ b/arch/powerpc/xmon/xmon.c > @@ -80,6 +80,7 @@ static int set_indicator_token = RTAS_UNKNOWN_SERVICE; > #endif > static unsigned long in_xmon __read_mostly = 0; > static int xmon_on = IS_ENABLED(CONFIG_XMON_DEFAULT); > +static bool xmon_is_ro = !IS_ENABLED(CONFIG_XMON_RW); > > static unsigned long adrs; > static int size = 1; > @@ -202,6 +203,8 @@ static void dump_tlb_book3e(void); > #define GETWORD(v) (((v)[0] << 24) + ((v)[1] << 16) + ((v)[2] << 8) + (v)[3]) > #endif > > +static const char *xmon_is_ro_warn = "xmon read-only mode: skipping write\n"; > + > static char *help_string = "\ > Commands:\n\ > b show breakpoints\n\ > @@ -989,6 +992,10 @@ cmds(struct pt_regs *excp) > memlocate(); > break; > case 'z': > + if (xmon_is_ro) { > + printf(xmon_is_ro_warn); > + break; > + } > memzcan(); > break; > case 'i': > @@ -1042,6 +1049,10 @@ cmds(struct pt_regs *excp) > set_lpp_cmd(); > break; > case 'b': > + if (xmon_is_ro) { > + printf(xmon_is_ro_warn); > + break; > + } > bpt_cmds(); > break; > case 'C': > @@ -1055,6 +1066,10 @@ cmds(struct pt_regs *excp) > bootcmds(); > break; > case 'p': > + if (xmon_is_ro) { > + printf(xmon_is_ro_warn); > + break; > + } > proccall(); > break; > case 'P': > @@ -1777,6 +1792,11 @@ read_spr(int n, unsigned long *vp) > static void > write_spr(int n, unsigned long val) > { > + if (xmon_is_ro) { > + printf(xmon_is_ro_warn); > + return; > + } > + > if (setjmp(bus_error_jmp) == 0) { > catch_spr_faults = 1; > sync(); > @@ -2016,6 +2036,12 @@ mwrite(unsigned long adrs, void *buf, int size) > char *p, *q; > > n = 0; > + > + if (xmon_is_ro) { > + printf(xmon_is_ro_warn); > + return n; > + } > + > if (setjmp(bus_error_jmp) == 0) { > catch_memory_errors = 1; > sync(); > @@ -2884,9 +2910,17 @@ memops(int cmd) > scanhex((void *)&mcount); > switch( cmd ){ > case 'm': > + if (xmon_is_ro) { > + printf(xmon_is_ro_warn); > + break; > + } > memmove((void *)mdest, (void *)msrc, mcount); > break; > case 's': > + if (xmon_is_ro) { > + printf(xmon_is_ro_warn); > + break; > + } > memset((void *)mdest, mval, mcount); > break; > case 'd': > @@ -3796,6 +3830,14 @@ static int __init early_parse_xmon(char *p) > } else if (strncmp(p, "on", 2) == 0) { > xmon_init(1); > xmon_on = 1; > + } else if (strncmp(p, "rw", 2) == 0) { > + xmon_init(1); > + xmon_on = 1; > + xmon_is_ro = false; > + } else if (strncmp(p, "ro", 2) == 0) { > + xmon_init(1); > + xmon_on = 1; > + xmon_is_ro = true; > } else if (strncmp(p, "off", 3) == 0) > xmon_on = 0; > else > -- > 2.21.0 >
On 8/4/19 1:08 pm, Christopher M. Riedl wrote: > Operations which write to memory and special purpose registers should be > restricted on systems with integrity guarantees (such as Secure Boot) > and, optionally, to avoid self-destructive behaviors. > > Add a config option, XMON_RW, to control default xmon behavior along > with kernel cmdline options xmon=ro and xmon=rw for explicit control. > Use XMON_RW instead of XMON in the condition to set PAGE_KERNEL_TEXT to > allow xmon in read-only mode alongside write-protected kernel text. > XMON_RW defaults to !STRICT_KERNEL_RWX. > > The following xmon operations are affected: > memops: > disable memmove > disable memset > disable memzcan > memex: > no-op'd mwrite > super_regs: > no-op'd write_spr > bpt_cmds: > disable > proc_call: > disable > > Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf> > --- > v1->v2: > Use bool type for xmon_is_ro flag > Replace XMON_RO with XMON_RW config option > Make XMON_RW dependent on STRICT_KERNEL_RWX > Use XMON_RW to control PAGE_KERNEL_TEXT > Add printf in xmon read-only mode when dropping/skipping writes > Disable memzcan (zero-fill memop) in xmon read-only mode > > arch/powerpc/Kconfig.debug | 10 +++++ > arch/powerpc/include/asm/book3s/32/pgtable.h | 5 ++- > arch/powerpc/include/asm/book3s/64/pgtable.h | 5 ++- > arch/powerpc/include/asm/nohash/pgtable.h | 5 ++- > arch/powerpc/xmon/xmon.c | 42 ++++++++++++++++++++ > 5 files changed, 61 insertions(+), 6 deletions(-) > > diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug > index 4e00cb0a5464..0c7f21476018 100644 > --- a/arch/powerpc/Kconfig.debug > +++ b/arch/powerpc/Kconfig.debug > @@ -117,6 +117,16 @@ config XMON_DISASSEMBLY > to say Y here, unless you're building for a memory-constrained > system. > > +config XMON_RW > + bool "Allow xmon read and write operations" "Allow xmon write operations" would be clearer. This option has no impact on read operations. > + depends on XMON > + default !STRICT_KERNEL_RWX > + help > + Allow xmon to read and write to memory and special-purpose registers. > + Conversely, prevent xmon write access when set to N. Read and write > + access can also be explicitly controlled with 'xmon=rw' or 'xmon=ro' > + (read-only) cmdline options. Default is !STRICT_KERNEL_RWX. This is an improvement but still doesn't clearly explain the relationship between selecting this option and using the cmdline options. > + > config DEBUGGER > bool > depends on KGDB || XMON > diff --git a/arch/powerpc/include/asm/book3s/32/pgtable.h b/arch/powerpc/include/asm/book3s/32/pgtable.h > index aa8406b8f7ba..615144ad667d 100644 > --- a/arch/powerpc/include/asm/book3s/32/pgtable.h > +++ b/arch/powerpc/include/asm/book3s/32/pgtable.h > @@ -86,8 +86,9 @@ static inline bool pte_user(pte_t pte) > * set breakpoints anywhere, so don't write protect the kernel text > * on platforms where such control is possible. > */ > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\ > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > + defined(CONFIG_DYNAMIC_FTRACE) > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > #else > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h > index 581f91be9dd4..bc4655122f6b 100644 > --- a/arch/powerpc/include/asm/book3s/64/pgtable.h > +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h > @@ -168,8 +168,9 @@ > * set breakpoints anywhere, so don't write protect the kernel text > * on platforms where such control is possible. > */ > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) || \ > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > + defined(CONFIG_DYNAMIC_FTRACE) We might need to rethink how the cmdline options work. What happens if CONFIG_XMON_RW=n, but "xmon=rw" on the command line? I think I agree with Oliver that we're not being clear whether this config option is selecting code to be compiled in, or whether it's just setting a default. > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > #else > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > diff --git a/arch/powerpc/include/asm/nohash/pgtable.h b/arch/powerpc/include/asm/nohash/pgtable.h > index 1ca1c1864b32..c052931bd243 100644 > --- a/arch/powerpc/include/asm/nohash/pgtable.h > +++ b/arch/powerpc/include/asm/nohash/pgtable.h > @@ -22,8 +22,9 @@ > * set breakpoints anywhere, so don't write protect the kernel text > * on platforms where such control is possible. > */ > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\ > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > + defined(CONFIG_DYNAMIC_FTRACE) > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > #else > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c > index a0f44f992360..224ca0b3506b 100644 > --- a/arch/powerpc/xmon/xmon.c > +++ b/arch/powerpc/xmon/xmon.c > @@ -80,6 +80,7 @@ static int set_indicator_token = RTAS_UNKNOWN_SERVICE; > #endif > static unsigned long in_xmon __read_mostly = 0; > static int xmon_on = IS_ENABLED(CONFIG_XMON_DEFAULT); > +static bool xmon_is_ro = !IS_ENABLED(CONFIG_XMON_RW); > > static unsigned long adrs; > static int size = 1; > @@ -202,6 +203,8 @@ static void dump_tlb_book3e(void); > #define GETWORD(v) (((v)[0] << 24) + ((v)[1] << 16) + ((v)[2] << 8) + (v)[3]) > #endif > > +static const char *xmon_is_ro_warn = "xmon read-only mode: skipping write\n"; Yay, error messages! Though something like: "Operation disabled: xmon is in read-only mode" would be clearer. > + > static char *help_string = "\ > Commands:\n\ > b show breakpoints\n\ > @@ -989,6 +992,10 @@ cmds(struct pt_regs *excp) > memlocate(); > break; > case 'z': > + if (xmon_is_ro) { > + printf(xmon_is_ro_warn); > + break; > + } > memzcan(); > break; > case 'i': > @@ -1042,6 +1049,10 @@ cmds(struct pt_regs *excp) > set_lpp_cmd(); > break; > case 'b': > + if (xmon_is_ro) { > + printf(xmon_is_ro_warn); > + break; > + } > bpt_cmds(); > break; > case 'C': > @@ -1055,6 +1066,10 @@ cmds(struct pt_regs *excp) > bootcmds(); > break; > case 'p': > + if (xmon_is_ro) { > + printf(xmon_is_ro_warn); > + break; > + } > proccall(); > break; > case 'P': > @@ -1777,6 +1792,11 @@ read_spr(int n, unsigned long *vp) > static void > write_spr(int n, unsigned long val) > { > + if (xmon_is_ro) { > + printf(xmon_is_ro_warn); > + return; > + } > + > if (setjmp(bus_error_jmp) == 0) { > catch_spr_faults = 1; > sync(); > @@ -2016,6 +2036,12 @@ mwrite(unsigned long adrs, void *buf, int size) > char *p, *q; > > n = 0; > + > + if (xmon_is_ro) { > + printf(xmon_is_ro_warn); > + return n; > + } > + > if (setjmp(bus_error_jmp) == 0) { > catch_memory_errors = 1; > sync(); > @@ -2884,9 +2910,17 @@ memops(int cmd) > scanhex((void *)&mcount); > switch( cmd ){ > case 'm': > + if (xmon_is_ro) { > + printf(xmon_is_ro_warn); > + break; > + } > memmove((void *)mdest, (void *)msrc, mcount); > break; > case 's': > + if (xmon_is_ro) { > + printf(xmon_is_ro_warn); > + break; > + } > memset((void *)mdest, mval, mcount); > break; > case 'd': > @@ -3796,6 +3830,14 @@ static int __init early_parse_xmon(char *p) > } else if (strncmp(p, "on", 2) == 0) { > xmon_init(1); > xmon_on = 1; > + } else if (strncmp(p, "rw", 2) == 0) { > + xmon_init(1); > + xmon_on = 1; > + xmon_is_ro = false; > + } else if (strncmp(p, "ro", 2) == 0) { > + xmon_init(1); > + xmon_on = 1; > + xmon_is_ro = true; > } else if (strncmp(p, "off", 3) == 0) > xmon_on = 0; > else >
> On April 8, 2019 at 1:34 AM Oliver <oohall@gmail.com> wrote: > > > On Mon, Apr 8, 2019 at 1:06 PM Christopher M. Riedl <cmr@informatik.wtf> wrote: > > > > Operations which write to memory and special purpose registers should be > > restricted on systems with integrity guarantees (such as Secure Boot) > > and, optionally, to avoid self-destructive behaviors. > > > > Add a config option, XMON_RW, to control default xmon behavior along > > with kernel cmdline options xmon=ro and xmon=rw for explicit control. > > Use XMON_RW instead of XMON in the condition to set PAGE_KERNEL_TEXT to > > allow xmon in read-only mode alongside write-protected kernel text. > > XMON_RW defaults to !STRICT_KERNEL_RWX. > > > > The following xmon operations are affected: > > memops: > > disable memmove > > disable memset > > disable memzcan > > memex: > > no-op'd mwrite > > super_regs: > > no-op'd write_spr > > bpt_cmds: > > disable > > proc_call: > > disable > > > > Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf> > > --- > > v1->v2: > > Use bool type for xmon_is_ro flag > > Replace XMON_RO with XMON_RW config option > > Make XMON_RW dependent on STRICT_KERNEL_RWX > Do you mean make it dependent on XMON? > Yeah that's really not clear at all -- XMON_RW is set based on the value of STRICT_KERNEL_RWX. > > > Use XMON_RW to control PAGE_KERNEL_TEXT > > Add printf in xmon read-only mode when dropping/skipping writes > > Disable memzcan (zero-fill memop) in xmon read-only mode > > > > arch/powerpc/Kconfig.debug | 10 +++++ > > arch/powerpc/include/asm/book3s/32/pgtable.h | 5 ++- > > arch/powerpc/include/asm/book3s/64/pgtable.h | 5 ++- > > arch/powerpc/include/asm/nohash/pgtable.h | 5 ++- > > arch/powerpc/xmon/xmon.c | 42 ++++++++++++++++++++ > > 5 files changed, 61 insertions(+), 6 deletions(-) > > > > diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug > > index 4e00cb0a5464..0c7f21476018 100644 > > --- a/arch/powerpc/Kconfig.debug > > +++ b/arch/powerpc/Kconfig.debug > > @@ -117,6 +117,16 @@ config XMON_DISASSEMBLY > > to say Y here, unless you're building for a memory-constrained > > system. > > > > > +config XMON_RW > > + bool "Allow xmon read and write operations" > > + depends on XMON > > + default !STRICT_KERNEL_RWX > > + help > > + Allow xmon to read and write to memory and special-purpose registers. > > + Conversely, prevent xmon write access when set to N. Read and write > > + access can also be explicitly controlled with 'xmon=rw' or 'xmon=ro' > > + (read-only) cmdline options. Default is !STRICT_KERNEL_RWX. > > Maybe I am a dumb, but I found this *extremely* confusing. > Conventionally Kconfig options will control what code is and is not > included in the kernel (see XMON_DISASSEMBLY) rather than changing the > default behaviour of code. It's not wrong to do so and I'm going to > assume that you were following the pattern of XMON_DEFAULT, but I > think you need to be a little more clear about what option actually > does. Renaming it to XMON_DEFAULT_RO_MODE and re-wording the > description to indicate it's a only a mode change would help a lot. > > Sorry if this comes across as pointless bikeshedding since it's the > opposite of what Christophe said in the last patch, but this was a bit > of a head scratcher. > If anyone is dumb here it's me for making this confusing :) I chatted with Michael Ellerman about this, so let me try to explain this more clearly. There are two things I am trying to address with XMON_RW: 1) provide a default access mode for xmon based on system "security" 2) replace XMON in the decision to write-protect kernel text at compile-time I think a single Kconfig for both of those things is sensible as ultimately the point is to allow xmon to operate in read-only mode on "secure" systems -- without violating any integrity/security guarantees (such as write-protected kernel text). Christophe suggested looking at STRICT_KERNEL_RWX and I think that option makes the most sense to base XMON_RW on since the description for STRICT_KERNEL_RWX states: > If this is set, kernel text and rodata memory will be made read-only, > and non-text memory will be made non-executable. This provides > protection against certain security exploits (e.g. executing the heap > or modifying text) > > These features are considered standard security practice these days. > You should say Y here in almost all cases. Considering this, does XMON_DEFAULT_RO_MODE really make things more clear? With that said, I will remove the 'xmon=rw' cmdline option as it really doesn't work since kernel text is write-protected at compile time. 'xmon=ro' remains for those who would optionally like to prevent shooting themselves in the foot/feet while using xmon. Hopefully this makes a bit more sense now? > > > config DEBUGGER > > bool > > depends on KGDB || XMON > > diff --git a/arch/powerpc/include/asm/book3s/32/pgtable.h b/arch/powerpc/include/asm/book3s/32/pgtable.h > > index aa8406b8f7ba..615144ad667d 100644 > > --- a/arch/powerpc/include/asm/book3s/32/pgtable.h > > +++ b/arch/powerpc/include/asm/book3s/32/pgtable.h > > @@ -86,8 +86,9 @@ static inline bool pte_user(pte_t pte) > > * set breakpoints anywhere, so don't write protect the kernel text > > * on platforms where such control is possible. > > */ > > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\ > > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > > + defined(CONFIG_DYNAMIC_FTRACE) > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > > #else > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > > diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h > > index 581f91be9dd4..bc4655122f6b 100644 > > --- a/arch/powerpc/include/asm/book3s/64/pgtable.h > > +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h > > @@ -168,8 +168,9 @@ > > * set breakpoints anywhere, so don't write protect the kernel text > > * on platforms where such control is possible. > > */ > > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) || \ > > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > > + defined(CONFIG_DYNAMIC_FTRACE) > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > > #else > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > > diff --git a/arch/powerpc/include/asm/nohash/pgtable.h b/arch/powerpc/include/asm/nohash/pgtable.h > > index 1ca1c1864b32..c052931bd243 100644 > > --- a/arch/powerpc/include/asm/nohash/pgtable.h > > +++ b/arch/powerpc/include/asm/nohash/pgtable.h > > @@ -22,8 +22,9 @@ > > * set breakpoints anywhere, so don't write protect the kernel text > > * on platforms where such control is possible. > > */ > > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\ > > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > > + defined(CONFIG_DYNAMIC_FTRACE) > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > > #else > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > > diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c > > index a0f44f992360..224ca0b3506b 100644 > > --- a/arch/powerpc/xmon/xmon.c > > +++ b/arch/powerpc/xmon/xmon.c > > @@ -80,6 +80,7 @@ static int set_indicator_token = RTAS_UNKNOWN_SERVICE; > > #endif > > static unsigned long in_xmon __read_mostly = 0; > > static int xmon_on = IS_ENABLED(CONFIG_XMON_DEFAULT); > > +static bool xmon_is_ro = !IS_ENABLED(CONFIG_XMON_RW); > > > > static unsigned long adrs; > > static int size = 1; > > @@ -202,6 +203,8 @@ static void dump_tlb_book3e(void); > > #define GETWORD(v) (((v)[0] << 24) + ((v)[1] << 16) + ((v)[2] << 8) + (v)[3]) > > #endif > > > > +static const char *xmon_is_ro_warn = "xmon read-only mode: skipping write\n"; > > + > > static char *help_string = "\ > > Commands:\n\ > > b show breakpoints\n\ > > @@ -989,6 +992,10 @@ cmds(struct pt_regs *excp) > > memlocate(); > > break; > > case 'z': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > memzcan(); > > break; > > case 'i': > > @@ -1042,6 +1049,10 @@ cmds(struct pt_regs *excp) > > set_lpp_cmd(); > > break; > > case 'b': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > bpt_cmds(); > > break; > > case 'C': > > @@ -1055,6 +1066,10 @@ cmds(struct pt_regs *excp) > > bootcmds(); > > break; > > case 'p': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > proccall(); > > break; > > case 'P': > > @@ -1777,6 +1792,11 @@ read_spr(int n, unsigned long *vp) > > static void > > write_spr(int n, unsigned long val) > > { > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + return; > > + } > > + > > if (setjmp(bus_error_jmp) == 0) { > > catch_spr_faults = 1; > > sync(); > > @@ -2016,6 +2036,12 @@ mwrite(unsigned long adrs, void *buf, int size) > > char *p, *q; > > > > n = 0; > > + > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + return n; > > + } > > + > > if (setjmp(bus_error_jmp) == 0) { > > catch_memory_errors = 1; > > sync(); > > @@ -2884,9 +2910,17 @@ memops(int cmd) > > scanhex((void *)&mcount); > > switch( cmd ){ > > case 'm': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > memmove((void *)mdest, (void *)msrc, mcount); > > break; > > case 's': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > memset((void *)mdest, mval, mcount); > > break; > > case 'd': > > @@ -3796,6 +3830,14 @@ static int __init early_parse_xmon(char *p) > > } else if (strncmp(p, "on", 2) == 0) { > > xmon_init(1); > > xmon_on = 1; > > + } else if (strncmp(p, "rw", 2) == 0) { > > + xmon_init(1); > > + xmon_on = 1; > > + xmon_is_ro = false; > > + } else if (strncmp(p, "ro", 2) == 0) { > > + xmon_init(1); > > + xmon_on = 1; > > + xmon_is_ro = true; > > } else if (strncmp(p, "off", 3) == 0) > > xmon_on = 0; > > else > > -- > > 2.21.0 > >
> On April 8, 2019 at 2:37 AM Andrew Donnellan <andrew.donnellan@au1.ibm.com> wrote: > > > On 8/4/19 1:08 pm, Christopher M. Riedl wrote: > > Operations which write to memory and special purpose registers should be > > restricted on systems with integrity guarantees (such as Secure Boot) > > and, optionally, to avoid self-destructive behaviors. > > > > Add a config option, XMON_RW, to control default xmon behavior along > > with kernel cmdline options xmon=ro and xmon=rw for explicit control. > > Use XMON_RW instead of XMON in the condition to set PAGE_KERNEL_TEXT to > > allow xmon in read-only mode alongside write-protected kernel text. > > XMON_RW defaults to !STRICT_KERNEL_RWX. > > > > The following xmon operations are affected: > > memops: > > disable memmove > > disable memset > > disable memzcan > > memex: > > no-op'd mwrite > > super_regs: > > no-op'd write_spr > > bpt_cmds: > > disable > > proc_call: > > disable > > > > Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf> > > --- > > v1->v2: > > Use bool type for xmon_is_ro flag > > Replace XMON_RO with XMON_RW config option > > Make XMON_RW dependent on STRICT_KERNEL_RWX > > Use XMON_RW to control PAGE_KERNEL_TEXT > > Add printf in xmon read-only mode when dropping/skipping writes > > Disable memzcan (zero-fill memop) in xmon read-only mode > > > > arch/powerpc/Kconfig.debug | 10 +++++ > > arch/powerpc/include/asm/book3s/32/pgtable.h | 5 ++- > > arch/powerpc/include/asm/book3s/64/pgtable.h | 5 ++- > > arch/powerpc/include/asm/nohash/pgtable.h | 5 ++- > > arch/powerpc/xmon/xmon.c | 42 ++++++++++++++++++++ > > 5 files changed, 61 insertions(+), 6 deletions(-) > > > > diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug > > index 4e00cb0a5464..0c7f21476018 100644 > > --- a/arch/powerpc/Kconfig.debug > > +++ b/arch/powerpc/Kconfig.debug > > @@ -117,6 +117,16 @@ config XMON_DISASSEMBLY > > to say Y here, unless you're building for a memory-constrained > > system. > > > > +config XMON_RW > > + bool "Allow xmon read and write operations" > > "Allow xmon write operations" would be clearer. This option has no > impact on read operations. > Agreed, if the option isn't renamed again I will fix this in the next version :) > > > + depends on XMON > > + default !STRICT_KERNEL_RWX > > + help > > + Allow xmon to read and write to memory and special-purpose registers. > > + Conversely, prevent xmon write access when set to N. Read and write > > + access can also be explicitly controlled with 'xmon=rw' or 'xmon=ro' > > + (read-only) cmdline options. Default is !STRICT_KERNEL_RWX. > > This is an improvement but still doesn't clearly explain the > relationship between selecting this option and using the cmdline options. > I will reword this in the next version. > > > + > > config DEBUGGER > > bool > > depends on KGDB || XMON > > diff --git a/arch/powerpc/include/asm/book3s/32/pgtable.h b/arch/powerpc/include/asm/book3s/32/pgtable.h > > index aa8406b8f7ba..615144ad667d 100644 > > --- a/arch/powerpc/include/asm/book3s/32/pgtable.h > > +++ b/arch/powerpc/include/asm/book3s/32/pgtable.h > > @@ -86,8 +86,9 @@ static inline bool pte_user(pte_t pte) > > * set breakpoints anywhere, so don't write protect the kernel text > > * on platforms where such control is possible. > > */ > > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\ > > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > > + defined(CONFIG_DYNAMIC_FTRACE) > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > > #else > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > > diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h > > index 581f91be9dd4..bc4655122f6b 100644 > > --- a/arch/powerpc/include/asm/book3s/64/pgtable.h > > +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h > > @@ -168,8 +168,9 @@ > > * set breakpoints anywhere, so don't write protect the kernel text > > * on platforms where such control is possible. > > */ > > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) || \ > > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > > + defined(CONFIG_DYNAMIC_FTRACE) > > We might need to rethink how the cmdline options work. > > What happens if CONFIG_XMON_RW=n, but "xmon=rw" on the command line? > > I think I agree with Oliver that we're not being clear whether this > config option is selecting code to be compiled in, or whether it's just > setting a default. > Please refer to my reply to Oliver's comments -- 'xmon=rw' is broken and will be removed in the next version. I think 'xmon=ro' still makes sense to optionally enable xmon's read- only mode even when writes are allowed on a non-secure system. > > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > > #else > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > > diff --git a/arch/powerpc/include/asm/nohash/pgtable.h b/arch/powerpc/include/asm/nohash/pgtable.h > > index 1ca1c1864b32..c052931bd243 100644 > > --- a/arch/powerpc/include/asm/nohash/pgtable.h > > +++ b/arch/powerpc/include/asm/nohash/pgtable.h > > @@ -22,8 +22,9 @@ > > * set breakpoints anywhere, so don't write protect the kernel text > > * on platforms where such control is possible. > > */ > > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\ > > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > > + defined(CONFIG_DYNAMIC_FTRACE) > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > > #else > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > > diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c > > index a0f44f992360..224ca0b3506b 100644 > > --- a/arch/powerpc/xmon/xmon.c > > +++ b/arch/powerpc/xmon/xmon.c > > @@ -80,6 +80,7 @@ static int set_indicator_token = RTAS_UNKNOWN_SERVICE; > > #endif > > static unsigned long in_xmon __read_mostly = 0; > > static int xmon_on = IS_ENABLED(CONFIG_XMON_DEFAULT); > > +static bool xmon_is_ro = !IS_ENABLED(CONFIG_XMON_RW); > > > > static unsigned long adrs; > > static int size = 1; > > @@ -202,6 +203,8 @@ static void dump_tlb_book3e(void); > > #define GETWORD(v) (((v)[0] << 24) + ((v)[1] << 16) + ((v)[2] << 8) + (v)[3]) > > #endif > > > > +static const char *xmon_is_ro_warn = "xmon read-only mode: skipping write\n"; > > Yay, error messages! Though something like: "Operation disabled: xmon is > in read-only mode" would be clearer. > Agreed, changing this in the next version. > > > + > > static char *help_string = "\ > > Commands:\n\ > > b show breakpoints\n\ > > @@ -989,6 +992,10 @@ cmds(struct pt_regs *excp) > > memlocate(); > > break; > > case 'z': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > memzcan(); > > break; > > case 'i': > > @@ -1042,6 +1049,10 @@ cmds(struct pt_regs *excp) > > set_lpp_cmd(); > > break; > > case 'b': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > bpt_cmds(); > > break; > > case 'C': > > @@ -1055,6 +1066,10 @@ cmds(struct pt_regs *excp) > > bootcmds(); > > break; > > case 'p': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > proccall(); > > break; > > case 'P': > > @@ -1777,6 +1792,11 @@ read_spr(int n, unsigned long *vp) > > static void > > write_spr(int n, unsigned long val) > > { > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + return; > > + } > > + > > if (setjmp(bus_error_jmp) == 0) { > > catch_spr_faults = 1; > > sync(); > > @@ -2016,6 +2036,12 @@ mwrite(unsigned long adrs, void *buf, int size) > > char *p, *q; > > > > n = 0; > > + > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + return n; > > + } > > + > > if (setjmp(bus_error_jmp) == 0) { > > catch_memory_errors = 1; > > sync(); > > @@ -2884,9 +2910,17 @@ memops(int cmd) > > scanhex((void *)&mcount); > > switch( cmd ){ > > case 'm': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > memmove((void *)mdest, (void *)msrc, mcount); > > break; > > case 's': > > + if (xmon_is_ro) { > > + printf(xmon_is_ro_warn); > > + break; > > + } > > memset((void *)mdest, mval, mcount); > > break; > > case 'd': > > @@ -3796,6 +3830,14 @@ static int __init early_parse_xmon(char *p) > > } else if (strncmp(p, "on", 2) == 0) { > > xmon_init(1); > > xmon_on = 1; > > + } else if (strncmp(p, "rw", 2) == 0) { > > + xmon_init(1); > > + xmon_on = 1; > > + xmon_is_ro = false; > > + } else if (strncmp(p, "ro", 2) == 0) { > > + xmon_init(1); > > + xmon_on = 1; > > + xmon_is_ro = true; > > } else if (strncmp(p, "off", 3) == 0) > > xmon_on = 0; > > else > > > > -- > Andrew Donnellan OzLabs, ADL Canberra > andrew.donnellan@au1.ibm.com IBM Australia Limited >
On Wed, Apr 10, 2019 at 12:35 PM Christopher M Riedl <cmr@informatik.wtf> wrote: > > > > On April 8, 2019 at 1:34 AM Oliver <oohall@gmail.com> wrote: > > > > > > On Mon, Apr 8, 2019 at 1:06 PM Christopher M. Riedl <cmr@informatik.wtf> wrote: > > > > > > Operations which write to memory and special purpose registers should be > > > restricted on systems with integrity guarantees (such as Secure Boot) > > > and, optionally, to avoid self-destructive behaviors. > > > > > > Add a config option, XMON_RW, to control default xmon behavior along > > > with kernel cmdline options xmon=ro and xmon=rw for explicit control. > > > Use XMON_RW instead of XMON in the condition to set PAGE_KERNEL_TEXT to > > > allow xmon in read-only mode alongside write-protected kernel text. > > > XMON_RW defaults to !STRICT_KERNEL_RWX. > > > > > > The following xmon operations are affected: > > > memops: > > > disable memmove > > > disable memset > > > disable memzcan > > > memex: > > > no-op'd mwrite > > > super_regs: > > > no-op'd write_spr > > > bpt_cmds: > > > disable > > > proc_call: > > > disable > > > > > > Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf> > > > --- > > > v1->v2: > > > Use bool type for xmon_is_ro flag > > > Replace XMON_RO with XMON_RW config option > > > Make XMON_RW dependent on STRICT_KERNEL_RWX > > Do you mean make it dependent on XMON? > > > > Yeah that's really not clear at all -- XMON_RW is set based on the value of > STRICT_KERNEL_RWX. > > > > > > Use XMON_RW to control PAGE_KERNEL_TEXT > > > Add printf in xmon read-only mode when dropping/skipping writes > > > Disable memzcan (zero-fill memop) in xmon read-only mode > > > > > > arch/powerpc/Kconfig.debug | 10 +++++ > > > arch/powerpc/include/asm/book3s/32/pgtable.h | 5 ++- > > > arch/powerpc/include/asm/book3s/64/pgtable.h | 5 ++- > > > arch/powerpc/include/asm/nohash/pgtable.h | 5 ++- > > > arch/powerpc/xmon/xmon.c | 42 ++++++++++++++++++++ > > > 5 files changed, 61 insertions(+), 6 deletions(-) > > > > > > diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug > > > index 4e00cb0a5464..0c7f21476018 100644 > > > --- a/arch/powerpc/Kconfig.debug > > > +++ b/arch/powerpc/Kconfig.debug > > > @@ -117,6 +117,16 @@ config XMON_DISASSEMBLY > > > to say Y here, unless you're building for a memory-constrained > > > system. > > > > > > > > +config XMON_RW > > > + bool "Allow xmon read and write operations" > > > + depends on XMON > > > + default !STRICT_KERNEL_RWX > > > + help > > > + Allow xmon to read and write to memory and special-purpose registers. > > > + Conversely, prevent xmon write access when set to N. Read and write > > > + access can also be explicitly controlled with 'xmon=rw' or 'xmon=ro' > > > + (read-only) cmdline options. Default is !STRICT_KERNEL_RWX. > > > > Maybe I am a dumb, but I found this *extremely* confusing. > > Conventionally Kconfig options will control what code is and is not > > included in the kernel (see XMON_DISASSEMBLY) rather than changing the > > default behaviour of code. It's not wrong to do so and I'm going to > > assume that you were following the pattern of XMON_DEFAULT, but I > > think you need to be a little more clear about what option actually > > does. Renaming it to XMON_DEFAULT_RO_MODE and re-wording the > > description to indicate it's a only a mode change would help a lot. > > > > Sorry if this comes across as pointless bikeshedding since it's the > > opposite of what Christophe said in the last patch, but this was a bit > > of a head scratcher. > > > > If anyone is dumb here it's me for making this confusing :) > I chatted with Michael Ellerman about this, so let me try to explain this more clearly. > > There are two things I am trying to address with XMON_RW: > 1) provide a default access mode for xmon based on system "security" > 2) replace XMON in the decision to write-protect kernel text at compile-time > > I think a single Kconfig for both of those things is sensible as ultimately the > point is to allow xmon to operate in read-only mode on "secure" systems -- without > violating any integrity/security guarantees (such as write-protected kernel text). > > Christophe suggested looking at STRICT_KERNEL_RWX and I think that option makes the > most sense to base XMON_RW on since the description for STRICT_KERNEL_RWX states: > > > If this is set, kernel text and rodata memory will be made read-only, > > and non-text memory will be made non-executable. This provides > > protection against certain security exploits (e.g. executing the heap > > or modifying text) > > > > These features are considered standard security practice these days. > > You should say Y here in almost all cases. > > Considering this, does XMON_DEFAULT_RO_MODE really make things more clear? Not really. > With that said, I will remove the 'xmon=rw' cmdline option as it really doesn't work > since kernel text is write-protected at compile time. I think you're overestimating the importance of being able to write to the kernel text. The only features that require a writeable text are using the mem commands to modify the kernel text (a bad idea) and software breakpoints. Funnily enough, enabling STRICT_RWX make software breakpoints work again since it enables a smarter implementation of patch_instruction() that uses a temporary RW mapping to do the patch. Considering that kprobes and dynamic ftrace are also implemented with patch_instruction() too we could just default to the newer implementation and force PAGE_KERNEL_TEXT to be PAGE_KERNEL_ROX all the time. > 'xmon=ro' remains for those who would optionally like to prevent shooting themselves > in the foot/feet while using xmon. > > Hopefully this makes a bit more sense now? > > > > config DEBUGGER > > > bool > > > depends on KGDB || XMON > > > diff --git a/arch/powerpc/include/asm/book3s/32/pgtable.h b/arch/powerpc/include/asm/book3s/32/pgtable.h > > > index aa8406b8f7ba..615144ad667d 100644 > > > --- a/arch/powerpc/include/asm/book3s/32/pgtable.h > > > +++ b/arch/powerpc/include/asm/book3s/32/pgtable.h > > > @@ -86,8 +86,9 @@ static inline bool pte_user(pte_t pte) > > > * set breakpoints anywhere, so don't write protect the kernel text > > > * on platforms where such control is possible. > > > */ > > > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\ > > > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > > > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > > > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > > > + defined(CONFIG_DYNAMIC_FTRACE) > > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > > > #else > > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > > > diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h > > > index 581f91be9dd4..bc4655122f6b 100644 > > > --- a/arch/powerpc/include/asm/book3s/64/pgtable.h > > > +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h > > > @@ -168,8 +168,9 @@ > > > * set breakpoints anywhere, so don't write protect the kernel text > > > * on platforms where such control is possible. > > > */ > > > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) || \ > > > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > > > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > > > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > > > + defined(CONFIG_DYNAMIC_FTRACE) > > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > > > #else > > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > > > diff --git a/arch/powerpc/include/asm/nohash/pgtable.h b/arch/powerpc/include/asm/nohash/pgtable.h > > > index 1ca1c1864b32..c052931bd243 100644 > > > --- a/arch/powerpc/include/asm/nohash/pgtable.h > > > +++ b/arch/powerpc/include/asm/nohash/pgtable.h > > > @@ -22,8 +22,9 @@ > > > * set breakpoints anywhere, so don't write protect the kernel text > > > * on platforms where such control is possible. > > > */ > > > -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\ > > > - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) > > > +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ > > > + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ > > > + defined(CONFIG_DYNAMIC_FTRACE) > > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_X > > > #else > > > #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX > > > diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c > > > index a0f44f992360..224ca0b3506b 100644 > > > --- a/arch/powerpc/xmon/xmon.c > > > +++ b/arch/powerpc/xmon/xmon.c > > > @@ -80,6 +80,7 @@ static int set_indicator_token = RTAS_UNKNOWN_SERVICE; > > > #endif > > > static unsigned long in_xmon __read_mostly = 0; > > > static int xmon_on = IS_ENABLED(CONFIG_XMON_DEFAULT); > > > +static bool xmon_is_ro = !IS_ENABLED(CONFIG_XMON_RW); > > > > > > static unsigned long adrs; > > > static int size = 1; > > > @@ -202,6 +203,8 @@ static void dump_tlb_book3e(void); > > > #define GETWORD(v) (((v)[0] << 24) + ((v)[1] << 16) + ((v)[2] << 8) + (v)[3]) > > > #endif > > > > > > +static const char *xmon_is_ro_warn = "xmon read-only mode: skipping write\n"; > > > + > > > static char *help_string = "\ > > > Commands:\n\ > > > b show breakpoints\n\ > > > @@ -989,6 +992,10 @@ cmds(struct pt_regs *excp) > > > memlocate(); > > > break; > > > case 'z': > > > + if (xmon_is_ro) { > > > + printf(xmon_is_ro_warn); > > > + break; > > > + } > > > memzcan(); > > > break; > > > case 'i': > > > @@ -1042,6 +1049,10 @@ cmds(struct pt_regs *excp) > > > set_lpp_cmd(); > > > break; > > > case 'b': > > > + if (xmon_is_ro) { > > > + printf(xmon_is_ro_warn); > > > + break; > > > + } > > > bpt_cmds(); > > > break; > > > case 'C': > > > @@ -1055,6 +1066,10 @@ cmds(struct pt_regs *excp) > > > bootcmds(); > > > break; > > > case 'p': > > > + if (xmon_is_ro) { > > > + printf(xmon_is_ro_warn); > > > + break; > > > + } > > > proccall(); > > > break; > > > case 'P': > > > @@ -1777,6 +1792,11 @@ read_spr(int n, unsigned long *vp) > > > static void > > > write_spr(int n, unsigned long val) > > > { > > > + if (xmon_is_ro) { > > > + printf(xmon_is_ro_warn); > > > + return; > > > + } > > > + > > > if (setjmp(bus_error_jmp) == 0) { > > > catch_spr_faults = 1; > > > sync(); > > > @@ -2016,6 +2036,12 @@ mwrite(unsigned long adrs, void *buf, int size) > > > char *p, *q; > > > > > > n = 0; > > > + > > > + if (xmon_is_ro) { > > > + printf(xmon_is_ro_warn); > > > + return n; > > > + } > > > + > > > if (setjmp(bus_error_jmp) == 0) { > > > catch_memory_errors = 1; > > > sync(); > > > @@ -2884,9 +2910,17 @@ memops(int cmd) > > > scanhex((void *)&mcount); > > > switch( cmd ){ > > > case 'm': > > > + if (xmon_is_ro) { > > > + printf(xmon_is_ro_warn); > > > + break; > > > + } > > > memmove((void *)mdest, (void *)msrc, mcount); > > > break; > > > case 's': > > > + if (xmon_is_ro) { > > > + printf(xmon_is_ro_warn); > > > + break; > > > + } > > > memset((void *)mdest, mval, mcount); > > > break; > > > case 'd': > > > @@ -3796,6 +3830,14 @@ static int __init early_parse_xmon(char *p) > > > } else if (strncmp(p, "on", 2) == 0) { > > > xmon_init(1); > > > xmon_on = 1; > > > + } else if (strncmp(p, "rw", 2) == 0) { > > > + xmon_init(1); > > > + xmon_on = 1; > > > + xmon_is_ro = false; > > > + } else if (strncmp(p, "ro", 2) == 0) { > > > + xmon_init(1); > > > + xmon_on = 1; > > > + xmon_is_ro = true; > > > } else if (strncmp(p, "off", 3) == 0) > > > xmon_on = 0; > > > else > > > -- > > > 2.21.0 > > >
Christopher M Riedl <cmr@informatik.wtf> writes: >> On April 8, 2019 at 1:34 AM Oliver <oohall@gmail.com> wrote: >> On Mon, Apr 8, 2019 at 1:06 PM Christopher M. Riedl <cmr@informatik.wtf> wrote: ... >> > >> > diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug >> > index 4e00cb0a5464..0c7f21476018 100644 >> > --- a/arch/powerpc/Kconfig.debug >> > +++ b/arch/powerpc/Kconfig.debug >> > @@ -117,6 +117,16 @@ config XMON_DISASSEMBLY >> > to say Y here, unless you're building for a memory-constrained >> > system. >> > >> >> > +config XMON_RW >> > + bool "Allow xmon read and write operations" >> > + depends on XMON >> > + default !STRICT_KERNEL_RWX >> > + help >> > + Allow xmon to read and write to memory and special-purpose registers. >> > + Conversely, prevent xmon write access when set to N. Read and write >> > + access can also be explicitly controlled with 'xmon=rw' or 'xmon=ro' >> > + (read-only) cmdline options. Default is !STRICT_KERNEL_RWX. >> >> Maybe I am a dumb, but I found this *extremely* confusing. >> Conventionally Kconfig options will control what code is and is not >> included in the kernel (see XMON_DISASSEMBLY) rather than changing the >> default behaviour of code. It's not wrong to do so and I'm going to >> assume that you were following the pattern of XMON_DEFAULT, but I >> think you need to be a little more clear about what option actually >> does. Renaming it to XMON_DEFAULT_RO_MODE and re-wording the >> description to indicate it's a only a mode change would help a lot. >> >> Sorry if this comes across as pointless bikeshedding since it's the >> opposite of what Christophe said in the last patch, but this was a bit >> of a head scratcher. > > If anyone is dumb here it's me for making this confusing :) > I chatted with Michael Ellerman about this, so let me try to explain this more clearly. Yeah it's my fault :) > There are two things I am trying to address with XMON_RW: > 1) provide a default access mode for xmon based on system "security" I think I've gone off this idea. Tying them together is just enforcing a linkage that people may not want. I think XMON_RW should just be an option that stands on its own. It should probably be default n, to give people a safe default. > 2) replace XMON in the decision to write-protect kernel text at compile-time We should do that as a separate patch. That's actually a bug in the current STRICT_KERNEL_RWX support. ie. STRICT_KERNEL_RWX should always give you PAGE_KERNEL_ROX, regardless of XMON or anything else. > I think a single Kconfig for both of those things is sensible as ultimately the > point is to allow xmon to operate in read-only mode on "secure" systems -- without > violating any integrity/security guarantees (such as write-protected kernel text). > > Christophe suggested looking at STRICT_KERNEL_RWX and I think that option makes the > most sense to base XMON_RW on since the description for STRICT_KERNEL_RWX states: Once we fix the bugs in STRICT_KERNEL_RWX people are going to enable that by default, so it will essentially be always on in future. > With that said, I will remove the 'xmon=rw' cmdline option as it really doesn't work > since kernel text is write-protected at compile time. I think 'xmon=rw' still makes sense. Only some of the RW functionality relies on being able to patch kernel text. And once you have proccall() you can just call a function to make it read/write anyway, or use memex to manually frob the page tables. cheers
> On April 11, 2019 at 8:37 AM Michael Ellerman <mpe@ellerman.id.au> wrote: > > > Christopher M Riedl <cmr@informatik.wtf> writes: > >> On April 8, 2019 at 1:34 AM Oliver <oohall@gmail.com> wrote: > >> On Mon, Apr 8, 2019 at 1:06 PM Christopher M. Riedl <cmr@informatik.wtf> wrote: > ... > >> > > >> > diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug > >> > index 4e00cb0a5464..0c7f21476018 100644 > >> > --- a/arch/powerpc/Kconfig.debug > >> > +++ b/arch/powerpc/Kconfig.debug > >> > @@ -117,6 +117,16 @@ config XMON_DISASSEMBLY > >> > to say Y here, unless you're building for a memory-constrained > >> > system. > >> > > >> > >> > +config XMON_RW > >> > + bool "Allow xmon read and write operations" > >> > + depends on XMON > >> > + default !STRICT_KERNEL_RWX > >> > + help > >> > + Allow xmon to read and write to memory and special-purpose registers. > >> > + Conversely, prevent xmon write access when set to N. Read and write > >> > + access can also be explicitly controlled with 'xmon=rw' or 'xmon=ro' > >> > + (read-only) cmdline options. Default is !STRICT_KERNEL_RWX. > >> > >> Maybe I am a dumb, but I found this *extremely* confusing. > >> Conventionally Kconfig options will control what code is and is not > >> included in the kernel (see XMON_DISASSEMBLY) rather than changing the > >> default behaviour of code. It's not wrong to do so and I'm going to > >> assume that you were following the pattern of XMON_DEFAULT, but I > >> think you need to be a little more clear about what option actually > >> does. Renaming it to XMON_DEFAULT_RO_MODE and re-wording the > >> description to indicate it's a only a mode change would help a lot. > >> > >> Sorry if this comes across as pointless bikeshedding since it's the > >> opposite of what Christophe said in the last patch, but this was a bit > >> of a head scratcher. > > > > If anyone is dumb here it's me for making this confusing :) > > I chatted with Michael Ellerman about this, so let me try to explain this more clearly. > > Yeah it's my fault :) > "Signed-off-by: Christopher M. Riedl" -- I take full responsibility hah. > > > There are two things I am trying to address with XMON_RW: > > 1) provide a default access mode for xmon based on system "security" > > I think I've gone off this idea. Tying them together is just enforcing a > linkage that people may not want. > > I think XMON_RW should just be an option that stands on its own. It > should probably be default n, to give people a safe default. > Next version includes this along with making it clear that this option provides the default mode for XMON. > > > 2) replace XMON in the decision to write-protect kernel text at compile-time > > We should do that as a separate patch. That's actually a bug in the > current STRICT_KERNEL_RWX support. > > ie. STRICT_KERNEL_RWX should always give you PAGE_KERNEL_ROX, regardless > of XMON or anything else. > > > I think a single Kconfig for both of those things is sensible as ultimately the > > point is to allow xmon to operate in read-only mode on "secure" systems -- without > > violating any integrity/security guarantees (such as write-protected kernel text). > > > > Christophe suggested looking at STRICT_KERNEL_RWX and I think that option makes the > > most sense to base XMON_RW on since the description for STRICT_KERNEL_RWX states: > > Once we fix the bugs in STRICT_KERNEL_RWX people are going to enable > that by default, so it will essentially be always on in future. > > > > With that said, I will remove the 'xmon=rw' cmdline option as it really doesn't work > > since kernel text is write-protected at compile time. > > I think 'xmon=rw' still makes sense. Only some of the RW functionality > relies on being able to patch kernel text. > > And once you have proccall() you can just call a function to make it > read/write anyway, or use memex to manually frob the page tables. > > cheers Great, adding this back in the next version.
diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug index 4e00cb0a5464..0c7f21476018 100644 --- a/arch/powerpc/Kconfig.debug +++ b/arch/powerpc/Kconfig.debug @@ -117,6 +117,16 @@ config XMON_DISASSEMBLY to say Y here, unless you're building for a memory-constrained system. +config XMON_RW + bool "Allow xmon read and write operations" + depends on XMON + default !STRICT_KERNEL_RWX + help + Allow xmon to read and write to memory and special-purpose registers. + Conversely, prevent xmon write access when set to N. Read and write + access can also be explicitly controlled with 'xmon=rw' or 'xmon=ro' + (read-only) cmdline options. Default is !STRICT_KERNEL_RWX. + config DEBUGGER bool depends on KGDB || XMON diff --git a/arch/powerpc/include/asm/book3s/32/pgtable.h b/arch/powerpc/include/asm/book3s/32/pgtable.h index aa8406b8f7ba..615144ad667d 100644 --- a/arch/powerpc/include/asm/book3s/32/pgtable.h +++ b/arch/powerpc/include/asm/book3s/32/pgtable.h @@ -86,8 +86,9 @@ static inline bool pte_user(pte_t pte) * set breakpoints anywhere, so don't write protect the kernel text * on platforms where such control is possible. */ -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\ - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ + defined(CONFIG_DYNAMIC_FTRACE) #define PAGE_KERNEL_TEXT PAGE_KERNEL_X #else #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h index 581f91be9dd4..bc4655122f6b 100644 --- a/arch/powerpc/include/asm/book3s/64/pgtable.h +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h @@ -168,8 +168,9 @@ * set breakpoints anywhere, so don't write protect the kernel text * on platforms where such control is possible. */ -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) || \ - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ + defined(CONFIG_DYNAMIC_FTRACE) #define PAGE_KERNEL_TEXT PAGE_KERNEL_X #else #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX diff --git a/arch/powerpc/include/asm/nohash/pgtable.h b/arch/powerpc/include/asm/nohash/pgtable.h index 1ca1c1864b32..c052931bd243 100644 --- a/arch/powerpc/include/asm/nohash/pgtable.h +++ b/arch/powerpc/include/asm/nohash/pgtable.h @@ -22,8 +22,9 @@ * set breakpoints anywhere, so don't write protect the kernel text * on platforms where such control is possible. */ -#if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\ - defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE) +#if defined(CONFIG_KGDB) || defined(CONFIG_XMON_RW) || \ + defined(CONFIG_BDI_SWITCH) || defined(CONFIG_KPROBES) || \ + defined(CONFIG_DYNAMIC_FTRACE) #define PAGE_KERNEL_TEXT PAGE_KERNEL_X #else #define PAGE_KERNEL_TEXT PAGE_KERNEL_ROX diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c index a0f44f992360..224ca0b3506b 100644 --- a/arch/powerpc/xmon/xmon.c +++ b/arch/powerpc/xmon/xmon.c @@ -80,6 +80,7 @@ static int set_indicator_token = RTAS_UNKNOWN_SERVICE; #endif static unsigned long in_xmon __read_mostly = 0; static int xmon_on = IS_ENABLED(CONFIG_XMON_DEFAULT); +static bool xmon_is_ro = !IS_ENABLED(CONFIG_XMON_RW); static unsigned long adrs; static int size = 1; @@ -202,6 +203,8 @@ static void dump_tlb_book3e(void); #define GETWORD(v) (((v)[0] << 24) + ((v)[1] << 16) + ((v)[2] << 8) + (v)[3]) #endif +static const char *xmon_is_ro_warn = "xmon read-only mode: skipping write\n"; + static char *help_string = "\ Commands:\n\ b show breakpoints\n\ @@ -989,6 +992,10 @@ cmds(struct pt_regs *excp) memlocate(); break; case 'z': + if (xmon_is_ro) { + printf(xmon_is_ro_warn); + break; + } memzcan(); break; case 'i': @@ -1042,6 +1049,10 @@ cmds(struct pt_regs *excp) set_lpp_cmd(); break; case 'b': + if (xmon_is_ro) { + printf(xmon_is_ro_warn); + break; + } bpt_cmds(); break; case 'C': @@ -1055,6 +1066,10 @@ cmds(struct pt_regs *excp) bootcmds(); break; case 'p': + if (xmon_is_ro) { + printf(xmon_is_ro_warn); + break; + } proccall(); break; case 'P': @@ -1777,6 +1792,11 @@ read_spr(int n, unsigned long *vp) static void write_spr(int n, unsigned long val) { + if (xmon_is_ro) { + printf(xmon_is_ro_warn); + return; + } + if (setjmp(bus_error_jmp) == 0) { catch_spr_faults = 1; sync(); @@ -2016,6 +2036,12 @@ mwrite(unsigned long adrs, void *buf, int size) char *p, *q; n = 0; + + if (xmon_is_ro) { + printf(xmon_is_ro_warn); + return n; + } + if (setjmp(bus_error_jmp) == 0) { catch_memory_errors = 1; sync(); @@ -2884,9 +2910,17 @@ memops(int cmd) scanhex((void *)&mcount); switch( cmd ){ case 'm': + if (xmon_is_ro) { + printf(xmon_is_ro_warn); + break; + } memmove((void *)mdest, (void *)msrc, mcount); break; case 's': + if (xmon_is_ro) { + printf(xmon_is_ro_warn); + break; + } memset((void *)mdest, mval, mcount); break; case 'd': @@ -3796,6 +3830,14 @@ static int __init early_parse_xmon(char *p) } else if (strncmp(p, "on", 2) == 0) { xmon_init(1); xmon_on = 1; + } else if (strncmp(p, "rw", 2) == 0) { + xmon_init(1); + xmon_on = 1; + xmon_is_ro = false; + } else if (strncmp(p, "ro", 2) == 0) { + xmon_init(1); + xmon_on = 1; + xmon_is_ro = true; } else if (strncmp(p, "off", 3) == 0) xmon_on = 0; else
Operations which write to memory and special purpose registers should be restricted on systems with integrity guarantees (such as Secure Boot) and, optionally, to avoid self-destructive behaviors. Add a config option, XMON_RW, to control default xmon behavior along with kernel cmdline options xmon=ro and xmon=rw for explicit control. Use XMON_RW instead of XMON in the condition to set PAGE_KERNEL_TEXT to allow xmon in read-only mode alongside write-protected kernel text. XMON_RW defaults to !STRICT_KERNEL_RWX. The following xmon operations are affected: memops: disable memmove disable memset disable memzcan memex: no-op'd mwrite super_regs: no-op'd write_spr bpt_cmds: disable proc_call: disable Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf> --- v1->v2: Use bool type for xmon_is_ro flag Replace XMON_RO with XMON_RW config option Make XMON_RW dependent on STRICT_KERNEL_RWX Use XMON_RW to control PAGE_KERNEL_TEXT Add printf in xmon read-only mode when dropping/skipping writes Disable memzcan (zero-fill memop) in xmon read-only mode arch/powerpc/Kconfig.debug | 10 +++++ arch/powerpc/include/asm/book3s/32/pgtable.h | 5 ++- arch/powerpc/include/asm/book3s/64/pgtable.h | 5 ++- arch/powerpc/include/asm/nohash/pgtable.h | 5 ++- arch/powerpc/xmon/xmon.c | 42 ++++++++++++++++++++ 5 files changed, 61 insertions(+), 6 deletions(-)