Message ID | 20201213183556.16976-1-ariel.marcovitch@gmail.com (mailing list archive) |
---|---|
State | Superseded |
Headers | show |
Series | powerpc: fix alignment bug whithin the init sections | expand |
Le 13/12/2020 à 19:35, Ariel Marcovitch a écrit : > This is a bug that can cause early crashes in configurations with a > .exit.text section smaller than a page and a .init.text section that > ends in the beginning of a physical page (this is kinda random, which > might explain why this wasn't really encountered before). It can cause, or it causes ? Did you encounter the issue ? > > The init sections are ordered like this: > .init.text > .exit.text > .init.data > > Currently, these sections aren't page aligned. > > Because the init code is mapped read-only at runtime and because the > .init.text section can potentially reside on the same physical page as > .init.data, the beginning of .init.data might be mapped read-only along > with .init.text. init code is mapped PAGE_KERNEL_TEXT. Whether PAGE_KERNEL_TEXT is read-only or not depends on the selected options. > > Then when the kernel tries to modify a variable in .init.data (like > kthreadd_done, used in kernel_init()) the kernel panics. > > To avoid this, I made these sections page aligned. Should write this unpersonal, something like "To avoid this, make these sections page aligned" > > Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext") > Signed-off-by: Ariel Marcovitch <ariel.marcovitch@gmail.com> > --- > arch/powerpc/kernel/vmlinux.lds.S | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S > index 326e113d2e45..e3a7c90c03f4 100644 > --- a/arch/powerpc/kernel/vmlinux.lds.S > +++ b/arch/powerpc/kernel/vmlinux.lds.S > @@ -179,6 +179,11 @@ SECTIONS > #endif > } :text > > + /* .init.text is made RO and .exit.text is not, so we must > + * ensure these sections reside in separate physical pages. > + */ > + . = ALIGN(PAGE_SIZE); > + In principle, as it is text, it should be made RO as well. But what happens at the begining doesn't really matter, anyway .exit.text should never be executed and is discarded together with init text. So, I think it is OK the live with it as is for the time being. Making it page aligned makes sense anyway. Should we make _einittext page aligned instead, just like _etext ? > /* .exit.text is discarded at runtime, not link time, > * to deal with references from __bug_table > */ > @@ -186,6 +191,8 @@ SECTIONS > EXIT_TEXT > } > > + . = ALIGN(PAGE_SIZE); > + Here for sure, as you explain in the coming log, this needs to be separated from init text. So making it aligned is a must. > .init.data : AT(ADDR(.init.data) - LOAD_OFFSET) { > INIT_DATA > } > > base-commit: 1398820fee515873379809a6415930ad0764b2f6 > Christophe
On Fri, Dec 18, 2020 at 5:39 PM Christophe Leroy < christophe.leroy@csgroup.eu> wrote: > It can cause, or it causes ? Did you encounter the issue ? > Yes, in configs that result in the section layout I described, the crush is consistent. > > > The init sections are ordered like this: > > .init.text > > .exit.text > > .init.data > > > > Currently, these sections aren't page aligned. > > > > Because the init code is mapped read-only at runtime and because the > > .init.text section can potentially reside on the same physical page as > > .init.data, the beginning of .init.data might be mapped read-only along > > with .init.text. > > init code is mapped PAGE_KERNEL_TEXT. > > Whether PAGE_KERNEL_TEXT is read-only or not depends on the selected > options. > You are right, of course. Should I change the commit message to 'might be mapped' or something? > > > Then when the kernel tries to modify a variable in .init.data (like > > kthreadd_done, used in kernel_init()) the kernel panics. > > > > To avoid this, I made these sections page aligned. > > Should write this unpersonal, something like "To avoid this, make these > sections page aligned" > Noted, thanks. > > > > > Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext") > > Signed-off-by: Ariel Marcovitch <ariel.marcovitch@gmail.com> > > --- > > arch/powerpc/kernel/vmlinux.lds.S | 7 +++++++ > > 1 file changed, 7 insertions(+) > > > > diff --git a/arch/powerpc/kernel/vmlinux.lds.S > b/arch/powerpc/kernel/vmlinux.lds.S > > index 326e113d2e45..e3a7c90c03f4 100644 > > --- a/arch/powerpc/kernel/vmlinux.lds.S > > +++ b/arch/powerpc/kernel/vmlinux.lds.S > > @@ -179,6 +179,11 @@ SECTIONS > > #endif > > } :text > > > > + /* .init.text is made RO and .exit.text is not, so we must > > + * ensure these sections reside in separate physical pages. > > + */ > > + . = ALIGN(PAGE_SIZE); > > + > > In principle, as it is text, it should be made RO as well. But what > happens at the begining doesn't > really matter, anyway .exit.text should never be executed and is discarded > together with init text. > So, I think it is OK the live with it as is for the time being. > Making it page aligned makes sense anyway. > > Should we make _einittext page aligned instead, just like _etext ? Yes, this will probably be better (because when _einittext is not aligned, the part of the page after _einittext is mapped RO implicitly, and it's hard to notice from the code). I suppose you mean something like this: _sinittext = .; INIT_TEXT + + . = ALIGN(.); _einittext = .; > /* .exit.text is discarded at runtime, not link time, > > * to deal with references from __bug_table > > */ > > @@ -186,6 +191,8 @@ SECTIONS > > EXIT_TEXT > > } > > > > + . = ALIGN(PAGE_SIZE); > > + > > Here for sure, as you explain in the coming log, this needs to be > separated from init text. So > making it aligned is a must. > > .init.data : AT(ADDR(.init.data) - LOAD_OFFSET) { > > INIT_DATA > > } > > > > base-commit: 1398820fee515873379809a6415930ad0764b2f6 > > > > Christophe > Thanks for your time, Ariel Marcovitch
On Fri, Dec 18, 2020 at 5:39 PM Christophe Leroy < christophe.leroy@csgroup.eu> wrote: > It can cause, or it causes ? Did you encounter the issue ? > Yes, in configs that result in the section layout I described, the crush is consistent. > > > The init sections are ordered like this: > > .init.text > > .exit.text > > .init.data > > > > Currently, these sections aren't page aligned. > > > > Because the init code is mapped read-only at runtime and because the > > .init.text section can potentially reside on the same physical page as > > .init.data, the beginning of .init.data might be mapped read-only along > > with .init.text. > > init code is mapped PAGE_KERNEL_TEXT. > > Whether PAGE_KERNEL_TEXT is read-only or not depends on the selected > options. > You are right, of course. Should I change the commit message to 'might be mapped' or something? > > > Then when the kernel tries to modify a variable in .init.data (like > > kthreadd_done, used in kernel_init()) the kernel panics. > > > > To avoid this, I made these sections page aligned. > > Should write this unpersonal, something like "To avoid this, make these > sections page aligned" > Noted, thanks. > > > > > Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext") > > Signed-off-by: Ariel Marcovitch <ariel.marcovitch@gmail.com> > > --- > > arch/powerpc/kernel/vmlinux.lds.S | 7 +++++++ > > 1 file changed, 7 insertions(+) > > > > diff --git a/arch/powerpc/kernel/vmlinux.lds.S > b/arch/powerpc/kernel/vmlinux.lds.S > > index 326e113d2e45..e3a7c90c03f4 100644 > > --- a/arch/powerpc/kernel/vmlinux.lds.S > > +++ b/arch/powerpc/kernel/vmlinux.lds.S > > @@ -179,6 +179,11 @@ SECTIONS > > #endif > > } :text > > > > + /* .init.text is made RO and .exit.text is not, so we must > > + * ensure these sections reside in separate physical pages. > > + */ > > + . = ALIGN(PAGE_SIZE); > > + > > In principle, as it is text, it should be made RO as well. But what > happens at the begining doesn't > really matter, anyway .exit.text should never be executed and is discarded > together with init text. > So, I think it is OK the live with it as is for the time being. > Making it page aligned makes sense anyway. > > Should we make _einittext page aligned instead, just like _etext ? Yes, this will probably be better (because when _einittext is not aligned, the part of the page after _einittext is mapped RO implicitly, and it's hard to notice from the code). I suppose you mean something like this: _sinittext = .; INIT_TEXT + + . = ALIGN(.); _einittext = .; > /* .exit.text is discarded at runtime, not link time, > > * to deal with references from __bug_table > > */ > > @@ -186,6 +191,8 @@ SECTIONS > > EXIT_TEXT > > } > > > > + . = ALIGN(PAGE_SIZE); > > + > > Here for sure, as you explain in the coming log, this needs to be > separated from init text. So > making it aligned is a must. > > .init.data : AT(ADDR(.init.data) - LOAD_OFFSET) { > > INIT_DATA > > } > > > > base-commit: 1398820fee515873379809a6415930ad0764b2f6 > > > > Christophe > Thanks for your time, Ariel Marcovitch
diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S index 326e113d2e45..e3a7c90c03f4 100644 --- a/arch/powerpc/kernel/vmlinux.lds.S +++ b/arch/powerpc/kernel/vmlinux.lds.S @@ -179,6 +179,11 @@ SECTIONS #endif } :text + /* .init.text is made RO and .exit.text is not, so we must + * ensure these sections reside in separate physical pages. + */ + . = ALIGN(PAGE_SIZE); + /* .exit.text is discarded at runtime, not link time, * to deal with references from __bug_table */ @@ -186,6 +191,8 @@ SECTIONS EXIT_TEXT } + . = ALIGN(PAGE_SIZE); + .init.data : AT(ADDR(.init.data) - LOAD_OFFSET) { INIT_DATA }
This is a bug that can cause early crashes in configurations with a .exit.text section smaller than a page and a .init.text section that ends in the beginning of a physical page (this is kinda random, which might explain why this wasn't really encountered before). The init sections are ordered like this: .init.text .exit.text .init.data Currently, these sections aren't page aligned. Because the init code is mapped read-only at runtime and because the .init.text section can potentially reside on the same physical page as .init.data, the beginning of .init.data might be mapped read-only along with .init.text. Then when the kernel tries to modify a variable in .init.data (like kthreadd_done, used in kernel_init()) the kernel panics. To avoid this, I made these sections page aligned. Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext") Signed-off-by: Ariel Marcovitch <ariel.marcovitch@gmail.com> --- arch/powerpc/kernel/vmlinux.lds.S | 7 +++++++ 1 file changed, 7 insertions(+) base-commit: 1398820fee515873379809a6415930ad0764b2f6