[v4] powerpc/xmon: add read-only mode

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_DEFAULT_RO_MODE, to set default xmon behavior.
The kernel cmdline options xmon=ro and xmon=rw override this default.

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>
Reviewed-by: Oliver O'Halloran <oohall@gmail.com>
---

v3->v4:
	Address Andrew's nitpick.

 arch/powerpc/Kconfig.debug |  8 ++++++++
 arch/powerpc/xmon/xmon.c   | 42 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+)

Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>

On 16/4/19 1:26 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_DEFAULT_RO_MODE, to set default xmon behavior.
> The kernel cmdline options xmon=ro and xmon=rw override this default.
> 
> 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>
> Reviewed-by: Oliver O'Halloran <oohall@gmail.com>
> ---
> 
> v3->v4:
> 	Address Andrew's nitpick.
> 
>   arch/powerpc/Kconfig.debug |  8 ++++++++
>   arch/powerpc/xmon/xmon.c   | 42 ++++++++++++++++++++++++++++++++++++++
>   2 files changed, 50 insertions(+)
> 
> diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
> index 4e00cb0a5464..326ac5ea3f72 100644
> --- a/arch/powerpc/Kconfig.debug
> +++ b/arch/powerpc/Kconfig.debug
> @@ -117,6 +117,14 @@ config XMON_DISASSEMBLY
>   	  to say Y here, unless you're building for a memory-constrained
>   	  system.
>   
> +config XMON_DEFAULT_RO_MODE
> +	bool "Restrict xmon to read-only operations by default"
> +	depends on XMON
> +	default y
> +	help
> +          Operate xmon in read-only mode. The cmdline options 'xmon=rw' and
> +          'xmon=ro' override this default.
> +
>   config DEBUGGER
>   	bool
>   	depends on KGDB || XMON
> diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
> index a0f44f992360..ce98c8049eb6 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_DEFAULT_RO_MODE);
>   
>   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_ro_msg = "Operation disabled: xmon in read-only mode\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_ro_msg);
> +					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_ro_msg);
> +				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_ro_msg);
> +				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_ro_msg);
> +		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_ro_msg);
> +		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_ro_msg);
> +			break;
> +		}
>   		memmove((void *)mdest, (void *)msrc, mcount);
>   		break;
>   	case 's':
> +		if (xmon_is_ro) {
> +			printf(xmon_ro_msg);
> +			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
>