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

[Andrew Donnellan]

On 29/3/19 3:21 pm, cmr wrote:
> Operations which write to memory should be restricted on secure systems
> and optionally to avoid self-destructive behaviors.

For reference:
  - https://github.com/linuxppc/issues/issues/219
  - https://github.com/linuxppc/issues/issues/232

Perhaps clarify what is meant here by "secure systems".

Otherwise commit message looks good.

> 
> Add a config option, XMON_RO, to control default xmon behavior along
> with kernel cmdline options xmon=ro and xmon=rw for explicit control.
> The default is to enable read-only mode.
> 
> The following xmon operations are affected:
> memops:
> 	disable memmove
> 	disable memset
> memex:
> 	no-op'd mwrite
> super_regs:
> 	no-op'd write_spr
> bpt_cmds:
> 	disable
> proc_call:
> 	disable
> 
> Signed-off-by: cmr <cmr at informatik.wtf>

You have a git config to fix :)

> ---
>   arch/powerpc/Kconfig.debug |  7 +++++++
>   arch/powerpc/xmon/xmon.c   | 24 ++++++++++++++++++++++++
>   2 files changed, 31 insertions(+)
> 
> diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
> index 4e00cb0a5464..33cc01adf4cb 100644
> --- a/arch/powerpc/Kconfig.debug
> +++ b/arch/powerpc/Kconfig.debug
> @@ -117,6 +117,13 @@ config XMON_DISASSEMBLY
>   	  to say Y here, unless you're building for a memory-constrained
>   	  system.
>   
> +config XMON_RO
> +	bool "Set xmon read-only mode"
> +	depends on XMON
> +	default y
> +	help
> +	  Disable state- and memory-altering write operations in xmon.

The meaning of this option is a bit unclear.

 From the code - it looks like what this option actually does is enable 
RO mode *by default*. In which case it should probably be called 
XMON_RO_DEFAULT and the description should note that RW mode can still 
be enabled via a cmdline option.

> +
>   config DEBUGGER
>   	bool
>   	depends on KGDB || XMON
> diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
> index a0f44f992360..c13ee73cdfd4 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 int xmon_ro = IS_ENABLED(CONFIG_XMON_RO);
>   
>   static unsigned long adrs;
>   static int size = 1;
> @@ -1042,6 +1043,8 @@ cmds(struct pt_regs *excp)
>   			set_lpp_cmd();
>   			break;
>   		case 'b':
> +			if (xmon_ro == 1)
> +				break;

For all these cases - it would be much better to print an error message 
somewhere when we abort due to read-only mode.

>   			bpt_cmds();
>   			break;
>   		case 'C':
> @@ -1055,6 +1058,8 @@ cmds(struct pt_regs *excp)
>   			bootcmds();
>   			break;
>   		case 'p':
> +			if (xmon_ro == 1)
> +				break;
>   			proccall();
>   			break;
>   		case 'P':
> @@ -1777,6 +1782,9 @@ read_spr(int n, unsigned long *vp)
>   static void
>   write_spr(int n, unsigned long val)
>   {
> +	if (xmon_ro == 1)
> +		return;
> +
>   	if (setjmp(bus_error_jmp) == 0) {
>   		catch_spr_faults = 1;
>   		sync();
> @@ -2016,6 +2024,10 @@ mwrite(unsigned long adrs, void *buf, int size)
>   	char *p, *q;
>   
>   	n = 0;
> +
> +	if (xmon_ro == 1)
> +		return n;
> +
>   	if (setjmp(bus_error_jmp) == 0) {
>   		catch_memory_errors = 1;
>   		sync();
> @@ -2884,9 +2896,13 @@ memops(int cmd)
>   	scanhex((void *)&mcount);
>   	switch( cmd ){
>   	case 'm':
> +		if (xmon_ro == 1)
> +			break;
>   		memmove((void *)mdest, (void *)msrc, mcount);
>   		break;
>   	case 's':
> +		if (xmon_ro == 1)
> +			break;
>   		memset((void *)mdest, mval, mcount);
>   		break;
>   	case 'd':
> @@ -3796,6 +3812,14 @@ 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_ro = 0;
> +	} else if (strncmp(p, "ro", 2) == 0) {
> +		xmon_init(1);
> +		xmon_on = 1;
> +		xmon_ro = 1;
>   	} else if (strncmp(p, "off", 3) == 0)
>   		xmon_on = 0;
>   	else
> 

-- 
Andrew Donnellan              OzLabs, ADL Canberra
andrew.donnellan at au1.ibm.com  IBM Australia Limited