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

[Christopher M Riedl]

> On March 29, 2019 at 12:49 AM Andrew Donnellan <andrew.donnellan at au1.ibm.com> wrote:
> 
> 
> 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.
> 

I will reword this for the next patch to reflect the verbiage in the referenced
github issue -- ie. Secure Boot and not violating secure boot integrity by using xmon.

> 
> > ---
> >   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.
>

Based on Christophe's feedback the default will change for this option in the
next patch. I will also add the cmdline options to the description for clarity.

>
> > +
> >   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.
> 

I included print messages initially but then thought about how xmon is intended
for "power" users. I can add print statements to avoid confusion and frustration
since the operations are just "silently" dropped -- *if* that aligns with xmon's
"philosophy".