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

[Andrew Donnellan]

On 4/4/19 12:02 am, Christopher M Riedl wrote:
> 
>> 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://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_linuxppc_issues_issues_219&d=DwICaQ&c=jf_iaSHvJObTbx-siA1ZOg&r=-pHOU8dm1U-U1crivyxKr_-xvZrIBB8YUqvA3el0Ee0&m=zNkGBUKLoTqdSUy_VUpM8VLTEqy7sJfIXpWU-ujc6Rc&s=9jgy61R_p5jvtwOKCMFfnhmJegzCIIomcf4I1BRvBPg&e=
>>    - https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_linuxppc_issues_issues_232&d=DwICaQ&c=jf_iaSHvJObTbx-siA1ZOg&r=-pHOU8dm1U-U1crivyxKr_-xvZrIBB8YUqvA3el0Ee0&m=zNkGBUKLoTqdSUy_VUpM8VLTEqy7sJfIXpWU-ujc6Rc&s=fFYm1ZTaEp6HbeZMV5JEmlbBtDwdehfiW1H3shFoFMM&e=
>>
>> 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.

Sounds good.

> 
>>
>>> ---
>>>    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.
> 

Yep, adding a description of the cmdline options is also a good idea.

>>
>>> +
>>>    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".
> 

Power users often want a straightforward self-explanatory UX more than 
anyone :)


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