Re: [RFC PATCH 1/2] perf: arm_spe: Fix consistency of PMSCR register bit CX

[German Gomez <german.gomez@arm.com>]

On 18/01/2022 14:04, German Gomez wrote:
> Hi Will,
>
> Many thanks for your comments
>
> On 18/01/2022 10:07, Will Deacon wrote:
>> On Mon, Jan 17, 2022 at 12:44:31PM +0000, German Gomez wrote:
>>> [...]
>>>
>>>   1. Run a process in the background with capability CAP_SYS_ADMIN in CPU0.
>>>
>>>     $ taskset --cpu-list 0 sudo dd if=/dev/random of=/dev/null &
>>>     [3] 3806
>>>
>>>   2. Begin a perf session _without_ capabilities (we shouldn't see CONTEXT packets).
>>>
>>>     $ perf record -e arm_spe_0// -C0 -- sleep 1
>>>     $ perf report -D | grep CONTEXT
>>>     .  0000000e:  65 df 0e 00 00                                  CONTEXT 0xedf el2
>>>     .  0000004e:  65 df 0e 00 00                                  CONTEXT 0xedf el2
>>>     .  0000008e:  65 df 0e 00 00                                  CONTEXT 0xedf el2
>>>     [...]
>>>
>>> As can be seen, the traces begin showing CONTEXT packets when the pid is
>>> 0xedf (3807).
>> So to be clear: we shouldn't be reporting these packets because 'perf'
>> doesn't have the right capabilities, but we evaluate that in the context of
>> 'dd' (running as root) and so incorrectly grant the permission. Correct?
> Yes, correct. My guess was that "perfmon_capable()" was being called
> under the assumption that it would always be evaluated in the context of
> 'perf'. Is that correct?
>
>>> This happens because the pmu start callback is run when
>>> the current process is not the owner of the perf session, so the CX
>>> register bit is set.
>> This doesn't really seem SPE-specific to me -- the perf_allow_*() helpers
>> also operate implicitly on the current task. How do other PMU drivers avoid
>> falling into this trap?
> I'm not as familiar with the other PMU drivers. I quickly tried grepping
> something related in the cs_etm drivers as they use CONTEXTIDR as well,
> but couldn't find references to perfmon_capable() or similar checks.
>
> Grepping for "perf_allow_" inside of drivers doesn't yield results.
> There's some gpu driver that has similar perfmon_capable() checks but
> unlike spe, they error out if they don't pass (drivers/gpu/drm/i915/i915_perf.c).

Just to expand a bit more on this (I missed grepping the other directories)

./arch/powerpc/perf/imc-pmu.c => perfmon_capable() only called on init
./kernel/events/core.c        => perfmon_capable() only called on init
./arch/x86/events/core.c      => perf_allow_*() function only called on init

As far as I see, currently spe seems to be the only PMU event that
checks capabilities in the start callback. 'perf' may not be the current
task in this callback.

>
>>> One way to fix this is by caching the value of the CX bit during the
>>> initialization of the PMU event, so that it remains consistent for the
>>> duration of the session.
>> It doesn't feel right to stash this in 'struct arm_spe_pmu' during event
>> initialisation -- wouldn't that allow perf to continue creating new events
>> with CX set, even if the paranoid sysctl was changed dynamically? Instead,
>> I think it would be better if the capabilities were stash in the event
>> itself somehow at initialisation time.
> I hadn't considered this. Makes more sense to store in the perf_event
> or via some type of mapping in the struct spe_pmu if not possible. Do
> you have any idea for the former? Or an idiomatic structure from the
> kernel for the later?
>
> Thanks,
> German
>
>> Will