Messages in this thread | | | Subject | Re: [RFC PATCH 1/2] perf: arm_spe: Fix consistency of PMSCR register bit CX | From | German Gomez <> | Date | Wed, 19 Jan 2022 11:27:50 +0000 |
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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
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