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From: Rob Herring <robh@kernel.org>
To: Mark Rutland <mark.rutland@arm.com>, Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Jiri Olsa <jolsa@redhat.com>, Ian Rogers <irogers@google.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>,
	Zachary.Leaf@arm.com, Raphael Gault <raphael.gault@arm.com>,
	Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Itaru Kitayama <itaru.kitayama@gmail.com>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v6 02/10] arm64: perf: Enable PMU counter direct access for perf event
Date: Thu, 8 Apr 2021 13:38:17 -0500	[thread overview]
Message-ID: <CAL_Jsq+H_asWWrHiCk-PBS8xDEGpBL1__dRyrPXdBYgRBBw2vA@mail.gmail.com> (raw)
In-Reply-To: <20210408110800.GA32792@C02TD0UTHF1T.local>

On Thu, Apr 8, 2021 at 6:08 AM Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Wed, Apr 07, 2021 at 01:44:37PM +0100, Will Deacon wrote:
> > [Moving Mark to To: since I'd like his view on this]
> >
> > On Thu, Apr 01, 2021 at 02:45:21PM -0500, Rob Herring wrote:
> > > On Wed, Mar 31, 2021 at 11:01 AM Will Deacon <will@kernel.org> wrote:
> > > >
> > > > On Tue, Mar 30, 2021 at 12:09:38PM -0500, Rob Herring wrote:
> > > > > On Tue, Mar 30, 2021 at 10:31 AM Will Deacon <will@kernel.org> wrote:
> > > > > >
> > > > > > On Wed, Mar 10, 2021 at 05:08:29PM -0700, Rob Herring wrote:
> > > > > > > From: Raphael Gault <raphael.gault@arm.com>
>
> > > > > > > +static void armv8pmu_event_unmapped(struct perf_event *event, struct mm_struct *mm)
> > > > > > > +{
> > > > > > > +     struct arm_pmu *armpmu = to_arm_pmu(event->pmu);
> > > > > > > +
> > > > > > > +     if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR))
> > > > > > > +             return;
> > > > > > > +
> > > > > > > +     if (atomic_dec_and_test(&mm->context.pmu_direct_access))
> > > > > > > +             on_each_cpu_mask(&armpmu->supported_cpus, refresh_pmuserenr, mm, 1);
> > > > > >
> > > > > > Given that the pmu_direct_access field is global per-mm, won't this go
> > > > > > wrong if multiple PMUs are opened by the same process but only a subset
> > > > > > are exposed to EL0? Perhaps pmu_direct_access should be treated as a mask
> > > > > > rather than a counter, so that we can 'and' it with the supported_cpus for
> > > > > > the PMU we're dealing with.
> > > > >
> > > > > It needs to be a count to support multiple events on the same PMU. If
> > > > > the event is not enabled for EL0, then we'd exit out on the
> > > > > ARMPMU_EL0_RD_CNTR check. So I think we're fine.
> > > >
> > > > I'm still not convinced; pmu_direct_access is shared between PMUs, so
> > > > testing the result of atomic_dec_and_test() just doesn't make sense to
> > > > me, as another PMU could be playing with the count.
> > >
> > > How is that a problem? Let's make a concrete example:
> > >
> > > map PMU1:event2 -> pmu_direct_access = 1 -> enable access
> > > map PMU2:event3 -> pmu_direct_access = 2
> > > map PMU1:event4 -> pmu_direct_access = 3
> > > unmap PMU2:event3 -> pmu_direct_access = 2
> > > unmap PMU1:event2 -> pmu_direct_access = 1
> > > unmap PMU1:event4 -> pmu_direct_access = 0 -> disable access
> > >
> > > The only issue I can see is PMU2 remains enabled for user access until
> > > the last unmap. But we're sharing the mm, so who cares? Also, in this
> > > scenario it is the user's problem to pin themselves to cores sharing a
> > > PMU. If the user doesn't do that, they get to keep the pieces.
> >
> > I guess I'm just worried about exposing the counters to userspace after
> > the PMU driver (or perf core?) thinks that they're no longer exposed in
> > case we leak other events.
>
> IMO that's not practically different from the single-PMU case (i.e.
> multi-PMU isn't material, either we have a concern with leaking or we
> don't); more on that below.
>
> While it looks odd to place this on the mm, I don't think it's the end
> of the world.
>
> > However, I'm not sure how this is supposed to work normally: what
> > happens if e.g. a privileged user has a per-cpu counter for a kernel
> > event while a task has a counter with direct access -- can that task
> > read the kernel event out of the PMU registers from userspace?
>
> Yes -- userspace could go read any counters even though it isn't
> supposed to, and could potentially infer information from those. It
> won't have access to the config registers or kernel data structures, so
> it isn't guaranteed to know what the even is or when it is
> context-switched/reprogrammed/etc.
>
> If we believe that's a problem, then it's difficult to do anything
> robust other than denying userspace access entirely, since disabling
> userspace access while in use would surprise applications, and denying
> privileged events would need some global state that we consult at event
> creation time (in addition to being an inversion of privilege).
>
> IIRC there was some fuss about this a while back on x86; I'll go dig and
> see what I can find, unless Peter has a memory...

Maybe this one[1].

Rob

[1] https://lore.kernel.org/lkml/20200730123815.18518-1-kan.liang@linux.intel.com/

WARNING: multiple messages have this Message-ID
From: Rob Herring <robh@kernel.org>
To: Mark Rutland <mark.rutland@arm.com>, Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	 Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Jiri Olsa <jolsa@redhat.com>, Ian Rogers <irogers@google.com>,
	 Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	 Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>,
	Zachary.Leaf@arm.com,  Raphael Gault <raphael.gault@arm.com>,
	Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	 Namhyung Kim <namhyung@kernel.org>,
	Itaru Kitayama <itaru.kitayama@gmail.com>,
	 linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v6 02/10] arm64: perf: Enable PMU counter direct access for perf event
Date: Thu, 8 Apr 2021 13:38:17 -0500	[thread overview]
Message-ID: <CAL_Jsq+H_asWWrHiCk-PBS8xDEGpBL1__dRyrPXdBYgRBBw2vA@mail.gmail.com> (raw)
In-Reply-To: <20210408110800.GA32792@C02TD0UTHF1T.local>

On Thu, Apr 8, 2021 at 6:08 AM Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Wed, Apr 07, 2021 at 01:44:37PM +0100, Will Deacon wrote:
> > [Moving Mark to To: since I'd like his view on this]
> >
> > On Thu, Apr 01, 2021 at 02:45:21PM -0500, Rob Herring wrote:
> > > On Wed, Mar 31, 2021 at 11:01 AM Will Deacon <will@kernel.org> wrote:
> > > >
> > > > On Tue, Mar 30, 2021 at 12:09:38PM -0500, Rob Herring wrote:
> > > > > On Tue, Mar 30, 2021 at 10:31 AM Will Deacon <will@kernel.org> wrote:
> > > > > >
> > > > > > On Wed, Mar 10, 2021 at 05:08:29PM -0700, Rob Herring wrote:
> > > > > > > From: Raphael Gault <raphael.gault@arm.com>
>
> > > > > > > +static void armv8pmu_event_unmapped(struct perf_event *event, struct mm_struct *mm)
> > > > > > > +{
> > > > > > > +     struct arm_pmu *armpmu = to_arm_pmu(event->pmu);
> > > > > > > +
> > > > > > > +     if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR))
> > > > > > > +             return;
> > > > > > > +
> > > > > > > +     if (atomic_dec_and_test(&mm->context.pmu_direct_access))
> > > > > > > +             on_each_cpu_mask(&armpmu->supported_cpus, refresh_pmuserenr, mm, 1);
> > > > > >
> > > > > > Given that the pmu_direct_access field is global per-mm, won't this go
> > > > > > wrong if multiple PMUs are opened by the same process but only a subset
> > > > > > are exposed to EL0? Perhaps pmu_direct_access should be treated as a mask
> > > > > > rather than a counter, so that we can 'and' it with the supported_cpus for
> > > > > > the PMU we're dealing with.
> > > > >
> > > > > It needs to be a count to support multiple events on the same PMU. If
> > > > > the event is not enabled for EL0, then we'd exit out on the
> > > > > ARMPMU_EL0_RD_CNTR check. So I think we're fine.
> > > >
> > > > I'm still not convinced; pmu_direct_access is shared between PMUs, so
> > > > testing the result of atomic_dec_and_test() just doesn't make sense to
> > > > me, as another PMU could be playing with the count.
> > >
> > > How is that a problem? Let's make a concrete example:
> > >
> > > map PMU1:event2 -> pmu_direct_access = 1 -> enable access
> > > map PMU2:event3 -> pmu_direct_access = 2
> > > map PMU1:event4 -> pmu_direct_access = 3
> > > unmap PMU2:event3 -> pmu_direct_access = 2
> > > unmap PMU1:event2 -> pmu_direct_access = 1
> > > unmap PMU1:event4 -> pmu_direct_access = 0 -> disable access
> > >
> > > The only issue I can see is PMU2 remains enabled for user access until
> > > the last unmap. But we're sharing the mm, so who cares? Also, in this
> > > scenario it is the user's problem to pin themselves to cores sharing a
> > > PMU. If the user doesn't do that, they get to keep the pieces.
> >
> > I guess I'm just worried about exposing the counters to userspace after
> > the PMU driver (or perf core?) thinks that they're no longer exposed in
> > case we leak other events.
>
> IMO that's not practically different from the single-PMU case (i.e.
> multi-PMU isn't material, either we have a concern with leaking or we
> don't); more on that below.
>
> While it looks odd to place this on the mm, I don't think it's the end
> of the world.
>
> > However, I'm not sure how this is supposed to work normally: what
> > happens if e.g. a privileged user has a per-cpu counter for a kernel
> > event while a task has a counter with direct access -- can that task
> > read the kernel event out of the PMU registers from userspace?
>
> Yes -- userspace could go read any counters even though it isn't
> supposed to, and could potentially infer information from those. It
> won't have access to the config registers or kernel data structures, so
> it isn't guaranteed to know what the even is or when it is
> context-switched/reprogrammed/etc.
>
> If we believe that's a problem, then it's difficult to do anything
> robust other than denying userspace access entirely, since disabling
> userspace access while in use would surprise applications, and denying
> privileged events would need some global state that we consult at event
> creation time (in addition to being an inversion of privilege).
>
> IIRC there was some fuss about this a while back on x86; I'll go dig and
> see what I can find, unless Peter has a memory...

Maybe this one[1].

Rob

[1] https://lore.kernel.org/lkml/20200730123815.18518-1-kan.liang@linux.intel.com/

_______________________________________________
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  reply	other threads:[~2021-04-08 18:38 UTC|newest]

Thread overview: 70+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-11  0:08 [PATCH v6 00/10] libperf and arm64 userspace counter access support Rob Herring
2021-03-11  0:08 ` Rob Herring
2021-03-11  0:08 ` [PATCH v6 01/10] arm64: pmu: Add function implementation to update event index in userpage Rob Herring
2021-03-11  0:08   ` Rob Herring
2021-03-30 15:30   ` Will Deacon
2021-03-30 15:30     ` Will Deacon
2021-03-11  0:08 ` [PATCH v6 02/10] arm64: perf: Enable PMU counter direct access for perf event Rob Herring
2021-03-11  0:08   ` Rob Herring
2021-03-30 11:30   ` Zachary Leaf
2021-03-30 11:30     ` Zachary Leaf
2021-03-30 15:31   ` Will Deacon
2021-03-30 15:31     ` Will Deacon
2021-03-30 17:09     ` Rob Herring
2021-03-30 17:09       ` Rob Herring
2021-03-30 21:08       ` Rob Herring
2021-03-30 21:08         ` Rob Herring
2021-03-31 15:38         ` Will Deacon
2021-03-31 15:38           ` Will Deacon
2021-03-31 17:52           ` Rob Herring
2021-03-31 17:52             ` Rob Herring
2021-04-01  9:04             ` Will Deacon
2021-04-01  9:04               ` Will Deacon
2021-03-31 16:00       ` Will Deacon
2021-03-31 16:00         ` Will Deacon
2021-04-01 19:45         ` Rob Herring
2021-04-01 19:45           ` Rob Herring
2021-04-07 12:44           ` Will Deacon
2021-04-07 12:44             ` Will Deacon
2021-04-08 11:08             ` Mark Rutland
2021-04-08 11:08               ` Mark Rutland
2021-04-08 18:38               ` Rob Herring [this message]
2021-04-08 18:38                 ` Rob Herring
2021-04-19 16:14                 ` Will Deacon
2021-04-19 16:14                   ` Will Deacon
2021-04-19 19:00                   ` Rob Herring
2021-04-19 19:00                     ` Rob Herring
2021-03-11  0:08 ` [PATCH v6 03/10] tools/include: Add an initial math64.h Rob Herring
2021-03-11  0:08   ` Rob Herring
2021-03-11  0:08 ` [PATCH v6 04/10] libperf: Add evsel mmap support Rob Herring
2021-03-11  0:08   ` Rob Herring
2021-03-12 13:58   ` Jiri Olsa
2021-03-12 13:58     ` Jiri Olsa
2021-03-12 14:34     ` Rob Herring
2021-03-12 14:34       ` Rob Herring
2021-03-12 18:29       ` Jiri Olsa
2021-03-12 18:29         ` Jiri Olsa
2021-03-31 22:06         ` Rob Herring
2021-03-31 22:06           ` Rob Herring
2021-03-11  0:08 ` [PATCH v6 05/10] libperf: tests: Add support for verbose printing Rob Herring
2021-03-11  0:08   ` Rob Herring
2021-03-11  0:08 ` [PATCH v6 06/10] libperf: Add support for user space counter access Rob Herring
2021-03-11  0:08   ` Rob Herring
2021-05-04 21:40   ` Ian Rogers
2021-05-04 21:40     ` Ian Rogers
2021-05-05  2:12     ` Rob Herring
2021-05-05  2:12       ` Rob Herring
2021-03-11  0:08 ` [PATCH v6 07/10] libperf: Add arm64 support to perf_mmap__read_self() Rob Herring
2021-03-11  0:08   ` Rob Herring
2021-03-11  0:08 ` [PATCH v6 08/10] perf: arm64: Add test for userspace counter access on heterogeneous systems Rob Herring
2021-03-11  0:08   ` Rob Herring
2021-03-15 16:09   ` Masayoshi Mizuma
2021-03-15 16:09     ` Masayoshi Mizuma
2021-03-11  0:08 ` [PATCH v6 09/10] perf: arm64: Add tests for 32-bit and 64-bit counter size userspace access Rob Herring
2021-03-11  0:08   ` Rob Herring
2021-03-11  0:08 ` [PATCH v6 10/10] Documentation: arm64: Document PMU counters access from userspace Rob Herring
2021-03-11  0:08   ` Rob Herring
2021-03-31 16:00   ` Will Deacon
2021-03-31 16:00     ` Will Deacon
2021-03-30 11:31 ` [PATCH v6 00/10] libperf and arm64 userspace counter access support Zachary Leaf
2021-03-30 11:31   ` Zachary Leaf

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