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From: Will Deacon <will@kernel.org>
To: Rob Herring <robh@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	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: Mon, 19 Apr 2021 17:14:29 +0100	[thread overview]
Message-ID: <20210419161429.GA30998@willie-the-truck> (raw)
In-Reply-To: <CAL_Jsq+H_asWWrHiCk-PBS8xDEGpBL1__dRyrPXdBYgRBBw2vA@mail.gmail.com>

On Thu, Apr 08, 2021 at 01:38:17PM -0500, Rob Herring wrote:
> 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:
> > > 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:
> > > 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.

Well, maybe. It looks the single-PMU case is exposed to the same issue,
but I think a solution needs to take into account the multi-PMU situation.

> > 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/

Going through the archives and talking to Peter, it looks like this is still
an active area of concern:

  - There are patches to clear "dirty" counters on context-switch. They were
    queued for 5.13 but broke -tip on Friday:

    https://lore.kernel.org/lkml/YHm%2FM4za2LpRYePw@hirez.programming.kicks-ass.net/

  - Per-cpu events cannot be protected in software:

    https://lore.kernel.org/lkml/CALCETrVVPzUd_hQ8xoomHn_wWRQJUvROeCt2do4_D4ROZoAVMg@mail.gmail.com/

    so without hardware support, we need a way to disable user access for
    people that care about this leakage

x86 has an "rdpmc" file exposed for the PMU device in sysfs which allows
access to be disabled. I don't think these patches add such a thing, and
that's where the fun with multi-PMU machines would come into play.

Will

WARNING: multiple messages have this Message-ID
From: Will Deacon <will@kernel.org>
To: Rob Herring <robh@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	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: Mon, 19 Apr 2021 17:14:29 +0100	[thread overview]
Message-ID: <20210419161429.GA30998@willie-the-truck> (raw)
In-Reply-To: <CAL_Jsq+H_asWWrHiCk-PBS8xDEGpBL1__dRyrPXdBYgRBBw2vA@mail.gmail.com>

On Thu, Apr 08, 2021 at 01:38:17PM -0500, Rob Herring wrote:
> 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:
> > > 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:
> > > 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.

Well, maybe. It looks the single-PMU case is exposed to the same issue,
but I think a solution needs to take into account the multi-PMU situation.

> > 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/

Going through the archives and talking to Peter, it looks like this is still
an active area of concern:

  - There are patches to clear "dirty" counters on context-switch. They were
    queued for 5.13 but broke -tip on Friday:

    https://lore.kernel.org/lkml/YHm%2FM4za2LpRYePw@hirez.programming.kicks-ass.net/

  - Per-cpu events cannot be protected in software:

    https://lore.kernel.org/lkml/CALCETrVVPzUd_hQ8xoomHn_wWRQJUvROeCt2do4_D4ROZoAVMg@mail.gmail.com/

    so without hardware support, we need a way to disable user access for
    people that care about this leakage

x86 has an "rdpmc" file exposed for the PMU device in sysfs which allows
access to be disabled. I don't think these patches add such a thing, and
that's where the fun with multi-PMU machines would come into play.

Will

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linux-arm-kernel@lists.infradead.org
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  reply	other threads:[~2021-04-19 16:14 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
2021-04-08 18:38                 ` Rob Herring
2021-04-19 16:14                 ` Will Deacon [this message]
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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