[PATCH AUTOSEL 4.19 25/25] perf/x86/intel: Fix race in intel_pmu_disable_event()

Sasha Levin
2019-05-16T11:41:12Z

From: Jiri Olsa <jolsa@kernel.org>

[ Upstream commit 6f55967ad9d9752813e36de6d5fdbd19741adfc7 ]

New race in x86_pmu_stop() was introduced by replacing the
atomic __test_and_clear_bit() of cpuc->active_mask by separate
test_bit() and __clear_bit() calls in the following commit:

3966c3feca3f ("x86/perf/amd: Remove need to check "running" bit in NMI handler")

The race causes panic for PEBS events with enabled callchains:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
...
RIP: 0010:perf_prepare_sample+0x8c/0x530
Call Trace:
<NMI>
perf_event_output_forward+0x2a/0x80
__perf_event_overflow+0x51/0xe0
handle_pmi_common+0x19e/0x240
intel_pmu_handle_irq+0xad/0x170
perf_event_nmi_handler+0x2e/0x50
nmi_handle+0x69/0x110
default_do_nmi+0x3e/0x100
do_nmi+0x11a/0x180
end_repeat_nmi+0x16/0x1a
RIP: 0010:native_write_msr+0x6/0x20
...
</NMI>
intel_pmu_disable_event+0x98/0xf0
x86_pmu_stop+0x6e/0xb0
x86_pmu_del+0x46/0x140
event_sched_out.isra.97+0x7e/0x160
...

The event is configured to make samples from PEBS drain code,
but when it's disabled, we'll go through NMI path instead,
where data->callchain will not get allocated and we'll crash:

x86_pmu_stop
test_bit(hwc->idx, cpuc->active_mask)
intel_pmu_disable_event(event)
{
...
intel_pmu_pebs_disable(event);
...

EVENT OVERFLOW -> <NMI>
intel_pmu_handle_irq
handle_pmi_common
TEST PASSES -> test_bit(bit, cpuc->active_mask))
perf_event_overflow
perf_prepare_sample
{
...
if (!(sample_type & __PERF_SAMPLE_CALLCHAIN_EARLY))
data->callchain = perf_callchain(event, regs);

CRASH -> size += data->callchain->nr;
}
</NMI>
...
x86_pmu_disable_event(event)
}

__clear_bit(hwc->idx, cpuc->active_mask);

Fixing this by disabling the event itself before setting
off the PEBS bit.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Arcari <darcari@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Lendacky Thomas <Thomas.Lendacky@amd.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Fixes: 3966c3feca3f ("x86/perf/amd: Remove need to check "running" bit in NMI handler")
Link: http://lkml.kernel.org/r/20190504151556.31031-1-jolsa@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/events/intel/core.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index a759e59990fbd..09c53bcbd497d 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -2074,15 +2074,19 @@ static void intel_pmu_disable_event(struct perf_event *event)
cpuc->intel_ctrl_host_mask &= ~(1ull << hwc->idx);
cpuc->intel_cp_status &= ~(1ull << hwc->idx);

- if (unlikely(event->attr.precise_ip))
- intel_pmu_pebs_disable(event);
-
if (unlikely(hwc->config_base == MSR_ARCH_PERFMON_FIXED_CTR_CTRL)) {
intel_pmu_disable_fixed(hwc);
return;
}

x86_pmu_disable_event(event);
+
+ /*
+ * Needs to be called after x86_pmu_disable_event,
+ * so we don't trigger the event without PEBS bit set.
+ */
+ if (unlikely(event->attr.precise_ip))
+ intel_pmu_pebs_disable(event);
}

static void intel_pmu_del_event(struct perf_event *event)
--
2.20.1