linux-kernel - [tip:perf/urgent] perf/x86/amd/ibs: Don't access non-started event

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Message-ID: <tip-d2753e6b4882a637a0e8fb3b9c2e15f33265300e@git.kernel.org>
Date:   Tue, 24 Jul 2018 08:01:26 -0700
From:   tip-bot for Thomas Gleixner <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     jolsa@...hat.com, vincent.weaver@...ne.edu, hpa@...or.com,
        pmenzel+linux-x86@...gen.mpg.de, linux-kernel@...r.kernel.org,
        pmenzel@...gen.mpg.de, tglx@...utronix.de, eranian@...gle.com,
        peterz@...radead.org, alexander.shishkin@...ux.intel.com,
        mingo@...nel.org, bp@...en8.de, acme@...hat.com,
        torvalds@...ux-foundation.org
Subject: [tip:perf/urgent] perf/x86/amd/ibs: Don't access non-started event

Commit-ID:  d2753e6b4882a637a0e8fb3b9c2e15f33265300e
Gitweb:     https://git.kernel.org/tip/d2753e6b4882a637a0e8fb3b9c2e15f33265300e
Author:     Thomas Gleixner <tglx@...utronix.de>
AuthorDate: Fri, 20 Jul 2018 10:39:07 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 24 Jul 2018 09:51:10 +0200

perf/x86/amd/ibs: Don't access non-started event

Paul Menzel reported the following bug:

> Enabling the undefined behavior sanitizer and building GNU/Linux 4.18-rc5+
> (with some unrelated commits) with GCC 8.1.0 from Debian Sid/unstable, the
> warning below is shown.
>
> > [    2.111913]
> > ================================================================================
> > [    2.111917] UBSAN: Undefined behaviour in arch/x86/events/amd/ibs.c:582:24
> > [    2.111919] member access within null pointer of type 'struct perf_event'
> > [    2.111926] CPU: 0 PID: 144 Comm: udevadm Not tainted 4.18.0-rc5-00316-g4864b68cedf2 #104
> > [    2.111928] Hardware name: ASROCK E350M1/E350M1, BIOS TIMELESS 01/01/1970
> > [    2.111930] Call Trace:
> > [    2.111943]  dump_stack+0x55/0x89
> > [    2.111949]  ubsan_epilogue+0xb/0x33
> > [    2.111953]  handle_null_ptr_deref+0x7f/0x90
> > [    2.111958]  __ubsan_handle_type_mismatch_v1+0x55/0x60
> > [    2.111964]  perf_ibs_handle_irq+0x596/0x620

The code dereferences event before checking the STARTED bit. Patch
below should cure the issue.

The warning should not trigger, if I analyzed the thing correctly.
(And Paul's testing confirms this.)

Reported-by: Paul Menzel <pmenzel@...gen.mpg.de>
Tested-by: Paul Menzel <pmenzel@...gen.mpg.de>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Jiri Olsa <jolsa@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul Menzel <pmenzel+linux-x86@...gen.mpg.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Stephane Eranian <eranian@...gle.com>
Cc: Vince Weaver <vincent.weaver@...ne.edu>
Link: http://lkml.kernel.org/r/alpine.DEB.2.21.1807200958390.1580@nanos.tec.linutronix.de
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/events/amd/ibs.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
index 4b98101209a1..d50bb4dc0650 100644
--- a/arch/x86/events/amd/ibs.c
+++ b/arch/x86/events/amd/ibs.c
@@ -579,7 +579,7 @@ static int perf_ibs_handle_irq(struct perf_ibs *perf_ibs, struct pt_regs *iregs)
 {
 	struct cpu_perf_ibs *pcpu = this_cpu_ptr(perf_ibs->pcpu);
 	struct perf_event *event = pcpu->event;
-	struct hw_perf_event *hwc = &event->hw;
+	struct hw_perf_event *hwc;
 	struct perf_sample_data data;
 	struct perf_raw_record raw;
 	struct pt_regs regs;
@@ -602,6 +602,10 @@ fail:
 		return 0;
 	}
 
+	if (WARN_ON_ONCE(!event))
+		goto fail;
+
+	hwc = &event->hw;
 	msr = hwc->config_base;
 	buf = ibs_data.regs;
 	rdmsrl(msr, *buf);