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

[ Upstream commit d2753e6 ]

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 <[email protected]>
Tested-by: Paul Menzel <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul Menzel <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Vince Weaver <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

arch/x86/events/amd/ibs.c

@@ -578,7 +578,7 @@ 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;
@@ -601,6 +601,10 @@ static int perf_ibs_handle_irq(struct perf_ibs *perf_ibs, struct pt_regs *iregs)
 		return 0;
 	}
 
+	if (WARN_ON_ONCE(!event))
+		goto fail;
+
+	hwc = &event->hw;
 	msr = hwc->config_base;
 	buf = ibs_data.regs;
 	rdmsrl(msr, *buf);