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SubjectRe: UBSAN: Undefined behaviour in arch/x86/events/amd/ibs.c:582:24: member access within null pointer of type 'struct perf_event'
Paul,

On Fri, 20 Jul 2018, Paul Menzel wrote:
> 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

Right, 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. Emphasis on *should*

Thanks,

tglx

8<--------------------
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 @@ 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);

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