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

[Thomas Gleixner]

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);