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

[Paul Menzel]

Dear Thomas,


Am 20.07.2018 um 10:39 schrieb Thomas Gleixner:

> 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*

> 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);

Thank you very much. The warning is gone after applying it.

Tested-by: Paul Menzel <pmenzel@xxxxxxxxxxxxx>


Kind regards,

Paul