linux-kernel - Re: perf: fuzzer triggers NULL pointer derefreence in x86_schedule

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Message-ID: <20150501125955.GF5029@twins.programming.kicks-ass.net>
Date:	Fri, 1 May 2015 14:59:55 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vince Weaver <vincent.weaver@...ne.edu>
Cc:	linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: perf: fuzzer triggers NULL pointer derefreence in
 x86_schedule_events

On Thu, Apr 30, 2015 at 03:08:56PM -0400, Vince Weaver wrote:
> 
> So the perf_fuzzer caught this after about a week of fuzzing on a Haswell 
> machine running a recent git kernel (pre 4.1-rc1 though).
> 
> We've seen this BUG before and various fixes were applied but apparently 
> it wasn't enough.
> 
> Sadly it doesn't seem to be reproducible.
> 
> validate_group() -> x86_pmu.schedule_events() -> ???? -> variable_test_bit()
>  (hard to tell which test bit with all the inlining going on).

Assuming you build with debug info addr2line -i can help, but I think I
found it by comparing the Code section below with my objdump -D output.

Its:
		/* constraint still honored */
		if (!test_bit(hwc->idx, c->idxmsk))
			break;

Which would seem to suggest c is NULL.

Lemme go figure out how that could happen.
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