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Date:   Wed, 6 Dec 2017 16:45:44 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Fengguang Wu <fengguang.wu@...el.com>,
        linux-kernel@...r.kernel.org, Wang Nan <wangnan0@...wei.com>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will.deacon@....com>, lkp@...org,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        kernel-team@....com
Subject: Re: BUG: KASAN: slab-out-of-bounds in perf_callchain_user+0x494/0x530

On Wed, Dec 06, 2017 at 11:31:30PM +0900, Namhyung Kim wrote:

> > There's also a race against put_callchain_buffers() there, consider:
> > 
> > 
> > 	get_callchain_buffers()		put_callchain_buffers()
> > 	  mutex_lock();
> > 	  inc()
> > 					  dec_and_test() // false
> > 
> > 	  dec() // 0
> > 
> > 
> > And the buffers leak.
> 
> Hmm.. did you mean that get_callchain_buffers() returns an error?

Yes, get_callchain_buffers() fails, but while doing so it has a
temporary increment on the count.

> AFAICS it cannot fail when it sees count > 1 (and callchain_cpus_
> entries is allocated).  

It can with your patch. We only test event_max_stack against the sysctl
after incrementing.

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