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Date: Wed, 6 Dec 2017 16:45:44 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Namhyung Kim <namhyung@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
        Fengguang Wu <fengguang.wu@intel.com>, linux-kernel@vger.kernel.org,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        Will Deacon <will.deacon@arm.com>, lkp@01.org,
        Dmitry Vyukov <dvyukov@google.com>, kasan-dev@googlegroups.com,
        kernel-team@lge.com
Subject: Re: BUG: KASAN: slab-out-of-bounds in perf_callchain_user+0x494/0x530
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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.