linux-kernel - Re: BUG: KASAN: slab-out-of-bounds in perf_callchain

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Date:   Wed, 6 Dec 2017 11:12:13 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Namhyung Kim <namhyung@...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

Em Wed, Dec 06, 2017 at 02:47:06PM +0100, Peter Zijlstra escreveu:
> On Tue, Dec 05, 2017 at 11:47:18PM +0900, Namhyung Kim wrote:
> > Sure, I mean the following code:
> > 
> > 	mutex_lock(&callchain_mutex);
> > 
> > 	count = atomic_inc_return(&nr_callchain_events);
> > 	if (WARN_ON_ONCE(count < 1)) {
> > 		err = -EINVAL;
> > 		goto exit;
> > 	}
> > 
> > 	if (count > 1) {
> > 		/* If the allocation failed, give up */
> > 		if (!callchain_cpus_entries)
> > 			err = -ENOMEM;
> > 
> > 		goto exit;
> > 	}
> > 
> > 	err = alloc_callchain_buffers();
> > exit:
> > 	if (err)
> > 		atomic_dec(&nr_callchain_events);
> > 
> > 	mutex_unlock(&callchain_mutex);
> > 
> > 
> > The callchain_cpus_entries is allocated in alloc_callchain_buffers()
> > only when the count is 1.  But if it failed to allocate, it decrease
> > the count so next event would try to allocate it again.  Thus it seems
> > not possible to see the callchain_cpus_entries being NULL in the
> > 'if (count > 1)' block.  If you want to make next event give up, it'd
> > need to take an additional count IMHO.
> 
> 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.

Yeah, this code is complicated, and there are several csets to consider,
by Frédéric that may help to understando why the code ended up like
that, I started from git blame going first to
9251f904f95175b4a1d8cbc0449e748f9edd7629, where the test seemed to make
sense, to then go back, but still reading this...

commit fc3b86d673e41ac66b4ba5b75a90c2fcafb90089
Author: Frederic Weisbecker <fweisbec@...il.com>
Date:   Fri Aug 2 18:29:54 2013 +0200

    perf: Roll back callchain buffer refcount under the callchain mutex

commit 90983b16078ab0fdc58f0dab3e8e3da79c9579a2
Author: Frederic Weisbecker <fweisbec@...il.com>
Date:   Tue Jul 23 02:31:00 2013 +0200

    perf: Sanitize get_callchain_buffer()

commit fd45c15f13e754f3c106427e857310f3e0813951
Author: Namhyung Kim <namhyung.kim@....com>
Date:   Fri Jan 20 10:12:45 2012 +0900

    perf: Don't call release_callchain_buffers() if allocation fails

commit 9251f904f95175b4a1d8cbc0449e748f9edd7629
Author: Borislav Petkov <borislav.petkov@....com>
Date:   Sun Oct 16 17:15:04 2011 +0200

    perf: Carve out callchain functionality