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Date: Tue, 6 Sep 2016 11:44:34 -0300
From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dmitry Vyukov <dvyukov@google.com>, Ingo Molnar <mingo@redhat.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
        "x86@kernel.org" <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: perf: out-of-bounds write in perf_callchain_store
Message-ID: <20160906144434.GB11557@kernel.org>
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Em Tue, Sep 06, 2016 at 04:36:08PM +0200, Peter Zijlstra escreveu:
> On Tue, Sep 06, 2016 at 03:42:40PM +0200, Dmitry Vyukov wrote:
> > Hello,
> > 
> > The following program trigger an out-of-bounds write in
> > perf_callchain_store (if run in a parallel loop):
> > 
> > https://gist.githubusercontent.com/dvyukov/c05d883e776a353a1d063b670f50bde6/raw/1c8906b1aacfbd8a0cc0b5cf0cc4d0535345e497/gistfile1.txt
> > 
> > 
> > BUG: KASAN: slab-out-of-bounds in perf_callchain_user+0xe65/0xfc0 at
> > addr ffff88003e162840
> > Write of size 8 by task syz-executor/22516
> > CPU: 0 PID: 22516 Comm: syz-executor Not tainted 4.8.0-rc5-next-20160905+ #14
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> >  ffffffff886b6fe0 ffff88003ec07738 ffffffff82db81a9 ffffffff00000000
> >  fffffbfff10d6dfc ffff88003e800a00 ffff88003e161740 ffff88003e163740
> >  0000000000000001 ffff88003e162840 ffff88003ec07760 ffffffff8180b2ec
> > Call Trace:
> >  [<ffffffff8180b9c7>] __asan_report_store8_noabort+0x17/0x20
> > mm/kasan/report.c:332
> >  [<     inline     >] perf_callchain_store include/linux/perf_event.h:1146
> >  [<ffffffff81014925>] perf_callchain_user+0xe65/0xfc0 arch/x86/events/core.c:2441
> >  [<ffffffff816c5f48>] get_perf_callchain+0x448/0x680 kernel/events/callchain.c:235
> >  [<ffffffff816c62cd>] perf_callchain+0x14d/0x1a0 kernel/events/callchain.c:191
> 
> Urgh, that callchain code is a pain with that context/entries
> separation. But I can't see an obvious overrun there.
> 
> But WTF is max_contexts a sysctl? that doesn't seen to make any kind of
> sense.
> 
> Acme, can you untangle that stuff and spot the fail?

I looked at it briefly some moments ago, couldn't find it so far, have
to look at what was behind adding a sysctl for that :-\

And yeah, that entry/ctx thing, IIRC, was done to reduce patch size,
probably needs some polishing to become clearer.

- Arnaldo