Re: perf: out-of-bounds write in perf_callchain_store

[Peter Zijlstra]

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?