Re: perf: fuzzer KASAN unwind_get_return_address

[Josh Poimboeuf]

On Tue, Nov 15, 2016 at 11:04:45AM -0800, Dmitry Vyukov wrote:
> On Tue, Nov 15, 2016 at 10:57 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Tue, Nov 15, 2016 at 12:43:56PM -0500, Vince Weaver wrote:
> >>
> >> Running on my haswell machine with the imc/uncore patch applied, the
> >> perf_fuzzer next tripped over this issue.
> >>
> >> [  202.034495] BAD LUCK: lost 371 message(s) from NMI context!
> >> [  202.034496] ==================================================================
> >> [  202.048327] BUG: KASAN: stack-out-of-bounds in unwind_get_return_address+0x35/0x80 at addr ffff8800cff0bd90
> >> [  202.058826] Read of size 8 by task perf_fuzzer/16254
> >> [  202.064186] page:ffffea00033fc2c0 count:1 mapcount:0 mapping:          (null) index:0x0^Ac
> >> [  202.073068] flags: 0x1ffff8000000400(reserved)
> >> [  202.077885] page dumped because: kasan: bad access detected
> >> [  202.083880] CPU: 4 PID: 16254 Comm: perf_fuzzer Not tainted 4.9.0-rc5+ #5
> >> [  202.091204] Hardware name: LENOVO 10AM000AUS/SHARKBAY, BIOS FBKT72AUS 01/26/2014
> >> [  202.099181]  ffff8800cff0b1d8^Ac ffffffff816bb796^Ac ffff8800cff0b270^Ac ffff8800cff0bd90^Ac
> >> [  202.107896]  ffff8800cff0b260^Ac ffffffff812fbe95^Ac 00007ffc9d1ab480^Ac 0000000000000000^Ac
> >> [  202.116638]  ffffffff8125117d^Ac 0000000000000092^Ac 0000000000000000^Ac ffff8800cff0b7c0^Ac
> >> [  202.125339] Call Trace:
> >> [  202.127994]  <NMI>  [<ffffffff816bb796>] dump_stack+0x63/0x8d
> >> [  202.134184]  [<ffffffff812fbe95>] kasan_report_error+0x495/0x4c0
> >> [  202.140680]  [<ffffffff8125117d>] ? perf_output_begin+0x28d/0x4c0
> >> [  202.147228]  [<ffffffff812fc319>] kasan_report+0x39/0x40
> >> [  202.152987]  [<ffffffff81095ce5>] ? unwind_get_return_address+0x35/0x80
> >> [  202.160094]  [<ffffffff812fa8fe>] __asan_load8+0x5e/0x70
> >> [  202.165859]  [<ffffffff81095ce5>] unwind_get_return_address+0x35/0x80
> >
> > Josh, any ideas?
> 
> I think this is a false positive due to imprecise unwind that hits a
> stack redzone.
> We probably need to use READ_ONCE_NOCHECK as in get_wchan.

I'm not so sure about that.  The unwind should be precise here: it
should only be looking at the frame pointers and return addresses on the
current task's stack.  I can't see any reason why it would be reading
into the KASAN stack redzone.

-- 
Josh