tree 1345c7ca57563006a0355bec1bbe689dd14ca67c
parent 3f1b6f9d49ba5a209d745fa2448657d8b66ed0c0
author Matthew Wilcox <mawilcox@microsoft.com> 1488561370 -0500
committer Matthew Wilcox <mawilcox@microsoft.com> 1488910703 -0500

ida: Free correct IDA bitmap

There's a relatively rare race where we look at the per-cpu preallocated
IDA bitmap, see it's NULL, allocate a new one, and atomically update it.
If the kmalloc() happened to sleep and we were rescheduled to a different
CPU, or an interrupt came in at the exact right time, another task
might have successfully allocated a bitmap and already deposited it.
I forgot what the semantics of cmpxchg() were and ended up freeing the
wrong bitmap leading to KASAN reporting a use-after-free.

Dmitry found the bug with syzkaller & wrote the patch.  I wrote the test
case that will reproduce the bug without his patch being applied.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>