Am 31.10.2017 um 18:58 schrieb Michel Dänzer:
On 25/10/17 05:43 PM, Michel Dänzer wrote:
KASAN caught another use-after-free on my development machine today, see
the attached dmesg excerpt. There haven't been any related changes in
amd-staging-drm-next since yesterday, so maybe userspace is just
tickling the kernel differently (e.g. piglit runs some more tests in
parallel now). It's not reproducible every time, but it just happened a
second time (with an amd-staging-drm-next commit from about a week ago).
I took a closer look, and I think I see what's happening. The
use-after-free happens at:
reservation_object_wait_timeout_rcu+0xe02/0xe90
ttm_bo_cleanup_refs_and_unlock+0x271/0x990 [ttm] (ttm_bo.c:530)
ttm_mem_evict_first+0x263/0x4a0 [ttm]
The memory was freed at:
[reservation_object_fini]
ttm_bo_cleanup_refs_and_unlock+0x517/0x990 [ttm] (ttm_bo.c:564)
ttm_mem_evict_first+0x263/0x4a0 [ttm]
So it's two processes handling the same BO in ttm_mem_evict_first ->
ttm_bo_cleanup_refs_and_unlock. The first one unreserved the BO before
calling reservation_object_wait_timeout_rcu. Meanwhile, the other one
manages to reserve the BO and get all the way to the end of
ttm_bo_cleanup_refs_and_unlock, destroying bo->ttm_resv. Then
reservation_object_wait_timeout_rcu in the first process still accesses
memory which bo->ttm_resv pointed to => boom.
Good catch. But this means that just grabbing another reference before
calling reservation_object_wait_timeout_rcu() and we should be on the
safe side, shouldn't we ?
Going to take a closer look tomorrow, today is a holiday here and I'm
actually ill again once more :(
Christian.
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