Am 27.07.20 um 07:40 schrieb Mazin Rezk:
This patch fixes a race condition that causes a use-after-free during
amdgpu_dm_atomic_commit_tail. This can occur when 2 non-blocking commits
are requested and the second one finishes before the first. Essentially,
this bug occurs when the following sequence of events happens:
1. Non-blocking commit #1 is requested w/ a new dm_state #1 and is
deferred to the workqueue.
2. Non-blocking commit #2 is requested w/ a new dm_state #2 and is
deferred to the workqueue.
3. Commit #2 starts before commit #1, dm_state #1 is used in the
commit_tail and commit #2 completes, freeing dm_state #1.
4. Commit #1 starts after commit #2 completes, uses the freed dm_state
1 and dereferences a freelist pointer while setting the context.
Well I only have a one mile high view on this, but why don't you let the work items execute in order?
That would be better anyway cause this way we don't trigger a cache line ping pong between CPUs.
Christian.
Since this bug has only been spotted with fast commits, this patch fixes
the bug by clearing the dm_state instead of using the old dc_state for
fast updates. In addition, since dm_state is only used for its dc_state
and amdgpu_dm_atomic_commit_tail will retain the dc_state if none is found,
removing the dm_state should not have any consequences in fast updates.
This use-after-free bug has existed for a while now, but only caused a
noticeable issue starting from 5.7-rc1 due to 3202fa62f ("slub: relocate
freelist pointer to middle of object") moving the freelist pointer from
dm_state->base (which was unused) to dm_state->context (which is
dereferenced).
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=207383
Fixes: bd200d190f45 ("drm/amd/display: Don't replace the dc_state for fast updates")
Reported-by: Duncan <1i5t5.duncan@xxxxxxx>
Signed-off-by: Mazin Rezk <mnrzk@xxxxxxxxxxxxxx>
---
 .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 36 ++++++++++++++-----
 1 file changed, 27 insertions(+), 9 deletions(-)
diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index 86ffa0c2880f..710edc70e37e 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -8717,20 +8717,38 @@ static int amdgpu_dm_atomic_check(struct drm_device *dev,
ÂÂÂÂÂÂÂÂÂÂ * the same resource. If we have a new DC context as part of
ÂÂÂÂÂÂÂÂÂÂ * the DM atomic state from validation we need to free it and
ÂÂÂÂÂÂÂÂÂÂ * retain the existing one instead.
+ÂÂÂÂÂÂÂÂ *
+ÂÂÂÂÂÂÂÂ * Furthermore, since the DM atomic state only contains the DC
+ÂÂÂÂÂÂÂÂ * context and can safely be annulled, we can free the state
+ÂÂÂÂÂÂÂÂ * and clear the associated private object now to free
+ÂÂÂÂÂÂÂÂ * some memory and avoid a possible use-after-free later.
ÂÂÂÂÂÂÂÂÂÂ */
-ÂÂÂÂÂÂÂ struct dm_atomic_state *new_dm_state, *old_dm_state;
-ÂÂÂÂÂÂÂ new_dm_state = dm_atomic_get_new_state(state);
-ÂÂÂÂÂÂÂ old_dm_state = dm_atomic_get_old_state(state);
+ÂÂÂÂÂÂÂ for (i = 0; i < state->num_private_objs; i++) {
+ÂÂÂÂÂÂÂÂÂÂÂ struct drm_private_obj *obj = state->private_objs[i].ptr;
-ÂÂÂÂÂÂÂ if (new_dm_state && old_dm_state) {
-ÂÂÂÂÂÂÂÂÂÂÂ if (new_dm_state->context)
-ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ dc_release_state(new_dm_state->context);
+ÂÂÂÂÂÂÂÂÂÂÂ if (obj->funcs == adev->dm.atomic_obj.funcs) {
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ int j = state->num_private_objs-1;
-ÂÂÂÂÂÂÂÂÂÂÂ new_dm_state->context = old_dm_state->context;
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ dm_atomic_destroy_state(obj,
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ state->private_objs[i].state);
+
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /* If i is not at the end of the array then the
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * last element needs to be moved to where i was
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * before the array can safely be truncated.
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ */
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (i != j)
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ state->private_objs[i] =
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ state->private_objs[j];
-ÂÂÂÂÂÂÂÂÂÂÂ if (old_dm_state->context)
-ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ dc_retain_state(old_dm_state->context);
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ state->private_objs[j].ptr = NULL;
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ state->private_objs[j].state = NULL;
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ state->private_objs[j].old_state = NULL;
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ state->private_objs[j].new_state = NULL;
+
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ state->num_private_objs = j;
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ break;
+ÂÂÂÂÂÂÂÂÂÂÂ }
ÂÂÂÂÂÂÂÂÂ }
ÂÂÂÂÂ }
--
2.27.0