drm/amd/display: Clear dm_state for fast updates

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.

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 3202fa62 ("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: bd200d19 ("drm/amd/display: Don't replace the dc_state for fast updates")
Reported-by: Duncan <1i5t5.duncan@cox.net>
Signed-off-by: Mazin Rezk <mnrzk@protonmail.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -8901,20 +8901,38 @@ static int amdgpu_dm_atomic_check(struct drm_device *dev,
 		 * the same resource. If we have a new DC context as part of
 		 * the DM atomic state from validation we need to free it and
 		 * retain the existing one instead.
+		 *
+		 * Furthermore, since the DM atomic state only contains the DC
+		 * context and can safely be annulled, we can free the state
+		 * and clear the associated private object now to free
+		 * some memory and avoid a possible use-after-free later.
 		 */
-		struct dm_atomic_state *new_dm_state, *old_dm_state;
 
-		new_dm_state = dm_atomic_get_new_state(state);
-		old_dm_state = dm_atomic_get_old_state(state);
+		for (i = 0; i < state->num_private_objs; i++) {
+			struct drm_private_obj *obj = state->private_objs[i].ptr;
 
-		if (new_dm_state && old_dm_state) {
-			if (new_dm_state->context)
-				dc_release_state(new_dm_state->context);
+			if (obj->funcs == adev->dm.atomic_obj.funcs) {
+				int j = state->num_private_objs-1;
 
-			new_dm_state->context = old_dm_state->context;
+				dm_atomic_destroy_state(obj,
+						state->private_objs[i].state);
+
+				/* If i is not at the end of the array then the
+				 * last element needs to be moved to where i was
+				 * before the array can safely be truncated.
+				 */
+				if (i != j)
+					state->private_objs[i] =
+						state->private_objs[j];
 
-			if (old_dm_state->context)
-				dc_retain_state(old_dm_state->context);
+				state->private_objs[j].ptr = NULL;
+				state->private_objs[j].state = NULL;
+				state->private_objs[j].old_state = NULL;
+				state->private_objs[j].new_state = NULL;
+
+				state->num_private_objs = j;
+				break;
+			}
 		}
 	}