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authorMazin Rezk <mnrzk@protonmail.com>2020-07-27 08:40:46 +0300
committerAlex Deucher <alexander.deucher@amd.com>2020-07-30 18:02:10 +0300
commitfde9f39ac7f1ffd799a96ffa1e06b2051f0898f1 (patch)
tree5b042a4d4ec57c6c3a2f3be163a4ed7b0adad9fa /drivers
parent543e8669ed9bfb30545fd52bc0e047ca4df7fb31 (diff)
downloadlinux-fde9f39ac7f1ffd799a96ffa1e06b2051f0898f1.tar.xz
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 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@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> Cc: stable@vger.kernel.org
Diffstat (limited to 'drivers')
-rw-r--r--drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c36
1 files 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,
* 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;
+ }
}
}