Re: [PATCH] drm/amd/display: Clear dm_state for fast updates

From: Kazlauskas, Nicholas
Date: Mon Jul 27 2020 - 09:27:06 EST

On 2020-07-27 1:40 a.m., Mazin Rezk wrote:
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

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,
* 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;
+ }

In the bug report itself I mentioned that I don't really like hacking around the DRM core for resolving this patch but to go into more specifics, it's really two issues of code maintenance:

1. It's iterating over internal structures and layout of private objects in the state and modifying the state. The core doesn't really guarantee how these things are going to be laid out and it may change in the future.

2. It's freeing an allocation we don't own from DM. DRM doesn't track this state elsewhere for purposes of freeing, but nothing is really stopping the core from doing this later down the line.

The implementation itself is correct from a technical perspective, but I'd rather it reside in DRM as a helper for code maintenance purposes.

Nicholas Kazlauskas



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