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

[Mazin Rezk]

On Monday, July 27, 2020 1:40 AM, Mazin Rezk <mnrzk at protonmail.com> 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
> 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 at cox.net>
> Signed-off-by: Mazin Rezk <mnrzk at protonmail.com>
> ---
>  .../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;
> +			}
>  		}
>  	}
>
> --
> 2.27.0
>

I have tested this on 5.8.0-rc6 w/ an RX 480 for 8 hours and I have not had
the crash described in the Bugzilla thread. I will also be running this
patch on my kernel for the next couple of days to further confirm that this
is working. In addition, I will ask the users in the Bugzilla thread to
test this patch and confirm that it works.

My previous attempt to patch this bug ("amdgpu_dm: fix nonblocking atomic
commit use-after-free") [1] was unsuccessful and this patch is meant to
replace it. That said, this isn't necessarily a v2 of that patch, since
this patch works very differently from the other patch.

[1] https://lkml.org/lkml/2020/7/23/1123

Thanks,
Mazin Rezk