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

["Kazlauskas, Nicholas" <nicholas.kazlauskas@amd.com>]

On 2020-07-27 9:39 a.m., Christian König wrote:
> 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.

We use the DRM helpers for managing drm_atomic_commit_state and those 
helpers internally push non-blocking commit work into the system unbound 
work queue.

While we could duplicate a copy of that code with nothing but the 
workqueue changed that isn't something I'd really like to maintain going 
forward.

Regards,
Nicholas Kazlauskas

> 
>>
>> 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>
>> ---
>>   .../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
>>
>