linux-kernel - Re: [PATCH v2] drm/scheduler: set entity to NULL in drm_sched_entity_pop

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Date:   Wed, 19 Apr 2023 10:53:12 +0100
From:   Steven Price <steven.price@....com>
To:     Lucas Stach <l.stach@...gutronix.de>,
        Danilo Krummrich <dakr@...hat.com>, luben.tuikov@....com,
        airlied@...il.com, daniel@...ll.ch, christian.koenig@....com
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2] drm/scheduler: set entity to NULL in
 drm_sched_entity_pop_job()

On 19/04/2023 10:44, Lucas Stach wrote:
> Hi Steven,
> 
> Am Mittwoch, dem 19.04.2023 um 10:39 +0100 schrieb Steven Price:
>> On 18/04/2023 11:04, Danilo Krummrich wrote:
>>> It already happend a few times that patches slipped through which
>>> implemented access to an entity through a job that was already removed
>>> from the entities queue. Since jobs and entities might have different
>>> lifecycles, this can potentially cause UAF bugs.
>>>
>>> In order to make it obvious that a jobs entity pointer shouldn't be
>>> accessed after drm_sched_entity_pop_job() was called successfully, set
>>> the jobs entity pointer to NULL once the job is removed from the entity
>>> queue.
>>>
>>> Moreover, debugging a potential NULL pointer dereference is way easier
>>> than potentially corrupted memory through a UAF.
>>>
>>> Signed-off-by: Danilo Krummrich <dakr@...hat.com>
>>
>> This triggers a splat for me (with Panfrost driver), the cause of which
>> is the following code in drm_sched_get_cleanup_job():
>>
>> 	if (job) {
>> 		job->entity->elapsed_ns += ktime_to_ns(
>> 			ktime_sub(job->s_fence->finished.timestamp,
>> 				  job->s_fence->scheduled.timestamp));
>> 	}
>>
>> which indeed is accessing entity after the job has been returned from
>> drm_sched_entity_pop_job(). And obviously job->entity is a NULL pointer
>> with this patch.
>>
>> I'm afraid I don't fully understand the lifecycle so I'm not sure if
>> this is simply exposing an existing bug in drm_sched_get_cleanup_job()
>> or if this commit needs to be reverted.
>>
> Not sure which tree you are on. The offending commit has been reverted
> in 6.3-rc5.

This is in drm-misc-next - I'm not sure which "offending commit" you are
referring to. I'm referring to:

96c7c2f4d5bd ("drm/scheduler: set entity to NULL in
drm_sched_entity_pop_job()")

which was merged yesterday to drm-misc-next (and is currently the top
commit).

Is there another commit which has been reverted elsewhere which is
conflicting?

Steve

> Regards,
> Lucas
> 
>> Thanks,
>>
>> Steve
>>
>>> ---
>>>  drivers/gpu/drm/scheduler/sched_entity.c | 6 ++++++
>>>  drivers/gpu/drm/scheduler/sched_main.c   | 4 ++++
>>>  2 files changed, 10 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
>>> index 15d04a0ec623..a9c6118e534b 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_entity.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_entity.c
>>> @@ -448,6 +448,12 @@ struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity)
>>>  			drm_sched_rq_update_fifo(entity, next->submit_ts);
>>>  	}
>>>  
>>> +	/* Jobs and entities might have different lifecycles. Since we're
>>> +	 * removing the job from the entities queue, set the jobs entity pointer
>>> +	 * to NULL to prevent any future access of the entity through this job.
>>> +	 */
>>> +	sched_job->entity = NULL;
>>> +
>>>  	return sched_job;
>>>  }
>>>  
>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>> index 9b16480686f6..e89a3e469cd5 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>> @@ -42,6 +42,10 @@
>>>   *    the hardware.
>>>   *
>>>   * The jobs in a entity are always scheduled in the order that they were pushed.
>>> + *
>>> + * Note that once a job was taken from the entities queue and pushed to the
>>> + * hardware, i.e. the pending queue, the entity must not be referenced anymore
>>> + * through the jobs entity pointer.
>>>   */
>>>  
>>>  #include <linux/kthread.h>
>>
>