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 10:39:31 +0100
From:   Steven Price <steven.price@....com>
To:     Danilo Krummrich <dakr@...hat.com>, luben.tuikov@....com,
        airlied@...il.com, daniel@...ll.ch, l.stach@...gutronix.de,
        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 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.

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>