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

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Message-ID: <04f039ac71f3c0685a849b492478d18ec6ea4d11.camel@pengutronix.de>
Date:   Wed, 19 Apr 2023 11:44:10 +0200
From:   Lucas Stach <l.stach@...gutronix.de>
To:     Steven Price <steven.price@....com>,
        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()

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.

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>
>