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

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Message-ID: <87ed5324a69b5f1cc1ab5be2483f224afacf5824.camel@pengutronix.de>
Date:   Wed, 19 Apr 2023 12:07:08 +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()

Am Mittwoch, dem 19.04.2023 um 10:53 +0100 schrieb Steven Price:
> 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?
> 
The commit which introduces the use-after-free, which is now upgraded
to the above NULL ptr dereference is df622729ddbf ("drm/scheduler:
track GPU active time per entity") and has been reverted in
baad10973fdb ("Revert "drm/scheduler: track GPU active time per
entity"). The revert is upstream in 6.3-rc5.

Regards,
Lucas


> 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>
> > > 
> > 
>