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

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Message-ID: <6ad72a7f-302f-4be1-0d53-00ff9dc37ef7@amd.com>
Date:   Fri, 31 Mar 2023 07:59:29 +0200
From:   Christian König <christian.koenig@....com>
To:     Danilo Krummrich <dakr@...hat.com>, luben.tuikov@....com,
        airlied@...il.com, daniel@...ll.ch, l.stach@...gutronix.de,
        "Prosyak, Vitaly" <Vitaly.Prosyak@....com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/scheduler: set entity to NULL in
 drm_sched_entity_pop_job()

Am 31.03.23 um 02:06 schrieb Danilo Krummrich:
> 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>

In general "YES PLEASE!", but I fear that this will break amdgpus reset 
sequence.

On the other hand when amdgpu still relies on that pointer it's clearly 
a bug (which I pointed out tons of times before).

Luben any opinion on that? Could you drive cleaning that up as well?

Thanks,
Christian.

> ---
> I'm aware that drivers could already use job->entity in arbitrary places, since
> they in control of when the entity is actually freed. A quick grep didn't give
> me any results where this would actually be the case, however maybe I also just
> didn't catch it.
>
> If, therefore, we don't want to set job->entity to NULL I think we should at
> least add a comment somewhere.
> ---
>
>   drivers/gpu/drm/scheduler/sched_entity.c | 6 ++++++
>   1 file changed, 6 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;
>   }
>