Re: [PATCH] drm/sched: Fix UB pointer dereference

[Philipp Stanner <pstanner@redhat.com>]

On Tue, 2024-08-27 at 11:00 +0200, Christian König wrote:
> Am 27.08.24 um 10:39 schrieb Danilo Krummrich:
> > On 8/27/24 9:45 AM, Philipp Stanner wrote:
> > > In drm_sched_job_init(), commit 56e449603f0a ("drm/sched: Convert
> > > the
> > > GPU scheduler to variable number of run-queues") implemented a
> > > call to
> > > drm_err(), which uses the job's scheduler pointer as a parameter.
> > > job->sched, however, is not yet valid as it gets set by
> > > drm_sched_job_arm(), which is always called after
> > > drm_sched_job_init().
> > > 
> > > Since the scheduler code has no control over how the API-User has
> > > allocated or set 'job', the pointer's dereference is undefined
> > > behavior.
> > > 
> > > Fix the UB by replacing drm_err() with pr_err().
> > > 
> > > Cc: <stable@vger.kernel.org>    # 6.7+
> > > Fixes: 56e449603f0a ("drm/sched: Convert the GPU scheduler to 
> > > variable number of run-queues")
> > > Reported-by: Danilo Krummrich <dakr@redhat.com>
> > > Closes: 
> > > https://lore.kernel.org/lkml/20231108022716.15250-1-dakr@redhat.com/
> > > Signed-off-by: Philipp Stanner <pstanner@redhat.com>
> > > ---
> > >   drivers/gpu/drm/scheduler/sched_main.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/scheduler/sched_main.c 
> > > b/drivers/gpu/drm/scheduler/sched_main.c
> > > index 7e90c9f95611..356c30fa24a8 100644
> > > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > > @@ -797,7 +797,7 @@ int drm_sched_job_init(struct drm_sched_job
> > > *job,
> > >            * or worse--a blank screen--leave a trail in the
> > >            * logs, so this can be debugged easier.
> > >            */
> > > -        drm_err(job->sched, "%s: entity has no rq!\n",
> > > __func__);
> > > +        pr_err("*ERROR* %s: entity has no rq!\n", __func__);
> > 
> > I don't think the "*ERROR*" string is necessary, it's pr_err()
> > already.
> 
> Good point. I will remove that and also add a comment why drm_err
> won't 
> work here before pushing it to drm-misc-fixes.

Well, as we're at it I want to point out that the exact same mechanism
occurs just a few lines below, from where I shamelessly copied it:

if (unlikely(!credits)) {
	pr_err("*ERROR* %s: credits cannot be 0!\n", __func__);


P.

> 
> Thanks,
> Christian.
> 
> > 
> > Otherwise,
> > 
> > Acked-by: Danilo Krummrich <dakr@kernel.org>
> > 
> > >           return -ENOENT;
> > >       }
>