Re: [PATCH 2/3] drm/scheduler: Fix UAF in drm_sched_fence_get_timeline_name

[Asahi Lina]

On 14/07/2023 17.43, Christian König wrote:
> Am 14.07.23 um 10:21 schrieb Asahi Lina:
> > A signaled scheduler fence can outlive its scheduler, since fences are
> > independencly reference counted. Therefore, we can't reference the
> > scheduler in the get_timeline_name() implementation.
> >
> > Fixes oopses on `cat /sys/kernel/debug/dma_buf/bufinfo` when shared
> > dma-bufs reference fences from GPU schedulers that no longer exist.
> >
> > Signed-off-by: Asahi Lina <lina@xxxxxxxxxxxxx>
> > ---
> >    drivers/gpu/drm/scheduler/sched_entity.c | 7 ++++++-
> >    drivers/gpu/drm/scheduler/sched_fence.c  | 4 +++-
> >    include/drm/gpu_scheduler.h              | 5 +++++
> >    3 files changed, 14 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
> > index b2bbc8a68b30..17f35b0b005a 100644
> > --- a/drivers/gpu/drm/scheduler/sched_entity.c
> > +++ b/drivers/gpu/drm/scheduler/sched_entity.c
> > @@ -389,7 +389,12 @@ static bool drm_sched_entity_add_dependency_cb(struct drm_sched_entity *entity)
> >    
> >    		/*
> >    		 * Fence is from the same scheduler, only need to wait for
> > -		 * it to be scheduled
> > +		 * it to be scheduled.
> > +		 *
> > +		 * Note: s_fence->sched could have been freed and reallocated
> > +		 * as another scheduler. This false positive case is okay, as if
> > +		 * the old scheduler was freed all of its jobs must have
> > +		 * signaled their completion fences.
>
> This is outright nonsense. As long as an entity for a scheduler exists
> it is not allowed to free up this scheduler.
>
> So this function can't be called like this.

As I already explained, the fences can outlive their scheduler. That 
means *this* entity certainly exists for *this* scheduler, but the 
*dependency* fence might have come from a past scheduler that was 
already destroyed along with all of its entities, and its address reused.

Christian, I'm really getting tired of your tone. I don't appreciate 
being told my comments are "outright nonsense" when you don't even take 
the time to understand what the issue is and what I'm trying to 
do/document. If you aren't interested in working with me, I'm just going 
to give up on drm_sched, wait until Rust gets workqueue support, and 
reimplement it in Rust. You can keep your broken fence lifetime 
semantics and I'll do my own thing.

~~ Lina