On Thu, Dec 4, 2025 at 1:27 AM Tvrtko Ursulin <[email protected]> wrote:
>
>
> On 04/12/2025 01:50, Chia-I Wu wrote:
> > Commit 506aa8b02a8d6 ("dma-fence: Add safe access helpers and document
> > the rules") details the dma-fence safe access rules. The most common
> > culprit is that drm_sched_fence_get_timeline_name may race with
> > group_free_queue.
> >
> > Fixes: d2624d90a0b77 ("drm/panthor: assign unique names to queues")
> > Signed-off-by: Chia-I Wu <[email protected]>
> > ---
> > drivers/gpu/drm/panthor/panthor_sched.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/panthor/panthor_sched.c
> > b/drivers/gpu/drm/panthor/panthor_sched.c
> > index 33b9ef537e359..a8b1347e4da71 100644
> > --- a/drivers/gpu/drm/panthor/panthor_sched.c
> > +++ b/drivers/gpu/drm/panthor/panthor_sched.c
> > @@ -23,6 +23,7 @@
> > #include <linux/module.h>
> > #include <linux/platform_device.h>
> > #include <linux/pm_runtime.h>
> > +#include <linux/rcupdate.h>
> >
> > #include "panthor_devfreq.h"
> > #include "panthor_device.h"
> > @@ -923,6 +924,9 @@ static void group_release_work(struct work_struct *work)
> > release_work);
> > u32 i;
> >
> > + /* dma-fences may still be accessing group->queues under rcu lock. */
> > + synchronize_rcu();
> > +
> > for (i = 0; i < group->queue_count; i++)
> > group_free_queue(group, group->queues[i]);
> >
>
> This handles the shared queue->fence_ctx.lock as well (which is also
> unsafe until Christian lands the inline lock, etc patch series) so it
> looks good to me as well.
Yeah, I will send v2 to drop the misleading "Fixes:" tag.
FWIW, the UAF I saw was from accessing the string returned by
static const char *drm_sched_fence_get_timeline_name(struct dma_fence *f)
{
struct drm_sched_fence *fence = to_drm_sched_fence(f);
return (const char *)fence->sched->name;
}
I thought it was "name" and added the "Fixes:" tag. But actually
"sched" was also freed by group_release_work.
>
> Just to mention an alternative could be to simply switch release_work to
> INIT_RCU_WORK/queue_rcu_work, but I am not sure if that has an advantage.
>
> Regards,
>
> Tvrtko
>
