drm/scheduler: Fix UAF in drm_sched_fence_get_timeline_name

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@asahilina.net>

Author: hoshinolina

drivers/gpu/drm/scheduler/sched_entity.c

@@ -368,7 +368,12 @@ 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.
 		 */
 		fence = dma_fence_get(&s_fence->scheduled);
 		dma_fence_put(entity->dependency);

drivers/gpu/drm/scheduler/sched_fence.c

@@ -66,7 +66,7 @@ static const char *drm_sched_fence_get_driver_name(struct dma_fence *fence)
 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;
+	return (const char *)fence->sched_name;
 }
 
 static void drm_sched_fence_free_rcu(struct rcu_head *rcu)
@@ -214,6 +214,8 @@ void drm_sched_fence_init(struct drm_sched_fence *fence,
 	unsigned seq;
 
 	fence->sched = entity->rq->sched;
+	strlcpy(fence->sched_name, entity->rq->sched->name,
+		sizeof(fence->sched_name));
 	seq = atomic_inc_return(&entity->fence_seq);
 	dma_fence_init(&fence->scheduled, &drm_sched_fence_ops_scheduled,
 		       &fence->lock, entity->fence_context, seq);

include/drm/gpu_scheduler.h

@@ -305,6 +305,11 @@ struct drm_sched_fence {
          * @lock: the lock used by the scheduled and the finished fences.
          */
 	spinlock_t			lock;
+        /**
+         * @sched_name: the name of the scheduler that owns this fence. We
+	 * keep a copy here since fences can outlive their scheduler.
+         */
+	char sched_name[16];
         /**
          * @owner: job owner for debugging
          */