drm/v3d: Fix a use-after-free race accessing the scheduler's fences.

Once we push the job, the scheduler could run it and free it.  So, if
we want to reference their fences, we need to grab them before then.
I haven't seen this happen in many days of conformance test runtime,
but let's still close the race.
Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 57692c94 ("drm/v3d: Introduce a new DRM driver for Broadcom V3D V3.x+")
Link: https://patchwork.freedesktop.org/patch/254119/Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>

drivers/gpu/drm/v3d/v3d_drv.h

@@ -198,6 +198,11 @@ struct v3d_exec_info {
 	 */
 	struct dma_fence *bin_done_fence;
 
+	/* Fence for when the scheduler considers the render to be
+	 * done, for when the BOs reservations should be complete.
+	 */
+	struct dma_fence *render_done_fence;
+
 	struct kref refcount;
 
 	/* This is the array of BOs that were looked up at the start of exec. */

drivers/gpu/drm/v3d/v3d_gem.c

@@ -209,7 +209,7 @@ v3d_flush_caches(struct v3d_dev *v3d)
 static void
 v3d_attach_object_fences(struct v3d_exec_info *exec)
 {
-	struct dma_fence *out_fence = &exec->render.base.s_fence->finished;
+	struct dma_fence *out_fence = exec->render_done_fence;
 	struct v3d_bo *bo;
 	int i;
 
@@ -409,6 +409,7 @@ v3d_exec_cleanup(struct kref *ref)
 	dma_fence_put(exec->render.done_fence);
 
 	dma_fence_put(exec->bin_done_fence);
+	dma_fence_put(exec->render_done_fence);
 
 	for (i = 0; i < exec->bo_count; i++)
 		drm_gem_object_put_unlocked(&exec->bo[i]->base);
@@ -572,6 +573,9 @@ v3d_submit_cl_ioctl(struct drm_device *dev, void *data,
 	if (ret)
 		goto fail_unreserve;
 
+	exec->render_done_fence =
+		dma_fence_get(&exec->render.base.s_fence->finished);
+
 	kref_get(&exec->refcount); /* put by scheduler job completion */
 	drm_sched_entity_push_job(&exec->render.base,
 				  &v3d_priv->sched_entity[V3D_RENDER]);
@@ -585,7 +589,7 @@ v3d_submit_cl_ioctl(struct drm_device *dev, void *data,
 	sync_out = drm_syncobj_find(file_priv, args->out_sync);
 	if (sync_out) {
 		drm_syncobj_replace_fence(sync_out, 0,
-					  &exec->render.base.s_fence->finished);
+					  exec->render_done_fence);
 		drm_syncobj_put(sync_out);
 	}