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

[Eric Anholt]

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 at anholt.net>
Fixes: 57692c94dcbe ("drm/v3d: Introduce a new DRM driver for Broadcom V3D V3.x+")
---
 drivers/gpu/drm/v3d/v3d_drv.h | 5 +++++
 drivers/gpu/drm/v3d/v3d_gem.c | 8 ++++++--
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/v3d/v3d_drv.h b/drivers/gpu/drm/v3d/v3d_drv.h
index 5042573e97f4..83c55ab6e1c0 100644
--- a/drivers/gpu/drm/v3d/v3d_drv.h
+++ b/drivers/gpu/drm/v3d/v3d_drv.h
@@ -204,6 +204,11 @@ 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. */
diff --git a/drivers/gpu/drm/v3d/v3d_gem.c b/drivers/gpu/drm/v3d/v3d_gem.c
index e1fcbb4cd0ae..c98fbfbdb68e 100644
--- a/drivers/gpu/drm/v3d/v3d_gem.c
+++ b/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);
@@ -574,6 +575,9 @@ 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]);
@@ -587,7 +591,7 @@ 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,
-					  &exec->render.base.s_fence->finished);
+					  exec->render_done_fence);
 		drm_syncobj_put(sync_out);
 	}
 
-- 
2.18.0