[PATCH] drm/gem-atomic-helper: Fix fence reference leaks in prepare_fb

[Ke Chen]

An atomic commit can set IN_FENCE_FD while disabling a plane, leaving
the new plane state with a fence but no framebuffer. In this case,
drm_gem_plane_helper_prepare_fb() takes an extra reference to
state->fence before checking state->fb and returns without dropping it.
Repeated commits therefore leak one fence reference each and keep the
affected fence objects alive.

dma_resv_get_singleton() also returns a referenced fence through new.
If dma_fence_chain_alloc() fails, ownership of new has not been
transferred to a chain, while the common error path only drops fence.

Take the explicit fence reference after the framebuffer check and drop
new explicitly if allocating the fence chain fails.

Fixes: 1ea28bc5542d ("drm: handle kernel fences in drm_gem_plane_helper_prepare_fb v2")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Ke Chen <Ke.Chen@xxxxxxx>
---
Tested with x86_64, arm64, and arm target-object builds using W=1.
Also checked the x86_64 target object with sparse C=1 and
CF=-D__CHECK_ENDIAN__. Runtime and allocation-failure fault-injection
testing were not performed.

 drivers/gpu/drm/drm_gem_atomic_helper.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_gem_atomic_helper.c b/drivers/gpu/drm/drm_gem_atomic_helper.c
index abef865c5f2c9..4c0a23f4b2970 100644
--- a/drivers/gpu/drm/drm_gem_atomic_helper.c
+++ b/drivers/gpu/drm/drm_gem_atomic_helper.c
@@ -114,7 +114,7 @@
 int drm_gem_plane_helper_prepare_fb(struct drm_plane *plane,
 				    struct drm_plane_state *state)
 {
-	struct dma_fence *fence = dma_fence_get(state->fence);
+	struct dma_fence *fence;
 	enum dma_resv_usage usage;
 	size_t i;
 	int ret;
@@ -122,6 +122,8 @@ int drm_gem_plane_helper_prepare_fb(struct drm_plane *plane,
 	if (!state->fb)
 		return 0;
 
+	fence = dma_fence_get(state->fence);
+
 	/*
 	 * Only add the kernel fences here if there is already a fence set via
 	 * explicit fencing interfaces on the atomic ioctl.
@@ -154,6 +156,7 @@ int drm_gem_plane_helper_prepare_fb(struct drm_plane *plane,
 			struct dma_fence_chain *chain = dma_fence_chain_alloc();
 
 			if (!chain) {
+				dma_fence_put(new);
 				ret = -ENOMEM;
 				goto error;
 			}

base-commit: 20839d02c0cf7437bc508d4c5430538d9dc4f428
-- 
2.34.1