[PATCH] drm/virtio: check virtio_gpu_array_lock_resv() return in cursor update

[Deepanshu Kartikey <kartikey406@gmail.com>]

virtio_gpu_cursor_plane_update() calls virtio_gpu_array_lock_resv()
but ignores its return value. The function can fail in two ways:

  - dma_resv_lock_interruptible() returns -ERESTARTSYS when a signal
    is delivered while waiting for the reservation lock.
  - dma_resv_reserve_fences() returns -ENOMEM if it fails to allocate
    a fence slot; in this case lock_resv unlocks before returning.

In both cases the resv lock is not held on return. The cursor path
proceeds to queue a fenced transfer command. The queue path then
walks the object array and calls dma_resv_add_fence() on the cursor
BO's reservation. dma_resv_add_fence() requires the resv lock to be
held; with lockdep enabled the missing lock trips
dma_resv_assert_held():

  WARNING: drivers/dma-buf/dma-resv.c:296 at dma_resv_add_fence+0x71e/0x840
  Call Trace:
   virtio_gpu_array_add_fence+0xcd/0x140
   virtio_gpu_queue_ctrl_sgs
   virtio_gpu_queue_fenced_ctrl_buffer+0x578/0xfb0
   virtio_gpu_cursor_plane_update+0x411/0xbc0
   drm_atomic_helper_commit_planes+0x497/0xf10
   ...
   drm_mode_cursor_ioctl+0xd4/0x110
   drm_ioctl+0x5e6/0xc60
   __x64_sys_ioctl+0x18e/0x210

Beyond the WARN, mutating the dma_resv fence list without the lock
races with concurrent readers/writers and can corrupt the list.

Check the return value of virtio_gpu_array_lock_resv(). On failure,
drop the references taken by virtio_gpu_array_add_obj() with
virtio_gpu_array_put_free() (which does not unlock, matching the
not-locked state) and return without queueing the command. A
skipped cursor frame is harmless; the WARN and the underlying race
are not.

The bug was reported by syzbot, triggered via fault injection
(fail_nth) on the DRM_IOCTL_MODE_CURSOR path, which forces the
-ENOMEM branch in dma_resv_reserve_fences().

Reported-by: syzbot+72bd3dd3a5d5f39a0271@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=72bd3dd3a5d5f39a0271
Fixes: 5cfd31c5b3a3 ("drm/virtio: fix virtio_gpu_cursor_plane_update().")
Cc: stable@vger.kernel.org
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 drivers/gpu/drm/virtio/virtgpu_plane.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_plane.c b/drivers/gpu/drm/virtio/virtgpu_plane.c
index a126d1b25f46..ca379b08b9ec 100644
--- a/drivers/gpu/drm/virtio/virtgpu_plane.c
+++ b/drivers/gpu/drm/virtio/virtgpu_plane.c
@@ -459,7 +459,10 @@ static void virtio_gpu_cursor_plane_update(struct drm_plane *plane,
 		if (!objs)
 			return;
 		virtio_gpu_array_add_obj(objs, vgfb->base.obj[0]);
-		virtio_gpu_array_lock_resv(objs);
+		if (virtio_gpu_array_lock_resv(objs)) {
+			virtio_gpu_array_put_free(objs);
+			return;
+		}
 		virtio_gpu_cmd_transfer_to_host_2d
 			(vgdev, 0,
 			 plane->state->crtc_w,
-- 
2.43.0