Hello,
On 5/10/26 08:30, Deepanshu Kartikey wrote:
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;}
Thanks for the patch. Atomic update shouldn't fail due to non-critical errors like on a signal interrupt. Could you please move this code that may fail in update() to .prepare/cleanup_fb() callbacks?