drm/virtio: use uninterruptible resv lock for plane updates

Overview

In the Linux kernel, the following vulnerability has been resolved:

drm/virtio: use uninterruptible resv lock for plane updates

virtio_gpu_cursor_plane_update() and virtio_gpu_resource_flush() lock the framebuffer BO's dma_resv via virtio_gpu_array_lock_resv() and ignore its return value. The function can fail with -EINTR from dma_resv_lock_interruptible() (signal during lock wait) or with -ENOMEM from dma_resv_reserve_fences() (fence slot allocation), leaving the resv lock not held. The queue path then walks the object array and calls dma_resv_add_fence(), which requires the lock held; with lockdep enabled this 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 virtio_gpu_queue_ctrl_sgs virtio_gpu_queue_fenced_ctrl_buffer virtio_gpu_cursor_plane_update drm_atomic_helper_commit_planes drm_atomic_helper_commit_tail commit_tail drm_atomic_helper_commit drm_atomic_commit drm_atomic_helper_update_plane __setplane_atomic drm_mode_cursor_universal drm_mode_cursor_common drm_mode_cursor_ioctl drm_ioctl __x64_sys_ioctl

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

Both call sites run inside the .atomic_update plane callback, which DRM atomic helpers do not allow to fail (by the time it runs, the commit has been signed off to userspace and there is no clean rollback path). Moving the lock acquisition to .prepare_fb was rejected because the broader lock scope deadlocks against other BO locking paths in the same atomic commit.

Introduce virtio_gpu_lock_one_resv_uninterruptible() that uses dma_resv_lock() instead of dma_resv_lock_interruptible(). This eliminates the -EINTR failure mode -- the realistic syzbot trigger -- without extending the lock hold across the commit. The helper locks a single BO and rejects nents > 1 with -EINVAL; both fix sites lock exactly one BO.

Use it from virtio_gpu_cursor_plane_update() and virtio_gpu_resource_flush(); check the return value to handle the remaining -ENOMEM case from dma_resv_reserve_fences() by freeing the objs and skipping the plane update for that frame. The framebuffer BOs touched here are not shared with other contexts and lock contention is expected to be brief, so the loss of signal-interruptibility is acceptable.

Other callers of virtio_gpu_array_lock_resv() (the ioctl paths) continue to use the interruptible variant.

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().

Affected

• Linux >= 5cfd31c5b3a321aed0c9621b7b45efa2942056f8 < c86077d512ee980cc91322211d35dbcd3175f64c
• Linux >= 5cfd31c5b3a321aed0c9621b7b45efa2942056f8 < 21ab64c77a30d56efc506c8fa2ad8959f8ce3d36
• Linux >= 5cfd31c5b3a321aed0c9621b7b45efa2942056f8 < 7930eee22cd3df61e85be8aa512032ab303b7167
• Linux >= 5cfd31c5b3a321aed0c9621b7b45efa2942056f8 < 8fadd01cf461fee5bb11506621339c548447e5c7
• Linux >= 5cfd31c5b3a321aed0c9621b7b45efa2942056f8 < a2359a411b15f495d12cfda6a7db6855ebb7f90f
• Linux >= 5cfd31c5b3a321aed0c9621b7b45efa2942056f8 < 9af1b6e175c82daf4b423da339a722d8e67a735a
• Linux 5.7

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.