Re: [PATCH v4] drm/virtio: use uninterruptible resv lock for plane updates

["Christian König" <christian.koenig@amd.com>]

On 5/19/26 10:22, Deepanshu Kartikey wrote:
> 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.

Well why are you trying to add a fence on an atomic mode set in the first place?

That is usually an illegal operation here.

Regards,
Christian.

> 
> 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().
> 
> 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>
> ---
> v4: Rename the helper to virtio_gpu_lock_one_resv_uninterruptible()
>     and reject objs->nents > 1 with -EINVAL. The v3 helper's
>     multi-object branch used drm_gem_lock_reservations(), which is
>     interruptible, contradicting the "uninterruptible" name; both
>     fix sites lock a single BO so the multi-object path is dropped.
>     (Dmitry Osipenko)
> v3: Drop the prepare_fb/cleanup_fb approach from v2 (it deadlocked
>     against virtio_gpu_resource_flush(), which also locks the BO in
>     the same atomic commit). Instead add an uninterruptible variant
>     of the resv lock helper and use it in both
>     virtio_gpu_cursor_plane_update() and virtio_gpu_resource_flush().
>     (Dmitry Osipenko)
> v2: Move resv lock acquisition from .atomic_update (which must not
>     fail) to .prepare_fb (which may), per maintainer review of v1.
>     The v1 approach of silently skipping the cursor update on lock
>     failure violated the atomic-commit contract with userspace.
> ---
>  drivers/gpu/drm/virtio/virtgpu_drv.h   |  1 +
>  drivers/gpu/drm/virtio/virtgpu_gem.c   | 17 +++++++++++++++++
>  drivers/gpu/drm/virtio/virtgpu_plane.c | 10 ++++++++--
>  3 files changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.h b/drivers/gpu/drm/virtio/virtgpu_drv.h
> index f17660a71a3e..2f3531950aa4 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_drv.h
> +++ b/drivers/gpu/drm/virtio/virtgpu_drv.h
> @@ -317,6 +317,7 @@ virtio_gpu_array_from_handles(struct drm_file *drm_file, u32 *handles, u32 nents
>  void virtio_gpu_array_add_obj(struct virtio_gpu_object_array *objs,
>  			      struct drm_gem_object *obj);
>  int virtio_gpu_array_lock_resv(struct virtio_gpu_object_array *objs);
> +int virtio_gpu_lock_one_resv_uninterruptible(struct virtio_gpu_object_array *objs);
>  void virtio_gpu_array_unlock_resv(struct virtio_gpu_object_array *objs);
>  void virtio_gpu_array_add_fence(struct virtio_gpu_object_array *objs,
>  				struct dma_fence *fence);
> diff --git a/drivers/gpu/drm/virtio/virtgpu_gem.c b/drivers/gpu/drm/virtio/virtgpu_gem.c
> index f22dc5c21cd4..435d37d36034 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_gem.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_gem.c
> @@ -238,6 +238,23 @@ int virtio_gpu_array_lock_resv(struct virtio_gpu_object_array *objs)
>  	return ret;
>  }
>  
> +int virtio_gpu_lock_one_resv_uninterruptible(struct virtio_gpu_object_array *objs)
> +{
> +	int ret;
> +
> +	if (objs->nents != 1)
> +		return -EINVAL;
> +
> +	dma_resv_lock(objs->objs[0]->resv, NULL);
> +
> +	ret = dma_resv_reserve_fences(objs->objs[0]->resv, 1);
> +	if (ret) {
> +		virtio_gpu_array_unlock_resv(objs);
> +		return ret;
> +	}
> +	return 0;
> +}
> +
>  void virtio_gpu_array_unlock_resv(struct virtio_gpu_object_array *objs)
>  {
>  	if (objs->nents == 1) {
> diff --git a/drivers/gpu/drm/virtio/virtgpu_plane.c b/drivers/gpu/drm/virtio/virtgpu_plane.c
> index a126d1b25f46..652352424744 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_plane.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_plane.c
> @@ -215,7 +215,10 @@ static void virtio_gpu_resource_flush(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_lock_one_resv_uninterruptible(objs)) {
> +			virtio_gpu_array_put_free(objs);
> +			return;
> +		}
>  		virtio_gpu_cmd_resource_flush(vgdev, bo->hw_res_handle, x, y,
>  					      width, height, objs,
>  					      vgplane_st->fence);
> @@ -459,7 +462,10 @@ 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_lock_one_resv_uninterruptible(objs)) {
> +			virtio_gpu_array_put_free(objs);
> +			return;
> +		}
>  		virtio_gpu_cmd_transfer_to_host_2d
>  			(vgdev, 0,
>  			 plane->state->crtc_w,