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 (v2) was rejected because the broader lock scope deadlocks against other BO locking paths in the same atomic commit. Introduce virtio_gpu_array_lock_resv_uninterruptible() that uses dma_resv_lock() instead of dma_resv_lock_interruptible() on the nents==1 path. This eliminates the -EINTR failure mode -- the realistic syzbot trigger -- without extending the lock hold across the commit. Use it from both virtio_gpu_cursor_plane_update() and virtio_gpu_resource_flush(); check the return value to handle the remaining -ENOMEM case 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: [email protected] Closes: https://syzkaller.appspot.com/bug?extid=72bd3dd3a5d5f39a0271 Fixes: 5cfd31c5b3a3 ("drm/virtio: fix virtio_gpu_cursor_plane_update().") Cc: [email protected] Signed-off-by: Deepanshu Kartikey <[email protected]> --- v3: Per maintainer feedback on v2 (lockup caused by the broader lock scope in prepare_fb conflicting with other BO locking in the same atomic commit): drop the prepare_fb/cleanup_fb approach, introduce an uninterruptible variant of virtio_gpu_array_lock_resv(), and use it in both virtio_gpu_cursor_plane_update() and virtio_gpu_resource_flush(). v2: Move resv lock acquisition from .atomic_update (which must not fail) to .prepare_fb (which may), per maintainer review of v1. The previous 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 | 24 ++++++++++++++++++++++++ drivers/gpu/drm/virtio/virtgpu_plane.c | 10 ++++++++-- 3 files changed, 33 insertions(+), 2 deletions(-) diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.h b/drivers/gpu/drm/virtio/virtgpu_drv.h index f17660a71a3e..43a7eb568e15 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_array_lock_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..08c4b7ef8d44 100644 --- a/drivers/gpu/drm/virtio/virtgpu_gem.c +++ b/drivers/gpu/drm/virtio/virtgpu_gem.c @@ -238,6 +238,30 @@ int virtio_gpu_array_lock_resv(struct virtio_gpu_object_array *objs) return ret; } +int virtio_gpu_array_lock_resv_uninterruptible(struct virtio_gpu_object_array *objs) +{ + unsigned int i; + int ret = 0; + + if (objs->nents == 1) { + dma_resv_lock(objs->objs[0]->resv, NULL); + } else { + ret = drm_gem_lock_reservations(objs->objs, objs->nents, + &objs->ticket); + if (ret) + return ret; + } + + for (i = 0; i < objs->nents; ++i) { + ret = dma_resv_reserve_fences(objs->objs[i]->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..ef118cb4f0fa 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_array_lock_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_array_lock_resv_uninterruptible(objs)) { + virtio_gpu_array_put_free(objs); + return; + } virtio_gpu_cmd_transfer_to_host_2d (vgdev, 0, plane->state->crtc_w, -- 2.43.0
