In [1], sashiko reported the following issue:
=== snip === Looking at how these fences are managed, drm_crtc_create_fence() creates a dma_fence without taking a reference to the drm_device or drm_crtc. Because the sync_file framework exposes this fence to userspace, the fence can outlive the CRTC.
The dma_fence contract requires that data accessed by dma_fence_ops (like get_driver_name) must remain valid for an RCU grace period after the fence is signaled. However, drm_crtc_cleanup() and the subsequent freeing of the device do not wait for an RCU grace period via synchronize_rcu().
If userspace calls ioctl(SYNC_IOC_FILE_INFO) concurrently with a device hot-unplug:
CPU1 (Userspace) sync_file_get_name() ops = rcu_dereference(fence->ops); if (!dma_fence_test_signaled_flag()) // Preempted or delayed here
CPU2 (Driver Teardown) Signals the fence (setting fence->ops = NULL) Destroys and frees the CRTC without waiting for an RCU grace period
CPU1 (Resumes) ops->get_driver_name(fence) -> drm_crtc_fence_get_driver_name() crtc = fence_to_crtc(fence); // Casts to the freed CRTC return crtc->dev->driver->name; // Use-after-free
...
Does the CRTC or DRM device need to be kept alive for the RCU grace period, or should the fence hold a proper reference to prevent the use-after-free when get_driver_name() and get_timeline_name() access the freed CRTC structure? === snap ===
I believe this to be a correct observation and this patch implements the suggestion of waiting for an RCU grace period before proceeding with destruction of the drm_crtc, so that get_driver_name() and get_timeline_name() can still work.
Link: https://sashiko.dev/#/patchset/20260618-linux-drm_crtc_fix2-v1-1-c03e77b36f3... Fixes: 6d6003c4b613 ("drm/fence: add fence timeline to drm_crtc") Cc: stable@vger.kernel.org Signed-off-by: André Draszik andre.draszik@linaro.org
--- v3: - Philipp: update kerneldoc, add Fixes:
v2: new patch --- drivers/gpu/drm/drm_crtc.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-)
diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c index 63ead8ba6756..e8e80c936852 100644 --- a/drivers/gpu/drm/drm_crtc.c +++ b/drivers/gpu/drm/drm_crtc.c @@ -493,14 +493,23 @@ EXPORT_SYMBOL(__drmm_crtc_alloc_with_planes); * drm_crtc_cleanup - Clean up the core crtc usage * @crtc: CRTC to cleanup * - * This function cleans up @crtc and removes it from the DRM mode setting - * core. Note that the function does *not* free the crtc structure itself, - * this is the responsibility of the caller. + * This function cleans up @crtc and removes it from the DRM mode setting core, + * after first waiting an RCU grace period to ensure @crtc->dev can safely be + * dereferenced by our dma_fence_ops. + * + * Note that the function does *not* free the crtc structure itself, this is the + * responsibility of the caller. */ void drm_crtc_cleanup(struct drm_crtc *crtc) { struct drm_device *dev = crtc->dev;
+ /* Ensure our dma_fence_ops remain valid for an RCU grace period after + * the fence is signaled. This is necessary because our dma_fence_ops + * dereference crtc->dev. + */ + synchronize_rcu(); + /* Note that the crtc_list is considered to be static; should we * remove the drm_crtc at runtime we would have to decrement all * the indices on the drm_crtc after us in the crtc_list.