On 21/07/2026 09:21, André Draszik wrote:
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.
Presumably these fence are exportable via syncobj and/or sync file? If so, your analysis seems correct to me. Fix as well looks it works to close the vulnerability, but the part I cannot help with, and which will need someone more familiar with the CRTC code and the associated object lifetimes, is to comment on whether a RCU delay in drm_crtc_cleanup() is acceptable or it must be made async one way or the other.
Regards,
Tvrtko
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
*/ void drm_crtc_cleanup(struct drm_crtc *crtc) { struct drm_device *dev = crtc->dev;
- responsibility of the caller.
- /* 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.