CVE-2025-38703: Use After Free

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

drm/xe: Make dma-fences compliant with the safe access rules

Xe can free some of the data pointed to by the dma-fences it exports. Most notably the timeline name can get freed if userspace closes the associated submit queue. At the same time the fence could have been exported to a third party (for example a sync_fence fd) which will then cause an use- after-free on subsequent access.

To make this safe we need to make the driver compliant with the newly documented dma-fence rules. Driver has to ensure a RCU grace period between signalling a fence and freeing any data pointed to by said fence.

For the timeline name we simply make the queue be freed via kfree_rcu and for the shared lock associated with multiple queues we add a RCU grace period before freeing the per GT structure holding the lock.