In the Linux kernel, the following vulnerability has been resolved:
can: bcm: defer rx_op deallocation to workqueue to fix thrtimer UAF
Commit f1b4e32aca08 (can: bcm: use call_rcu() instead of costly synchronize_rcu()) replaced synchronize_rcu() in bcm_delete_rx_op() with call_rcu() and introduced the RX_NO_AUTOTIMER flag.
However, this flag check was omitted for thrtimer in the packet rx fast-path. During BCM RX operation teardown, a concurrent RCU reader (bcm_rx_handler) can race and re-arm thrtimer via bcm_rx_update_and_send() after call_rcu() has been scheduled. Once the RCU grace period elapses, bcm_op is freed. The subsequently firing thrtimer then dereferences the deallocated op, causing a UAF.
Adding flag checks to the rx fast-path (bcm_rx_update_and_send) does not fully close the TOCTOU race and introduces latency for every CAN frame. Conversely, calling hrtimer_cancel() directly inside the RCU callback (softirq context) is fatal as hrtimer_cancel() can sleep, triggering a scheduling while atomic panic.
Resolve this by deferring the timer cancellation and memory free to a dedicated unbound workqueue (bcm_wq). The RCU callback now queues a work item to bcm_wq, which safely cancels both timers and deallocates memory in sleepable process context. A dedicated workqueue is used to prevent system-wide WQ saturation and is cleanly flushed/destroyed on module unload to avoid rmmod page faults.
Since the deferred work can now outlive the calling context by an unbounded amount, also take a reference on op->sk when it is assigned and drop it only once the deferred work has cancelled both timers, so a socket can no longer be freed out from under a still-armed timer whose callback (bcm_send_to_user()) dereferences op->sk.
References
- https://git.kernel.org/stable/c/3cf4fd5316f449811d8baf1bc6978ef5a7b743a9
- https://git.kernel.org/stable/c/4177762f70646ac48a2af382e45a795cbd295198
- https://git.kernel.org/stable/c/68973f9db76144825e4f35dfdc80fb8279eb2d57
- https://git.kernel.org/stable/c/6fd08e8d826c3aa4cc7021f5f9cdbb7fa7441d3f
- https://git.kernel.org/stable/c/cd830e0bc25ee2d38cbfbdbb3cd77c5f53b2b6d5
- https://git.kernel.org/stable/c/ce2d4b121fb7545e1ed588e860c8e5fd5ad45224