can: bcm: defer rx_op deallocation to workqueue to fix thrtimer UAF

Overview

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.

Affected

• Linux >= 85cd41070df992d3c0dfd828866fdd243d3b774a < de5fce46637de05bef56ec08528127676eb6fc9b
• Linux >= f34f2a18e47b73e48f90a757e1f4aaa8c7d665a1 < 036a8c320ca11bc912e8027adcfad14b326f067e
• Linux >= f1b4e32aca0811aa011c76e5d6cf2fa19224b386 < 3cf4fd5316f449811d8baf1bc6978ef5a7b743a9
• Linux >= f1b4e32aca0811aa011c76e5d6cf2fa19224b386 < 4177762f70646ac48a2af382e45a795cbd295198
• Linux >= f1b4e32aca0811aa011c76e5d6cf2fa19224b386 < 6fd08e8d826c3aa4cc7021f5f9cdbb7fa7441d3f
• Linux >= f1b4e32aca0811aa011c76e5d6cf2fa19224b386 < cd830e0bc25ee2d38cbfbdbb3cd77c5f53b2b6d5
• Linux >= f1b4e32aca0811aa011c76e5d6cf2fa19224b386 < ce2d4b121fb7545e1ed588e860c8e5fd5ad45224
• Linux >= f1b4e32aca0811aa011c76e5d6cf2fa19224b386 < 68973f9db76144825e4f35dfdc80fb8279eb2d57
• Linux fbac09a3b8890003c0c55294c00709f3ae5501bb
• Linux 5b48f5711f1c630841ab78dcc061de902f0e37bf
• Linux edb4baffb9483141a50fb7f7146cfe4a4c0c2db8
• Linux >= 5.10.130 < 5.10.265
• Linux >= 5.15.54 < 5.15.216
• Linux >= 4.19.252 < 4.20
• Linux >= 5.4.205 < 5.5
• Linux >= 5.18.11 < 5.19
• Linux 5.19

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.