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CVE-2026-72123
can: bcm: defer rx_op deallocation to workqueue to fix thrtimer UAF
Published 2026-08-15 (2 months ago) by the Linux kernel CNA. Subsystem net/can. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-72123 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixed | 6.1.180-1 | Update the kernel and reboot. Fixed in 6.1.180-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.100-1 | Update the kernel and reboot. Fixed in 6.12.100-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
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.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
net/can/bcm.c
- What this is
- CAN NETWORK LAYERMAINTAINERS section matched on net/can/ · linux-can@vger.kernel.org · Maintained
- Kernel module
- can_bcm
Check it on the machine
grep -wE 'CONFIG_CAN|CONFIG_CAN_BCM' /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
lsmod | grep -w can_bcmPrints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.
Show whether Debian builds this, per release
Debian builds this, but not on every flavour. At least one release below builds the code on some of its kernel flavours and not on others, so the answer depends on the architecture and flavour installed.
- bookworm, trixie, forky, sidMnot built on amd64/cloud-amd64, arm64/cloud-arm64
- M
- a module where it is built at all; some flavours do not build it
- Build switches
- CONFIG_CAN, CONFIG_CAN_BCM
- Reads as
- all on means the file is compiled; any one off means it is not.
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-08-15 (2 months ago)source
- CVSS
- 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.17%, higher than 5.8% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/can
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.10.130 | 5.10.265 | de5fce46637d |
| 5.15.54 | 5.15.216 | 036a8c320ca1 |
| 5.19 | 6.1.178 | 3cf4fd5316f4 |
| 5.19 | 6.6.145 | 4177762f7064 |
| 5.19 | 6.12.97 | 6fd08e8d826c |
| 5.19 | 6.18.40 | cd830e0bc25e |
| 5.19 | 7.1.5 | ce2d4b121fb7 |
| 5.19 | 7.2 | 68973f9db761 |
| 4.19.252 | 0 | 0 |
| 5.4.205 | 0 | 0 |
| 5.18.11 | 0 | 0 |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- Exploitation requires local BCM socket syscalls (sendmsg with RX_SETUP/SETTIMER/ival2 throttling and RX_DELETE); CAN frames can be injected locally via loopback without physical bus access.
- AC:LAttack complexity, low
- The attacker controls both sides of the race by concurrently issuing RX_DELETE teardown and delivering matching CAN frames (e.g., via threads or paired BCM/RAW sockets), making the thrtimer re-arm UAF reliably triggerable.
- PR:LPrivileges required, low
- No kernel capability check gates AF_CAN/CAN_BCM socket creation or bcm_sendmsg opcodes; any unprivileged local user with socket access (including via user namespaces with vcan) can reach the vulnerable path.
- UI:NUser interaction, none
- No victim interaction is required; the attacker self-triggers the race through their own BCM socket operations and locally injected CAN traffic.
- S:UScope, unchanged
- Impact is confined to kernel memory corruption and local privilege escalation within the same kernel security boundary, not a cross-boundary escape such as VM or IOMMU bypass.
- C:HConfidentiality impact, high
- The thrtimer UAF dereferences freed bcm_op fields (locks, frame buffers, op->sk) in bcm_rx_thr_handler/bcm_send_to_user, enabling attacker-controlled heap reuse and arbitrary kernel memory disclosure.
- I:HIntegrity impact, high
- UAF on the bcm_op slab object allows heap spraying to corrupt function pointers, timers, and sock structures, providing a standard path to arbitrary kernel writes and local privilege escalation.
- A:HAvailability impact, high
- The freed-op dereference in the hrtimer softirq path causes kernel oops/panic under normal exploitation attempts, and successful memory corruption can crash or hang the system.