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CVE-2026-17523
can: bcm: switch timer to HRTIMER_MODE_SOFT and remove hrtimer_tasklet
Published 2026-09-09 (40 days 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-2026-17523 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 | 5.4.6-1 | Update the kernel and reboot. Fixed in 5.4.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 5.4.6-1 | Update the kernel and reboot. Fixed in 5.4.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 5.4.6-1 | Update the kernel and reboot. Fixed in 5.4.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 5.4.6-1 | Update the kernel and reboot. Fixed in 5.4.6-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
This patch switches the timer to HRTIMER_MODE_SOFT, which executed the timer callback in softirq context and removes the hrtimer_tasklet.
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
- 2026-09-09 (40 days 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
- 0.15%, higher than 3.5% 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 |
|---|---|---|
| 2.6.29 | 4.19.226 | 79305a826f87 |
| 2.6.29 | 5.4 | bf74aa86e111 |
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
- The UAF is in bcm_remove_op() hrtimer/tasklet teardown, reached only via local PF_CAN/CAN_BCM syscalls (socket/connect/sendmsg TX_SETUP or RX_SETUP with STARTTIMER, then close or TX_DELETE/RX_DELETE). Remote IP services or CAN bus frames alone cannot close the socket or delete the op.
- AC:LAttack complexity, low
- The attacker controls both sides of the race: they arm cyclic TX/RX hrtimers with attacker-chosen microsecond intervals, then delete the op or close the socket, hitting the TOCTOU between tasklet-state checks and hrtimer_active() in bcm_remove_op(). The window is retryable until the scheduled tasklet runs on the freed object.
- PR:LPrivileges required, low
- can_create(), bcm_connect(), and bcm_sendmsg() have no capability checks. RX_SETUP with ifindex 0 starts timeout timers without a CAN device, so any unprivileged local user (including via user namespaces / vcan) can create BCM ops and trigger teardown.
- UI:NUser interaction, none
- Exploitation requires only the attacker opening a BCM socket, installing timer-armed TX_SETUP/RX_SETUP ops, and closing or deleting them; no victim action such as mounting a filesystem or opening a file is needed.
- S:UScope, unchanged
- The use-after-free corrupts kernel heap state in the same kernel security domain, enabling local privilege escalation or crash; it does not by itself cross a VM, guest-to-host, or IOMMU boundary.
- C:HConfidentiality impact, high
- bcm_op is a kmalloc object that embeds tasklet_struct; after the hrtimer hardirq callback schedules a tasklet that bcm_remove_op() misses, the tasklet runs on freed memory. Heap reuse of tsklet.func/data and UAF reads in bcm_send_to_user()/bcm_can_tx() enable arbitrary kernel disclosure.
- I:HIntegrity impact, high
- The same UAF lets a sprayed tasklet_struct hijack tsklet.func for arbitrary kernel execution, and bcm_tx_timeout_tsklet()/bcm_can_tx() write through freed op fields (frames, sock, timers), providing arbitrary write and control-flow hijack primitives.
- A:HAvailability impact, high
- A scheduled tasklet running after kfree(op) dereferences the freed bcm_op (and its frames/sk pointers), causing kernel oops/panic even when the UAF is not fully turned into code execution.