CVE-2026-23225 sched/mmcid: Don't assume CID is CPU owned on mode switch
A use-after-free vulnerability in the Linux kernel scheduler allows local attackers to execute arbitrary code or escalate privileges via an out-of-bounds access.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-23225?
CVE-2026-23225 is a high-severity use after free vulnerability in Linux, published on 18 Feb 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
sched/mmcid: Don't assume CID is CPU owned on mode switch
Shinichiro reported a KASAN UAF, which is actually an out of bounds access in the MMCID management code.
CPU0 CPU1 T1 runs in userspace T0: fork(T4) -> Switch to per CPU CID mode fixup() set MM_CID_TRANSIT on T1/CPU1 T4 exit() T3 exit() T2 exit() T1 exit() switch to per task mode ---> Out of bounds access.
As T1 has not scheduled after T0 set the TRANSIT bit, it exits with the TRANSIT bit set. sched_mm_cid_remove_user() clears the TRANSIT bit in the task and drops the CID, but it does not touch the per CPU storage. That's functionally correct because a CID is only owned by the CPU when the ONCPU bit is set, which is mutually exclusive with the TRANSIT flag.
Now sched_mm_cid_exit() assumes that the CID is CPU owned because the prior mode was per CPU. It invokes mm_drop_cid_on_cpu() which clears the not set ONCPU bit and then invokes clear_bit() with an insanely large bit number because TRANSIT is set (bit 29).
Prevent that by actually validating that the CID is CPU owned in mm_drop_cid_on_cpu().
Which versions are affected by CVE-2026-23225?
How do I fix CVE-2026-23225?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2026-23225 being exploited?
There is no known exploitation of CVE-2026-23225 in the wild and no public exploit code.
Sources
Also known as GHSA-p525-h9pq-233r, ROOT-OS-DEBIAN-11-CVE-2026-23225, ROOT-OS-DEBIAN-12-CVE-2026-23225, ROOT-OS-DEBIAN-13-CVE-2026-23225, UBUNTU-CVE-2026-23225.