CVE-2026-23225

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 MMCIDTRANSIT 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. schedmmcidremoveuser() 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 schedmmcidexit() assumes that the CID is CPU owned because the prior mode was per CPU. It invokes mmdropcidoncpu() which clears the not set ONCPU bit and then invokes clearbit() with an insanely large bit number because TRANSIT is set (bit 29).

Prevent that by actually validating that the CID is CPU owned in mmdropcidoncpu().