CVE-2026-89903

Status: UPDATED  |  Advisory ID: CVE-2026-89903

Key Details

Affected Products, Subsystems & Sectors

What to Know

In the Linux kernel, the following vulnerability has been resolved:

LoongArch: Do not save/restore percpu base register in rethook trampoline

The rethook trampoline saves $r21 ($u0), the percpu base, into its frame

at entry and restores it at exit. Inbetween rethook_trampoline_handler()

may schedule via preempt_enable_notrace().

If the task migrates to another CPU, the frame's $r21 holds the old

CPU's percpu base, and restoring it poisons $r21 on the new CPU. Until

the next user->kernel transition heals $r21, all this_cpu_*() accesses

(runqueues, RCU per-CPU data, timer tick programming, FPU ownership)

hit the wrong CPU's percpu area.

Under kretprobe-heavy preemptible load this can corrupt scheduler and

timer state: scheduling-while-atomic splats, wrong-CPU RCU warnings,

WARN_ON_ONCE(rq != this_rq()) in nohz_balance_exit_idle(), and CPUs

parking in the idle loop with the constant timer never re-armed (hard

lockup). Reproduces on a Loongson-3A6000 with kretprobes on VFS paths

plus heavy file churn (OS install / unsquashfs).

By convention $r21 always holds the current CPU's percpu base in kernel

mode: SAVE_SOME() at exception entry reloads it only when coming from

user mode, and RESTORE_SOME() restores it only when returning to user

mode; the context-switch path never writes it. Therefore the live $r21

at trampoline exit is already correct, and nothing inbetween can change

it legitimately (kernel C code cannot write a global register variable).

The same flaw existed even in the pre-rethook kretprobe trampoline since

v6.3; it was carried over when rethook replaced it. Drop both the save

and the restore here. Drop the restore is enough to solve the issue, and

drop the save is to keep the code tidy and no need to clear it. (NVD)

What to Do

Monitor Linux's web page for any future patch releases.

References