# CVE-2026-89903: LoongArch: Do not save/restore percpu base register in rethook trampoline

> A race condition in the LoongArch kernel rethook trampoline can cause CPU state corruption and system lockups during task migration.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **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:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-101280, BELL-CVE-2026-89903, DEBIAN-CVE-2026-89903, GHSA-fx3m-r2wg-wx6v, UBUNTU-CVE-2026-89903

## What is CVE-2026-89903?

CVE-2026-89903 is a high-severity vulnerability in Linux, published on 16 Sep 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:

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.

## Which versions are affected by CVE-2026-89903?


## How do I fix CVE-2026-89903?

Apply the vendor's patch.

## Is CVE-2026-89903 being exploited?

There is no known exploitation of CVE-2026-89903 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89903 and fix it if it is affected.

What it is: CVE-2026-89903 is a high-severity vulnerability in Linux, published on 16 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-89903 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-89903/ (Markdown: https://insights.sbomify.com/CVE-2026-89903/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89903
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89903
- OSV: https://osv.dev/vulnerability/CVE-2026-89903

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89903/
Summaries are generated and may contain mistakes; the official description above is the record.