# CVE-2026-90211: bpf, s390: Clear fetch destination on faulting arena atomic

> A missing register clear in the Linux kernel s390x BPF JIT allows local attackers to read uninitialized memory during faulting atomic operations.

- **Severity:** Not scored
- **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:** 17 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-90211, DEBIAN-CVE-2026-90211, GHSA-xjxr-6fhh-44vq, UBUNTU-CVE-2026-90211

## What is CVE-2026-90211?

CVE-2026-90211 is a vulnerability in Linux, published on 17 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:

bpf, s390: Clear fetch destination on faulting arena atomic

Same missing register clear as on riscv64. A RMW atomic on an arena pointer
is converted to BPF_PROBE_ATOMIC and gets an exception table entry, but
bpf_jit_probe_atomic_pre() only fills in the arena base and the probe
offset, leaving probe->reg at the -1 that bpf_jit_probe_init() set, which
bpf_jit_probe_post() writes into the entry and ex_handler_bpf() then reads
back as "there is nothing to clear".

That is right for a plain BPF_{ADD,AND,OR,XOR}, which only writes memory,
but an RMW carrying BPF_FETCH also reads the old value into a register:
src_reg for BPF_{ADD,AND,OR,XOR} | BPF_FETCH and BPF_XCHG, and r0 for
BPF_CMPXCHG. So on a fault over an unmapped arena page the program resumes
at the landing pad with whatever that register held before the atomic
instead of the 0 that every other BPF_PROBE_* access delivers.

Fill probe->reg in from bpf_atomic_load_reg(). Unlike x86-64 and arm64,
s390x does not report arena violations from its exception handler, so there
is no access direction to correct here, only the missing register clear.

## Which versions are affected by CVE-2026-90211?


## How do I fix CVE-2026-90211?

Apply the vendor's patch.

## Is CVE-2026-90211 being exploited?

There is no known exploitation of CVE-2026-90211 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90211 and fix it if it is affected.

What it is: CVE-2026-90211 is a vulnerability in Linux, published on 17 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-2026-90211 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-2026-90211/ (Markdown: https://insights.sbomify.com/CVE-2026-90211/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90211
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90211
- OSV: https://osv.dev/vulnerability/CVE-2026-90211

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90211/
Summaries are generated and may contain mistakes; the official description above is the record.