# CVE-2026-68438: smp: Make CSD lock acquisition atomic for debug mode

> A race condition in Linux kernel SMP CSD lock acquisition during debug mode can cause system hangs or panics.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 12 Aug 2026
- **Updated:** 17 Aug 2026
- **Also known as:** DEBIAN-CVE-2026-68438, GHSA-26r5-rg9x-853c, UBUNTU-CVE-2026-68438

## What is CVE-2026-68438?

CVE-2026-68438 is a vulnerability in Linux, published on 12 Aug 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:

smp: Make CSD lock acquisition atomic for debug mode

Commit b0473dcd4b1d ("smp: Improve smp_call_function_single()
CSD-lock diagnostics") changed smp_call_function_single() so that,
when CSD lock debugging is enabled, async !wait calls use the
destination CPU csd_data. That improves diagnostics, but it also removes
the single-writer property that made the old csd_lock() safe: multiple
CPUs can now prepare the same destination CPU CSD concurrently.

csd_lock() currently waits for CSD_FLAG_LOCK to clear and then sets the
bit with a non-atomic read-modify-write. Two senders can both see an
unlocked CSD, set the bit, overwrite the callback fields, and enqueue
the same llist node. Re-adding a node that is already the queue head can
make node->next point to itself, leaving the target CPU stuck walking
call_single_queue. Later synchronous work, such as a TLB shootdown, can
then remain queued and trigger soft-lockup warnings or panics.

Keep the single csd_lock() implementation, but when CSD lock debugging is
enabled, acquire CSD_FLAG_LOCK with try_cmpxchg_acquire(). This makes the
destination CPU CSD a real atomic lock in the only configuration where it
can be shared by multiple remote senders, while preserving the existing
non-debug fast path.

## Which versions are affected by CVE-2026-68438?


## How do I fix CVE-2026-68438?

Apply the vendor's patch.

## Is CVE-2026-68438 being exploited?

There is no known exploitation of CVE-2026-68438 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-68438 and fix it if it is affected.

What it is: CVE-2026-68438 is a vulnerability in Linux, published on 12 Aug 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-2026-68438 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-2026-68438/ (Markdown: https://insights.sbomify.com/CVE-2026-68438/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-68438
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-68438
- OSV: https://osv.dev/vulnerability/CVE-2026-68438

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-68438/
Summaries are generated and may contain mistakes; the official description above is the record.