# CVE-2026-43121: io_uring/zcrx: fix user_ref race between scrub and refill paths

> A race condition in the Linux kernel io_uring zero-copy receive path allows double-free leading to out-of-bounds writes and potential privilege escalation.

- **Severity:** Medium 4.7 (`CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/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:** 6 May 2026
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2026-43121, DEBIAN-CVE-2026-43121, GHSA-77p3-mmmp-fjj2, UBUNTU-CVE-2026-43121

## What is CVE-2026-43121?

CVE-2026-43121 is a medium-severity vulnerability in Linux kernel, published on 6 May 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:

io_uring/zcrx: fix user_ref race between scrub and refill paths

The io_zcrx_put_niov_uref() function uses a non-atomic
check-then-decrement pattern (atomic_read followed by separate
atomic_dec) to manipulate user_refs. This is serialized against other
callers by rq_lock, but io_zcrx_scrub() modifies the same counter with
atomic_xchg() WITHOUT holding rq_lock.

On SMP systems, the following race exists:

  CPU0 (refill, holds rq_lock)          CPU1 (scrub, no rq_lock)
  put_niov_uref:
    atomic_read(uref) - 1
    // window opens
                                        atomic_xchg(uref, 0) - 1
                                        return_niov_freelist(niov) [PUSH #1]
    // window closes
    atomic_dec(uref) - wraps to -1
    returns true
    return_niov(niov)
    return_niov_freelist(niov)           [PUSH #2: DOUBLE-FREE]

The same niov is pushed to the freelist twice, causing free_count to
exceed nr_iovs. Subsequent freelist pushes then perform an out-of-bounds
write (a u32 value) past the kvmalloc'd freelist array into the adjacent
slab object.

Fix this by replacing the non-atomic read-then-dec in
io_zcrx_put_niov_uref() with an atomic_try_cmpxchg loop that atomically
tests and decrements user_refs. This makes the operation safe against
concurrent atomic_xchg from scrub without requiring scrub to acquire
rq_lock.

[pavel: removed a warning and a comment]

Weakness: CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization)

## Which versions are affected by CVE-2026-43121?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-43121?

Apply the vendor's patch.

## Is CVE-2026-43121 being exploited?

There is no known exploitation of CVE-2026-43121 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-43121 and fix it if it is affected.

What it is: CVE-2026-43121 is a medium-severity vulnerability in Linux kernel, published on 6 May 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-2026-43121 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2026-43121/ (Markdown: https://insights.sbomify.com/CVE-2026-43121/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-43121
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-43121
- OSV: https://osv.dev/vulnerability/CVE-2026-43121

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-43121/
Summaries are generated and may contain mistakes; the official description above is the record.