# CVE-2021-3491: Linux kernel io_uring PROVIDE_BUFFERS MAX_RW_COUNT bypass

> A bypass of MAX_RW_COUNT in io_uring PROVIDE_BUFFERS allows heap overflow and arbitrary code execution in the Linux kernel.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/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:** 4 Jun 2021
- **Updated:** 16 Sep 2024
- **Also known as:** A-190877100, DEBIAN-CVE-2021-3491, GHSA-q4c9-gg27-f3pq, PUB-A-190877100, UBUNTU-CVE-2021-3491

## What is CVE-2021-3491?

CVE-2021-3491 is a high-severity vulnerability in Canonical Ubuntu Linux, published on 4 Jun 2021. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

The io_uring subsystem in the Linux kernel allowed the MAX_RW_COUNT limit to be bypassed in the PROVIDE_BUFFERS operation, which led to negative values being usedin mem_rw when reading /proc/<PID>/mem. This could be used to create a heap overflow leading to arbitrary code execution in the kernel. It was addressed via commit d1f82808877b ("io_uring: truncate lengths larger than MAX_RW_COUNT on provide buffers") (v5.13-rc1) and backported to the stable kernels in v5.12.4, v5.11.21, and v5.10.37. It was introduced in ddf0322db79c ("io_uring: add IORING_OP_PROVIDE_BUFFERS") (v5.7-rc1).

Weakness: CWE-787 (Out-of-bounds Write), CWE-131 (Incorrect Calculation of Buffer Size)

## Which versions are affected by CVE-2021-3491?

It affects 2 products.


Products: Canonical Ubuntu Linux, Linux kernel.

## How do I fix CVE-2021-3491?

Apply the vendor's patch.

## Is CVE-2021-3491 being exploited?

There is no known exploitation of CVE-2021-3491 in the wild and no public exploit code.

## Other vulnerabilities in Canonical Ubuntu Linux

- [CVE-2020-11651](https://insights.sbomify.com/CVE-2020-11651/): SaltStack Salt Authentication Bypass (critical 9.8, exploited in the wild)
- [CVE-2020-7247](https://insights.sbomify.com/CVE-2020-7247/): OpenSMTPD Remote Code Execution (critical 9.8, exploited in the wild)
- [CVE-2019-16928](https://insights.sbomify.com/CVE-2019-16928/): Exim Out-of-bounds Write (critical 9.8, exploited in the wild)
- [CVE-2018-6789](https://insights.sbomify.com/CVE-2018-6789/): Exim Buffer Overflow (critical 9.8, exploited in the wild)
- [CVE-2015-2590](https://insights.sbomify.com/CVE-2015-2590/): Oracle Java SE and Java SE Embedded Remote Code Execution (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-3491 and fix it if it is affected.

What it is: CVE-2021-3491 is a high-severity vulnerability in Canonical Ubuntu Linux, published on 4 Jun 2021. 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-2021-3491 in the wild and no public exploit code.

Affected product: Canonical Ubuntu 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-2021-3491/ (Markdown: https://insights.sbomify.com/CVE-2021-3491/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-3491
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-3491
- OSV: https://osv.dev/vulnerability/CVE-2021-3491

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-3491/
Summaries are generated and may contain mistakes; the official description above is the record.