# CVE-2021-29657: Use after free in n/a

> A use-after-free in the Linux kernel KVM module allows a guest to bypass host MSR access controls via a race condition.

- **Severity:** High 7.4 (`CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **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:** 22 Jul 2021
- **Updated:** 3 Aug 2024
- **Also known as:** AZL-6554, DEBIAN-CVE-2021-29657, GHSA-m46m-jr5m-g95h, UBUNTU-CVE-2021-29657

## What is CVE-2021-29657?

CVE-2021-29657 is a high-severity use after free vulnerability in Linux kernel, published on 22 Jul 2021. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

arch/x86/kvm/svm/nested.c in the Linux kernel before 5.11.12 has a use-after-free in which an AMD KVM guest can bypass access control on host OS MSRs when there are nested guests, aka CID-a58d9166a756. This occurs because of a TOCTOU race condition associated with a VMCB12 double fetch in nested_svm_vmrun.

Weakness: CWE-416 (Use After Free), CWE-367 (Time-of-check Time-of-use (TOCTOU) Race Condition)

## Which versions are affected by CVE-2021-29657?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-29657?

Apply the vendor's patch.

## Is CVE-2021-29657 being exploited?

There is no known exploitation of CVE-2021-29657 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-2021-29657 and fix it if it is affected.

What it is: CVE-2021-29657 is a high-severity use after free vulnerability in Linux kernel, published on 22 Jul 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-2021-29657 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-2021-29657/ (Markdown: https://insights.sbomify.com/CVE-2021-29657/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-29657
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-29657
- OSV: https://osv.dev/vulnerability/CVE-2021-29657

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-29657/
Summaries are generated and may contain mistakes; the official description above is the record.