# CVE-2026-99120: KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 28 Oct 2026
- **Updated:** 28 Oct 2026
- **Also known as:** BELL-CVE-2026-99120, DEBIAN-CVE-2026-99120, GHSA-r4wm-h5f5-6qhj, UBUNTU-CVE-2026-99120

## What is CVE-2026-99120?

CVE-2026-99120 is a vulnerability in Linux, published on 28 Oct 2026. 

### Official description

In the Linux kernel, the following vulnerability has been resolved:

KVM: x86/mmu: Cope with the SS bit being set in #NPF exit codes

When running Hyper-V from Windows 11, KVM is seeing the SS bit set
in nested page fault error codes produced for shadowed NPT.  The AMD
manual is unclear about when SS is set, including whether it can be set
when SupervisorShadowStackEn (bit 6) is not set in the MISC_CTL field
of the VMCB.

Nevertheless, the unexpected SS bit causes out of bounds accesses in
permission_fault(), seen as either WARNs or UBSAN reports:

  kernel: UBSAN: array-index-out-of-bounds in arch/x86/kvm/mmu.h:225:27
  kernel: index 34 is out of range for type 'u16 [16]'

(in this case, the page fault error code is 34*2=0x44, i.e. SS|U; the
report shows SS|U|W happening as well).  Fix the handling of unknown
error codes by dropping SS and (with a WARN) any bits not among those
that permission_fault() expects to receive.

At the same time, given the uncertainty about when hardware sets SS,
follow the processor's behavior when constructing the fault's error
code, and preserve the SS bit as passed to FNAME(walk_addr_generic).
While the behavior is surprising, there's a possibility that Hyper-V
expects it (Hyper-V does not even enable CET unless it finds MBEC/GMET!),
so retain the information when reflecting the fault to L1 instead of
unconditionally discarding it.

Bisection shows that this started happening with commit 687ee95c1b6d ("KVM:
nSVM: enable GMET for guests"), even though SHSTK was available to guests
since Linux 6.18.  However, this is not because bit 1 of MISC_CTL (GMET)
somehow enables reporting the SS bit; rather, it's simply that Hyper-V
never uses CET unless GMET is also present.  The WARN is reproducible
even with gmet=0, with some hacking of kvm-unit-tests's cet.c testcase
to ensure that the first access to the shadow stack triggers a page fault.

## Which versions are affected by CVE-2026-99120?


## How do I fix CVE-2026-99120?

Upgrade to a fixed version.

## Is CVE-2026-99120 being exploited?

There is no known exploitation of CVE-2026-99120 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-99120 and fix it if it is affected.

What it is: CVE-2026-99120 is a vulnerability in Linux, published on 28 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-99120 in the wild and no public exploit code.

Affected product: 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-2026-99120/ (Markdown: https://insights.sbomify.com/CVE-2026-99120/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-99120
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-99120
- OSV: https://osv.dev/vulnerability/CVE-2026-99120

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-99120/
Summaries are generated and may contain mistakes; the official description above is the record.