# CVE-2026-98639: KVM: riscv: Fix NACL hfence entry update order

> A race condition in Linux KVM RISC-V allows incorrect TLB flush ranges, potentially leading to data exposure or modification by a local attacker.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 Oct 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-98639, DEBIAN-CVE-2026-98639, GHSA-2qhr-3xrg-mw89, UBUNTU-CVE-2026-98639

## What is CVE-2026-98639?

CVE-2026-98639 is a vulnerability in Linux, published on 18 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

KVM: riscv: Fix NACL hfence entry update order

The SBI v3.0 specification (section 15.1.2) requires a nested HFENCE
entry to be populated as follows:
1) find an unused entry with Config.Pending == 0
2) update the Page_Number and Page_Count words
3) update the Config word with Config.Pending set

__kvm_riscv_nacl_hfence() writes the Config word first, so the SBI
implementation (or NACL hardware) can observe a pending entry with
pnum/pcount values left over from the previous use of that entry,
resulting in incorrect TLB flush ranges.

Write pnum and pcount first and the Config word last. Since the
consumer is an external agent on coherent shared memory, use
WRITE_ONCE() to stop the compiler from reordering the stores and
smp_wmb() to make the parameter words globally visible before the
Pending bit is set.

## Which versions are affected by CVE-2026-98639?


## How do I fix CVE-2026-98639?

Apply the vendor's patch.

## Is CVE-2026-98639 being exploited?

There is no known exploitation of CVE-2026-98639 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98639 and fix it if it is affected.

What it is: CVE-2026-98639 is a vulnerability in Linux, published on 18 Oct 2026. 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-2026-98639 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-2026-98639/ (Markdown: https://insights.sbomify.com/CVE-2026-98639/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98639
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98639
- OSV: https://osv.dev/vulnerability/CVE-2026-98639

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98639/
Summaries are generated and may contain mistakes; the official description above is the record.