# CVE-2026-98038: bpf: Keep refcount_acquire nullable for borrowed RCU kptrs

> A logic error in the Linux kernel BPF verifier allows null pointer dereferences when handling borrowed RCU pointers, potentially causing a system crash.

- **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:** 25 Sep 2026
- **Updated:** 21 Oct 2026
- **Also known as:** BELL-CVE-2026-98038, DEBIAN-CVE-2026-98038, GHSA-cwcr-7vrq-384j, UBUNTU-CVE-2026-98038

## What is CVE-2026-98038?

CVE-2026-98038 is a vulnerability in Linux, published on 25 Sep 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:

bpf: Keep refcount_acquire nullable for borrowed RCU kptrs

bpf_refcount_acquire() is fallible for a borrowed reference because the
object may have reached a zero refcount. The verifier therefore keeps
KF_RET_NULL on the return value unless the argument is an owning reference.

An RCU-protected load of a local kptr is marked MEM_ALLOC, but it only
receives NON_OWN_REF when the pointee contains a graph node. A refcounted
object without a graph node consequently looks like an owning reference
even though the loaded register has no acquired reference state. If the
program drops the last real reference while remaining in the RCU critical
section, refcount_inc_not_zero() returns NULL while the verifier treats the
result as non-NULL.

Only classify the argument as owning when it is backed by a verifier-tracked
reference. This retains the non-NULL return for pointers from bpf_obj_new(),
bpf_kptr_xchg(), or an earlier successful acquisition, while requiring a
NULL check for borrowed RCU kptrs.

[ kkd: Rewrote commit log ]

## Which versions are affected by CVE-2026-98038?


## How do I fix CVE-2026-98038?

Apply the vendor's patch.

## Is CVE-2026-98038 being exploited?

There is no known exploitation of CVE-2026-98038 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98038 and fix it if it is affected.

What it is: CVE-2026-98038 is a vulnerability in Linux, published on 25 Sep 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-2026-98038 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-2026-98038/ (Markdown: https://insights.sbomify.com/CVE-2026-98038/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98038
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98038
- OSV: https://osv.dev/vulnerability/CVE-2026-98038

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98038/
Summaries are generated and may contain mistakes; the official description above is the record.