# CVE-2026-89991: bpf: Fix infinite loop in pcpu_freelist push with one possible CPU

> A local attacker can cause a kernel infinite loop and system hang by triggering a specific BPF freelist condition on single-CPU systems.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-89991, DEBIAN-CVE-2026-89991, GHSA-c23v-6rx9-2699, UBUNTU-CVE-2026-89991

## What is CVE-2026-89991?

CVE-2026-89991 is a denial of service vulnerability in Linux, published on 16 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: Fix infinite loop in pcpu_freelist push with one possible CPU

__pcpu_freelist_push() can loop forever when only one CPU is possible
and an NMI re-enters pcpu_freelist_push() while the interrupted context
holds that CPU's freelist lock.

After the current-CPU fast path fails, the fallback loop walks
cpu_possible_mask while skipping the current CPU. With CONFIG_SMP=n, or
when an SMP kernel is limited to one possible CPU with nr_cpus=1 or
possible_cpus=1, there are no other possible CPUs to examine. The loop
therefore makes no lock acquisition attempt and can never make progress.

The following stack was observed on a UP system:

  NMI context:
    pcpu_freelist_push
    free_htab_elem
    htab_map_delete_elem
    [perf-event BPF program]
    __perf_event_overflow
    perf_event_nmi_handler
    exc_nmi

  Interrupted context:
    __pcpu_freelist_push
    pcpu_freelist_push
    free_htab_elem
    htab_map_delete_elem
    [raw_tp/sys_enter BPF program]
    __bpf_trace_sys_enter
    do_syscall_64

raw_res_spin_lock() detects the same-CPU recursive acquisition and
returns -EDEADLK, but the subsequent fallback loop has no candidate head
on a system with one possible CPU.

Restore the extra fallback head that existed before the rqspinlock
conversion. Keep the current-CPU fast path, then try the other possible
CPUs and finally the extra head. The additional head lets a push, which
cannot fail without losing a preallocated element, make progress when the
only per-CPU head is held by the interrupted context.

Also check the extra head from the pop path so that nodes placed there
can be reused.

## Which versions are affected by CVE-2026-89991?


## How do I fix CVE-2026-89991?

Apply the vendor's patch.

## Is CVE-2026-89991 being exploited?

There is no known exploitation of CVE-2026-89991 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89991 and fix it if it is affected.

What it is: CVE-2026-89991 is a denial of service vulnerability in Linux, published on 16 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-2026-89991 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-2026-89991/ (Markdown: https://insights.sbomify.com/CVE-2026-89991/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89991
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89991
- OSV: https://osv.dev/vulnerability/CVE-2026-89991

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89991/
Summaries are generated and may contain mistakes; the official description above is the record.