# CVE-2026-98789: bpf: Fix objects stuck in free_by_rcu_ttrace

> A flaw in the Linux kernel BPF subsystem allows unbounded memory retention, leading to resource exhaustion and denial of service.

- **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:** AZL-109236, BELL-CVE-2026-98789, DEBIAN-CVE-2026-98789, GHSA-49x9-888x-jjxr, UBUNTU-CVE-2026-98789

## What is CVE-2026-98789?

CVE-2026-98789 is a denial of service 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:

bpf: Fix objects stuck in free_by_rcu_ttrace

do_call_rcu_ttrace() returns early when call_rcu_ttrace_in_progress is set
and leaves the objects in free_by_rcu_ttrace. __free_rcu() frees
waiting_for_gp_ttrace only and clears the flag. Hence the objects that
free_bulk() or __free_by_rcu() added while RCU tasks trace GP was in flight
stay in free_by_rcu_ttrace until free_bulk() or alloc_bulk() is called for
the same bpf_mem_cache again, which may never happen. The number of such
objects is not bounded.

Turn call_rcu_ttrace_in_progress into three states:
0 - idle
1 - __free_rcu() is queued
2 - __free_rcu() is queued and free_by_rcu_ttrace got more objects since

do_call_rcu_ttrace() sets 2. __free_rcu() does cmpxchg(1 -> 0) and starts
the next GP when it fails. It cannot clear the flag first and check
free_by_rcu_ttrace later, since bpf_mem_alloc_destroy() frees bpf_mem_cache
without waiting for RCU callbacks when the flag is zero.

Now __free_rcu() queues itself, so the one that didn't see 'draining' may
do call_rcu_tasks_trace() after rcu_barrier_tasks_trace() in
free_mem_alloc(). Queue it under rcu_read_lock() and do synchronize_rcu()
before the barriers. Calling rcu_barrier_tasks_trace() twice works too, but
creating and destroying hash maps in a loop on many cpus slows down to one
free_mem_alloc() per GP and kworkers pile up.

## Which versions are affected by CVE-2026-98789?


## How do I fix CVE-2026-98789?

Apply the vendor's patch.

## Is CVE-2026-98789 being exploited?

There is no known exploitation of CVE-2026-98789 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98789 and fix it if it is affected.

What it is: CVE-2026-98789 is a denial of service 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-2026-98789 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-2026-98789/ (Markdown: https://insights.sbomify.com/CVE-2026-98789/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98789
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98789
- OSV: https://osv.dev/vulnerability/CVE-2026-98789

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98789/
Summaries are generated and may contain mistakes; the official description above is the record.