# CVE-2026-90248: net/sched: cls_api: fix teardown of an adopted proto on insert-race loss

> A race condition in the Linux kernel traffic control subsystem allows local attackers to cause denial of service by triggering incorrect teardown of active classifiers.

- **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:** 17 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-102564, BELL-CVE-2026-90248, DEBIAN-CVE-2026-90248, GHSA-rm9p-rr32-f7j6, UBUNTU-CVE-2026-90248

## What is CVE-2026-90248?

CVE-2026-90248 is a vulnerability in Linux, published on 17 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:

net/sched: cls_api: fix teardown of an adopted proto on insert-race loss

In tc_new_tfilter() the create branch sets tp_created = 1 before calling
tcf_chain_tp_insert_unique(). When the caller loses the race (another
request inserted a proto at the same chain/prio first), insert_unique()
destroys the caller's own tp_new and returns the winner's proto with an
extra reference. tp_created was never cleared, so the loser's errout
path treated the winner's live proto as its own and called
tcf_chain_tp_delete_empty() on it, silently unlinking an active
classifier that the winning request already advertised via
RTM_NEWTFILTER.

Track the outcome of the insert step in a single tri-state variable so
each errout path reacts correctly:

- TP_NOT_CREATED: no proto created; pursue the old path.
- TP_CREATED: proto inserted successfully; same code path as before.
- TP_NOT_OWNED: New - lost the insert race; tp is another request's proto
  (chain ref already released by tp_new's destroy)

Both errout reactions are single expressions derived from the state.

This fix is motivated by the Sashiko's automated review of Patch
(net/sched: cls_api: Always acquire rtnl_lock when destroying locked
classifiers) [1][2]. The review identified the silent-unlink behaviour of
an adopted proto's teardown when a request loses the
tcf_chain_tp_insert_unique() race.

[1] https://sashiko.dev/#/patchset/20260801125632.360365-1-jhs%40mojatatu.com
[2] https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260801125632.360365-1-jhs%40mojatatu.com

## Which versions are affected by CVE-2026-90248?


## How do I fix CVE-2026-90248?

Apply the vendor's patch.

## Is CVE-2026-90248 being exploited?

There is no known exploitation of CVE-2026-90248 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90248 and fix it if it is affected.

What it is: CVE-2026-90248 is a vulnerability in Linux, published on 17 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-2026-90248 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-2026-90248/ (Markdown: https://insights.sbomify.com/CVE-2026-90248/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90248
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90248
- OSV: https://osv.dev/vulnerability/CVE-2026-90248

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90248/
Summaries are generated and may contain mistakes; the official description above is the record.