# CVE-2026-98861: fprobe: Use guard(rcu_sched_notrace) and check rcu_is_watching()

> A use-after-free in the Linux kernel fprobe subsystem allows local attackers with high privileges to execute arbitrary code or escalate privileges.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **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-2026-98861, DEBIAN-CVE-2026-98861, GHSA-7564-v46w-3pmr, UBUNTU-CVE-2026-98861

## What is CVE-2026-98861?

CVE-2026-98861 is an use after free 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:

fprobe: Use guard(rcu_sched_notrace) and check rcu_is_watching()

unregister_fprobe() and unregister_fprobe_async() (used by BPF
kprobe-multi) rely on standard RCU grace periods (synchronize_rcu()
and call_rcu()) to wait until in-flight fprobe handlers complete before
freeing the fprobe.

However, if an fprobe handler executes while RCU is not watching (such
as in the idle loop or nohz_full extended quiescent states), standard
RCU does not track preemption-disabled sections. Consequently,
synchronize_rcu() does not wait for those executions, which can lead
to a use-after-free if the fprobe is freed immediately after
unregistration. Ensure handlers exit early when !rcu_is_watching().

Furthermore, fprobe_fgraph_entry() and fprobe_ftrace_entry() previously
used guard(rcu)() and rcu_read_lock(), which invoke lockdep on every
hit under CONFIG_PROVE_LOCKING. This adds overhead and can cause lockdep
recursion if probed functions interact with lockdep.

Since rhltable_lookup() and rhl_for_each_entry_rcu() use
rcu_dereference_all_check() (which checks rcu_read_lock_any_held()),
holding preemption disabled via rcu_read_lock_sched_notrace() is fully
valid and sufficient so long as rcu_is_watching() is true.

Define and use guard(rcu_sched_notrace)() across fprobe_ftrace_entry(),
fprobe_fgraph_entry(), and fprobe_return(). This eliminates fast-path
rcu_read_lock() and lockdep overhead while guaranteeing safe grace
period synchronization.

## Which versions are affected by CVE-2026-98861?


## How do I fix CVE-2026-98861?

Apply the vendor's patch.

## Is CVE-2026-98861 being exploited?

There is no known exploitation of CVE-2026-98861 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98861 and fix it if it is affected.

What it is: CVE-2026-98861 is an use after free 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-2026-98861 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-2026-98861/ (Markdown: https://insights.sbomify.com/CVE-2026-98861/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98861
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98861
- OSV: https://osv.dev/vulnerability/CVE-2026-98861

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98861/
Summaries are generated and may contain mistakes; the official description above is the record.