# CVE-2026-98536: writeback: report a Tasks-RCU quiescent state per cgwb drain pass

> A missing RCU quiescent state in the Linux kernel writeback drain loop can cause system hangs and panics under specific preemption configurations.

- **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:** 18 Oct 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-108755, BELL-CVE-2026-98536, DEBIAN-CVE-2026-98536, GHSA-hq8j-r79p-42p5, UBUNTU-CVE-2026-98536

## What is CVE-2026-98536?

CVE-2026-98536 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:

writeback: report a Tasks-RCU quiescent state per cgwb drain pass

cleanup_offline_cgwbs_workfn() drains a dying cgwb by calling
cleanup_offline_cgwb() until it returns false, with a cond_resched()
between passes.  On a CONFIG_PREEMPTION kernel that cond_resched() does
nothing: _cond_resched() is a plain "return 0", and under PREEMPT_DYNAMIC
the full and lazy modes disable it.  Since commit 7dadeaa6e851 ("sched:
Further restrict the preemption modes") those are the only two models on
the architectures with PREEMPT_LAZY support, arm64 and x86 among them, so
the drain loop never reports a Tasks-RCU quiescent state.

A worker draining a cgwb with millions of attached inodes runs for
minutes.  On a 6.18 arm64 host in lazy mode the cgwb worker drained one
dying cgroup's writeback domain for over 11 minutes.  A BPF program unlink
(bpf_trampoline_unlink_prog -> bpf_trampoline_update ->
unregister_ftrace_direct -> ftrace_shutdown -> synchronize_rcu_tasks())
waited on that grace period while holding the trampoline mutex, 42 tasks
queued behind it in D state, and the hung task detector fired at 614 s and
panicked the host.  Any BPF or ftrace detach during a long drain inherits
the drain's length.

Fix this by calling cond_resched_tasks_rcu_qs() so we do not stall out
anybody who calls sycnrhonize_rcu_tasks().  We put this in a do { } while
loop because if we have many small cgroups cleanup_offline_cgwb() will
return false and we will never call cond_resched_tasks_rcu_qs(), creating
the same problem.

## Which versions are affected by CVE-2026-98536?


## How do I fix CVE-2026-98536?

Apply the vendor's patch.

## Is CVE-2026-98536 being exploited?

There is no known exploitation of CVE-2026-98536 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98536 and fix it if it is affected.

What it is: CVE-2026-98536 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-2026-98536 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-2026-98536/ (Markdown: https://insights.sbomify.com/CVE-2026-98536/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98536
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98536
- OSV: https://osv.dev/vulnerability/CVE-2026-98536

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98536/
Summaries are generated and may contain mistakes; the official description above is the record.