---
id: "CVE-2026-98536"
kind: "vulnerability"
aliases: ["CVE-2026-98536"]
title: "Linux kernel: writeback drain loop can stall Tasks-RCU grace periods"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "other"
products: []
ecosystems: []
tags: ["bpf", "linux-kernel", "other", "self-hosted"]
not_affected_if: ["default-config"]
published_at: "2026-10-18T16:17:05+00:00"
first_seen_at: "2026-10-23T21:57:15+00:00"
updated_at: "2026-10-25T05:23:47+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98536", "first_seen_at": "2026-10-23T21:57:15+00:00"}]
---

# Linux kernel: writeback drain loop can stall Tasks-RCU grace periods

The writeback cleanup worker for dying cgroups can run for minutes without reporting a Tasks-RCU quiescent state, so callers waiting on that grace period (such as BPF or ftrace detach) can hang. A host can be hung-task panicked as a result, which is a denial of service rather than code execution.

_Summary, check and fix written by a model from the sources; labelled as such._

## Timeline

- 2026-10-23T21:57:15+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-98536

## From the sources

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.

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