CVE-2026-98536: writeback: report a Tasks-RCU quiescent state per cgwb drain pass
In the Linux kernel, the following vulnerability has been resolved:
writeback: report a Tasks-RCU quiescent state per cgwb drain pass
cleanupofflinecgwbsworkfn() drains a dying cgwb by calling cleanupofflinecgwb() until it returns false, with a condresched() between passes. On a CONFIGPREEMPTION kernel that condresched() does nothing: condresched() is a plain "return 0", and under PREEMPTDYNAMIC 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 PREEMPTLAZY 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 (bpftrampolineunlinkprog -> bpftrampolineupdate -> unregisterftracedirect -> ftraceshutdown -> synchronizercutasks()) 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 condreschedtasksrcuqs() so we do not stall out anybody who calls sycnrhonizercutasks(). We put this in a do { } while loop because if we have many small cgroups cleanupofflinecgwb() will return false and we will never call condreschedtasksrcuqs(), creating the same problem.
Affected Software
Remediation
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Call cond_resched_tasks_rcu_qs() during each cleanup_offline_cgwb() drain pass, using a do-while loop so the Tasks-RCU quiescent state is reported even when many small cgroups cause cleanup_offline_cgwb() to return false.
Event History
Frequently Asked Questions
Which systems are most exposed to this issue?
Systems using CONFIG_PREEMPTION are affected when cond_resched() is ineffective. The description specifically identifies PREEMPT_DYNAMIC full and lazy modes, including arm64 and x86 architectures with PREEMPT_LAZY support.
What workload is needed to trigger a severe impact?
A dying cgroup writeback domain must have a very large number of attached inodes, potentially millions, causing its cleanup worker to run for minutes. A BPF program unlink can then block in a Tasks-RCU grace period while the worker fails to report a quiescent state.
What operational symptoms could indicate exploitation or exposure?
BPF program unlink activity may stall while holding the trampoline mutex, with other tasks queueing behind it in D state. The hung-task detector can fire and panic the host after a prolonged stall; the reported example observed a drain lasting more than 11 minutes and a hung-task event at 614 seconds.
Is this limited to a particular architecture or preemption mode?
No. arm64 lazy mode is the documented example, and x86 is also named among architectures with PREEMPT_LAZY support. The underlying condition is a CONFIG_PREEMPTION configuration where cond_resched() does not provide the needed Tasks-RCU quiescent state, notably PREEMPT_DYNAMIC full and lazy modes.