GHSA-HQ8J-R79P-42P5

Vulnerability from github – Published: 2026-10-18 18:31 – Updated: 2026-10-18 18:31
VLAI
Details

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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{
  "affected": [],
  "aliases": [
    "CVE-2026-98536"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-10-18T16:17:05Z",
    "severity": null
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nwriteback: report a Tasks-RCU quiescent state per cgwb drain pass\n\ncleanup_offline_cgwbs_workfn() drains a dying cgwb by calling\ncleanup_offline_cgwb() until it returns false, with a cond_resched()\nbetween passes.  On a CONFIG_PREEMPTION kernel that cond_resched() does\nnothing: _cond_resched() is a plain \"return 0\", and under PREEMPT_DYNAMIC\nthe full and lazy modes disable it.  Since commit 7dadeaa6e851 (\"sched:\nFurther restrict the preemption modes\") those are the only two models on\nthe architectures with PREEMPT_LAZY support, arm64 and x86 among them, so\nthe drain loop never reports a Tasks-RCU quiescent state.\n\nA worker draining a cgwb with millions of attached inodes runs for\nminutes.  On a 6.18 arm64 host in lazy mode the cgwb worker drained one\ndying cgroup\u0027s writeback domain for over 11 minutes.  A BPF program unlink\n(bpf_trampoline_unlink_prog -\u003e bpf_trampoline_update -\u003e\nunregister_ftrace_direct -\u003e ftrace_shutdown -\u003e synchronize_rcu_tasks())\nwaited on that grace period while holding the trampoline mutex, 42 tasks\nqueued behind it in D state, and the hung task detector fired at 614 s and\npanicked the host.  Any BPF or ftrace detach during a long drain inherits\nthe drain\u0027s length.\n\nFix this by calling cond_resched_tasks_rcu_qs() so we do not stall out\nanybody who calls sycnrhonize_rcu_tasks().  We put this in a do { } while\nloop because if we have many small cgroups cleanup_offline_cgwb() will\nreturn false and we will never call cond_resched_tasks_rcu_qs(), creating\nthe same problem.",
  "id": "GHSA-hq8j-r79p-42p5",
  "modified": "2026-10-18T18:31:53Z",
  "published": "2026-10-18T18:31:53Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98536"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/260f586a80e7f624dd7f069898a75a904f34d373"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/407a5d205179a4ab186571b0e16ec42725dc77bc"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/42462b5bb0dec57db7f85d0f77655c737d45754d"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/4eb889649eaf65f4a5727b04ba1c017b7d05bb16"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/863049b4018237d1590cd13b16e9fe9c0faad2c6"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/ebf1555ad2821be06f87a88ffd9a2cab8d2200e1"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/fd3bf32279df98751364b3609eb100a3e42f253b"
    }
  ],
  "schema_version": "1.4.0",
  "severity": []
}



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