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

{
  "id": "e392803c7b2908e0167e84f07054a0926d547e7fd69010c7ba4b2f9629cc2d4a",
  "url": "https://osv.dev/vulnerability/CVE-2026-98536",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "writeback: report a Tasks-RCU quiescent state per cgwb drain pass",
    "summary": "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 w",
    "evidence": "osv:CVE-2026-98536",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "5.15.222",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.1.189",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.6.158",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.12.112",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.18.55",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "7.2.9",
            "evidence": "/affected/1/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "unknown",
    "structured": {
      "id": "CVE-2026-98536",
      "aliases": [
        "CVE-2026-98536"
      ],
      "affected": [
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          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-98536.json"
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              "type": "ECOSYSTEM",
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                  "introduced": "5.14.0"
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          "package": {
            "name": "Kernel",
            "ecosystem": "Linux"
          },
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-98536.json"
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      ],
      "modified": "2026-10-28T02:30:44.626777845Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/260f586a80e7f624dd7f069898a75a904f34d373",
        "https://git.kernel.org/stable/c/407a5d205179a4ab186571b0e16ec42725dc77bc",
        "https://git.kernel.org/stable/c/42462b5bb0dec57db7f85d0f77655c737d45754d",
        "https://git.kernel.org/stable/c/4eb889649eaf65f4a5727b04ba1c017b7d05bb16",
        "https://git.kernel.org/stable/c/863049b4018237d1590cd13b16e9fe9c0faad2c6",
        "https://git.kernel.org/stable/c/ebf1555ad2821be06f87a88ffd9a2cab8d2200e1",
        "https://git.kernel.org/stable/c/fd3bf32279df98751364b3609eb100a3e42f253b",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98536.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-98536",
        "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"
      ],
      "provider_metadata": {
        "cna_assigner": "Linux",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98536.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-98536",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": null
  },
  "source_id": "core-osv",
  "document_id": 512538,
  "published_at": "2026-10-18T15:18:04.013+00:00"
}