CVE-2026-98163 (GCVE-0-2026-98163)
Vulnerability from cvelistv5
Published
2026-09-26 08:31
Modified
2026-10-19 09:14
Severity ?
7.8 (High) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: cgroup: Avoid iteration of dying tasks with zero refcount The commit 260fbcb92bbea ("cgroup: Move dying_tasks cleanup from cgroup_task_release() to cgroup_task_free()") extended the lifetime of tasks on the dying_tasks list. The iterators have provision to go through dying_tasks because of dying threadgroup leaders or explicit CSS_TASK_ITER_WITH_DEAD, however, it was expected that such tasks can obtain a new reference (that is possible before cgroup_task_release()/put_task_struct_rcu_user()). The tasks after cgroup_task_release() and before cgroup_task_free() are subject to race when they may or may not have ->usage count > 0. The race window is between css_task_iter_next() invocations when css_set_lock is released and we may arrive at a new ->task_pos. The iterator should not attempt to resurrect tasks whose ->usage count dropped to zero. (When that happens, __put_task_struct_rcu_cb() is already imminent and the returned task_struct would could be used after free.) As for the fix, we cannot simply check the signal->live count of a task on the dying list because that won't distinguish regular zombies waiting to be reaped from RCU remnant tasks that are going to be free'd. Therefore add an extra check to rule out ->usage==0 tasks from any iteration. The repeat: loop in css_task_iter_advance() doesn't consider ->usage count, so add a new loop to css_task_iter_next() to skip de-used tasks on the dying_list. Rough illustration of the possible race R (reader of cgroup.procs) T (thread) L (group leader) --------------------------------- -------------------------------- -------------------------------- L exits, signal->live > 0 cgroup_task_dead(L) css_set_skip_task_iters() // skips only cset->tasks list_add_tail(&L->cg_list, &cset->dying_tasks) css_task_iter_next() take css_set_lock css_task_iter_advance() leader && signal->live != 0 => it->task_pos = &L->cg_list release css_set_lock T exits --signal->live == 0 cgroup_task_dead(T) // css_set_lock release_task(T) cgroup_task_release(T) release_task(L) // zap_leader cgroup_task_release(L) put_task_struct_rcu_user(L) ...RCU... put_task_struct(L) L->usage = 0 /* L still on dying_tasks */ ...RCU... __put_task_struct(L) css_task_iter_next() // another iteration take css_set_lock it->task_pos = &L->cg_list get_task_struct(L) => addition on 0 drop css_set_lock cgroup_task_free(L) css_set_skip_task_iters() // dying skip comes too late free_task(L) cgroup_procs_show() task_pid_vnr(L)
References
Impacted products
Vendor Product Version
Linux Linux Version: 260fbcb92bbeacfcd050410fdc2d24ab15044400
Version: 260fbcb92bbeacfcd050410fdc2d24ab15044400
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{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "kernel/cgroup/cgroup.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "828938118d6c2bb711301748c3e39e4bed6a62f5",
              "status": "affected",
              "version": "260fbcb92bbeacfcd050410fdc2d24ab15044400",
              "versionType": "git"
            },
            {
              "lessThan": "057dac23d329d5c5ed62352f2659a39fd46c6d4a",
              "status": "affected",
              "version": "260fbcb92bbeacfcd050410fdc2d24ab15044400",
              "versionType": "git"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "kernel/cgroup/cgroup.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "6.19"
            },
            {
              "lessThan": "6.19",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.2.*",
              "status": "unaffected",
              "version": "7.2.8",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.3",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.2.8",
                  "versionStartIncluding": "6.19",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.3",
                  "versionStartIncluding": "6.19",
                  "vulnerable": true
                }
              ],
              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\ncgroup: Avoid iteration of dying tasks with zero refcount\n\nThe commit 260fbcb92bbea (\"cgroup: Move dying_tasks cleanup from\ncgroup_task_release() to cgroup_task_free()\") extended the lifetime of\ntasks on the dying_tasks list.\nThe iterators have provision to go through dying_tasks because of\ndying threadgroup leaders or explicit CSS_TASK_ITER_WITH_DEAD, however,\nit was expected that such tasks can obtain a new reference (that is\npossible before cgroup_task_release()/put_task_struct_rcu_user()).\nThe tasks after cgroup_task_release() and before cgroup_task_free()\nare subject to race when they may or may not have -\u003eusage count \u003e 0.\n\nThe race window is between css_task_iter_next() invocations\nwhen css_set_lock is released and we may arrive at a new -\u003etask_pos.\nThe iterator should not attempt to resurrect tasks whose -\u003eusage count\ndropped to zero. (When that happens, __put_task_struct_rcu_cb() is\nalready imminent and the returned task_struct would could be used\nafter free.)\n\nAs for the fix, we cannot simply check the signal-\u003elive count of a task\non the dying list because that won\u0027t distinguish regular zombies waiting\nto be reaped from RCU remnant tasks that are going to be free\u0027d.\nTherefore add an extra check to rule out -\u003eusage==0 tasks from any\niteration.\n\nThe repeat: loop in css_task_iter_advance() doesn\u0027t consider -\u003eusage\ncount, so add a new loop to css_task_iter_next() to skip de-used tasks\non the dying_list.\n\nRough illustration of the possible race\n\n  R (reader of cgroup.procs)         T (thread)                       L (group leader)\n  ---------------------------------  -------------------------------- --------------------------------\n                                                                      L exits, signal-\u003elive \u003e 0\n                                                                      cgroup_task_dead(L)\n                                                                        css_set_skip_task_iters() // skips only cset-\u003etasks\n                                                                        list_add_tail(\u0026L-\u003ecg_list, \u0026cset-\u003edying_tasks)\n  css_task_iter_next()\n    take css_set_lock\n    css_task_iter_advance()\n      leader \u0026\u0026 signal-\u003elive != 0\n      =\u003e it-\u003etask_pos = \u0026L-\u003ecg_list\n    release css_set_lock\n                                     T exits\n                                     --signal-\u003elive == 0\n\t\t\t\t     cgroup_task_dead(T) // css_set_lock\n                                     release_task(T)\n                                       cgroup_task_release(T)\n                                       release_task(L) // zap_leader\n                                         cgroup_task_release(L)\n                                         put_task_struct_rcu_user(L)\n                                         ...RCU...\n                                         put_task_struct(L)\n                                           L-\u003eusage = 0\n                                           /* L still on dying_tasks */\n                                           ...RCU...\n                                           __put_task_struct(L)\n  css_task_iter_next() // another iteration\n    take css_set_lock\n    it-\u003etask_pos = \u0026L-\u003ecg_list\n    get_task_struct(L)\n      =\u003e addition on 0\n    drop css_set_lock\n                                           cgroup_task_free(L)\n                                             css_set_skip_task_iters() // dying skip comes too late\n                                           free_task(L)\n  cgroup_procs_show()\n    task_pid_vnr(L)"
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 7.8,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - The bug is in css_task_iter_next() (kernel/cgroup/cgroup.c), reached by a local read() of a cgroup.procs/cgroup.threads seq file via cgroup_procs_start/next/show. No remote peer supplies any input to this path.\nAC:L - The attacker controls both sides of the race: one thread repeatedly reads its own cgroup.procs while the attacker\u0027s own multithreaded process exits its group leader and then its last thread, dropping the leader\u0027s -\u003eusage to zero while it stays on dying_tasks. The attacker can retry until the reader lands on that task.\nPR:L - cgroup_file_mode() makes cgroup.procs S_IRUGO, so any unprivileged local user can read it. Creating and exiting threads in their own cgroup needs no capability, so a basic local account is enough.\nUI:N - The attacker triggers the race entirely with their own processes and their own reads of cgroup.procs. No other user or administrator has to do anything.\nS:U - The use-after-free corrupts a kernel task_struct within the same kernel security authority. It does not cross a VM, IOMMU or similar boundary.\nC:H - get_task_struct() on a zero-refcount task makes the iterator keep a task_struct that free_task() releases. cgroup_procs_show() then dereferences it via task_pid_vnr(), so a reclaimed object\u0027s contents are read and reported back to userspace.\nI:H - The iterator later calls put_task_struct() on the freed task_struct, decrementing -\u003eusage in memory that may have been reallocated. This use-after-free write on reclaimed slab memory can be groomed into a corruption primitive.\nA:H - The refcount_t \"addition on 0\" warning, followed by dereferencing and releasing a freed task_struct in css_task_iter_next()/css_task_iter_end(), causes an oops, or a panic with panic_on_warn/KASAN, and an unprivileged user can repeat it."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-10-19T09:14:56.823Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/828938118d6c2bb711301748c3e39e4bed6a62f5"
        },
        {
          "url": "https://git.kernel.org/stable/c/057dac23d329d5c5ed62352f2659a39fd46c6d4a"
        }
      ],
      "title": "cgroup: Avoid iteration of dying tasks with zero refcount",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-98163",
    "datePublished": "2026-09-26T08:31:50.899Z",
    "dateReserved": "2026-09-25T10:25:14.321Z",
    "dateUpdated": "2026-10-19T09:14:56.823Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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