CVE-2026-98163
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.
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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
Detalles técnicos trazas, registros y código del informe original
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)CVSS
NVD no ha asignado puntuación CVSS a esta CVE (habitual desde el cambio de política de abril de 2026).
Probabilidad de explotación (EPSS)
- Probabilidad de explotación en los próximos 30 días: 0.18%
- Percentil entre todas las CVEs puntuadas: 7
- Fecha de la puntuación: 1/10/2026
EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).
Tecnologías afectadas (1)
⚠ Inferidas por IA a partir de la descripción — NVD aún no ha analizado esta CVE; no son CPE verificados.
Referencias
JSON original (NVD)
Mostrar
{
"id": "CVE-2026-98163",
"cveTags": [],
"metrics": {},
"affected": [
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"affectedData": [
{
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"product": "Linux",
"versions": [
{
"status": "affected",
"version": "260fbcb92bbeacfcd050410fdc2d24ab15044400",
"lessThan": "828938118d6c2bb711301748c3e39e4bed6a62f5",
"versionType": "git"
},
{
"status": "affected",
"version": "260fbcb92bbeacfcd050410fdc2d24ab15044400",
"lessThan": "057dac23d329d5c5ed62352f2659a39fd46c6d4a",
"versionType": "git"
}
],
"programFiles": [
"kernel/cgroup/cgroup.c"
],
"defaultStatus": "unaffected"
},
{
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"product": "Linux",
"versions": [
{
"status": "affected",
"version": "6.19"
},
{
"status": "unaffected",
"version": "0",
"lessThan": "6.19",
"versionType": "semver"
},
{
"status": "unaffected",
"version": "7.2.8",
"versionType": "semver",
"lessThanOrEqual": "7.2.*"
},
{
"status": "unaffected",
"version": "7.3-rc4",
"versionType": "original_commit_for_fix",
"lessThanOrEqual": "*"
}
],
"programFiles": [
"kernel/cgroup/cgroup.c"
],
"defaultStatus": "affected"
}
]
}
],
"published": "2026-09-26T09:16:38.303",
"references": [
{
"url": "https://git.kernel.org/stable/c/057dac23d329d5c5ed62352f2659a39fd46c6d4a",
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
},
{
"url": "https://git.kernel.org/stable/c/828938118d6c2bb711301748c3e39e4bed6a62f5",
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
}
],
"vulnStatus": "Awaiting Analysis",
"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 ->usage count > 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 ->task_pos.\nThe iterator should not attempt to resurrect tasks whose ->usage 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->live count of a task\non the dying list because that won't distinguish regular zombies waiting\nto be reaped from RCU remnant tasks that are going to be free'd.\nTherefore add an extra check to rule out ->usage==0 tasks from any\niteration.\n\nThe repeat: loop in css_task_iter_advance() doesn't consider ->usage\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->live > 0\n cgroup_task_dead(L)\n css_set_skip_task_iters() // skips only cset->tasks\n list_add_tail(&L->cg_list, &cset->dying_tasks)\n css_task_iter_next()\n take css_set_lock\n css_task_iter_advance()\n leader && signal->live != 0\n => it->task_pos = &L->cg_list\n release css_set_lock\n T exits\n --signal->live == 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->usage = 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->task_pos = &L->cg_list\n get_task_struct(L)\n => 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)"
}
],
"lastModified": "2026-09-30T14:10:59.253",
"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
}