ghsa-m59q-rgx4-6vq4
Vulnerability from github
In the Linux kernel, the following vulnerability has been resolved:
sched/fair: Prevent dead task groups from regaining cfs_rq's
Kevin is reporting crashes which point to a use-after-free of a cfs_rq in update_blocked_averages(). Initial debugging revealed that we've live cfs_rq's (on_list=1) in an about to be kfree()'d task group in free_fair_sched_group(). However, it was unclear how that can happen.
His kernel config happened to lead to a layout of struct sched_entity that put the 'my_q' member directly into the middle of the object which makes it incidentally overlap with SLUB's freelist pointer. That, in combination with SLAB_FREELIST_HARDENED's freelist pointer mangling, leads to a reliable access violation in form of a #GP which made the UAF fail fast.
Michal seems to have run into the same issue[1]. He already correctly diagnosed that commit a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle") is causing the preconditions for the UAF to happen by re-adding cfs_rq's also to task groups that have no more running tasks, i.e. also to dead ones. His analysis, however, misses the real root cause and it cannot be seen from the crash backtrace only, as the real offender is tg_unthrottle_up() getting called via sched_cfs_period_timer() via the timer interrupt at an inconvenient time.
When unregister_fair_sched_group() unlinks all cfs_rq's from the dying task group, it doesn't protect itself from getting interrupted. If the timer interrupt triggers while we iterate over all CPUs or after unregister_fair_sched_group() has finished but prior to unlinking the task group, sched_cfs_period_timer() will execute and walk the list of task groups, trying to unthrottle cfs_rq's, i.e. re-add them to the dying task group. These will later -- in free_fair_sched_group() -- be kfree()'ed while still being linked, leading to the fireworks Kevin and Michal are seeing.
To fix this race, ensure the dying task group gets unlinked first. However, simply switching the order of unregistering and unlinking the task group isn't sufficient, as concurrent RCU walkers might still see it, as can be seen below:
CPU1: CPU2:
: timer IRQ:
: do_sched_cfs_period_timer():
: :
: distribute_cfs_runtime():
: rcu_read_lock();
: :
: unthrottle_cfs_rq():
sched_offline_group(): :
: walk_tg_tree_from(…,tg_unthrottle_up,…):
list_del_rcu(&tg->list); :
(1) : list_for_each_entry_rcu(child, &parent->children, siblings)
: :
(2) list_del_rcu(&tg->siblings); :
: tg_unthrottle_up():
unregister_fair_sched_group(): struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
: :
list_del_leaf_cfs_rq(tg->cfs_rq[cpu]); :
: :
: if (!cfs_rq_is_decayed(cfs_rq) || cfs_rq->nr_running)
(3) : list_add_leaf_cfs_rq(cfs_rq);
: :
: :
: :
: :
:
---truncated---
{
"affected": [],
"aliases": [
"CVE-2021-47209"
],
"database_specific": {
"cwe_ids": [
"CWE-416"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2024-04-10T19:15:48Z",
"severity": "MODERATE"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nsched/fair: Prevent dead task groups from regaining cfs_rq\u0027s\n\nKevin is reporting crashes which point to a use-after-free of a cfs_rq\nin update_blocked_averages(). Initial debugging revealed that we\u0027ve\nlive cfs_rq\u0027s (on_list=1) in an about to be kfree()\u0027d task group in\nfree_fair_sched_group(). However, it was unclear how that can happen.\n\nHis kernel config happened to lead to a layout of struct sched_entity\nthat put the \u0027my_q\u0027 member directly into the middle of the object\nwhich makes it incidentally overlap with SLUB\u0027s freelist pointer.\nThat, in combination with SLAB_FREELIST_HARDENED\u0027s freelist pointer\nmangling, leads to a reliable access violation in form of a #GP which\nmade the UAF fail fast.\n\nMichal seems to have run into the same issue[1]. He already correctly\ndiagnosed that commit a7b359fc6a37 (\"sched/fair: Correctly insert\ncfs_rq\u0027s to list on unthrottle\") is causing the preconditions for the\nUAF to happen by re-adding cfs_rq\u0027s also to task groups that have no\nmore running tasks, i.e. also to dead ones. His analysis, however,\nmisses the real root cause and it cannot be seen from the crash\nbacktrace only, as the real offender is tg_unthrottle_up() getting\ncalled via sched_cfs_period_timer() via the timer interrupt at an\ninconvenient time.\n\nWhen unregister_fair_sched_group() unlinks all cfs_rq\u0027s from the dying\ntask group, it doesn\u0027t protect itself from getting interrupted. If the\ntimer interrupt triggers while we iterate over all CPUs or after\nunregister_fair_sched_group() has finished but prior to unlinking the\ntask group, sched_cfs_period_timer() will execute and walk the list of\ntask groups, trying to unthrottle cfs_rq\u0027s, i.e. re-add them to the\ndying task group. These will later -- in free_fair_sched_group() -- be\nkfree()\u0027ed while still being linked, leading to the fireworks Kevin\nand Michal are seeing.\n\nTo fix this race, ensure the dying task group gets unlinked first.\nHowever, simply switching the order of unregistering and unlinking the\ntask group isn\u0027t sufficient, as concurrent RCU walkers might still see\nit, as can be seen below:\n\n CPU1: CPU2:\n : timer IRQ:\n : do_sched_cfs_period_timer():\n : :\n : distribute_cfs_runtime():\n : rcu_read_lock();\n : :\n : unthrottle_cfs_rq():\n sched_offline_group(): :\n : walk_tg_tree_from(\u2026,tg_unthrottle_up,\u2026):\n list_del_rcu(\u0026tg-\u003elist); :\n (1) : list_for_each_entry_rcu(child, \u0026parent-\u003echildren, siblings)\n : :\n (2) list_del_rcu(\u0026tg-\u003esiblings); :\n : tg_unthrottle_up():\n unregister_fair_sched_group(): struct cfs_rq *cfs_rq = tg-\u003ecfs_rq[cpu_of(rq)];\n : :\n list_del_leaf_cfs_rq(tg-\u003ecfs_rq[cpu]); :\n : :\n : if (!cfs_rq_is_decayed(cfs_rq) || cfs_rq-\u003enr_running)\n (3) : list_add_leaf_cfs_rq(cfs_rq);\n : :\n : :\n : :\n : :\n : \n---truncated---",
"id": "GHSA-m59q-rgx4-6vq4",
"modified": "2025-03-27T21:31:08Z",
"published": "2024-04-10T21:30:32Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2021-47209"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/512e21c150c1c3ee298852660f3a796e267e62ec"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/b027789e5e50494c2325cc70c8642e7fd6059479"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
]
}
Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
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- Not confirmed: The user expresses doubt about the veracity of the vulnerability.
- Not patched: This vulnerability was not successfully patched by the user reporting the sighting.