GHSA-X7Q8-XCW2-7MFG
Vulnerability from github – Published: 2026-04-24 15:32 – Updated: 2026-06-01 18:31In the Linux kernel, the following vulnerability has been resolved:
mm: blk-cgroup: fix use-after-free in cgwb_release_workfn()
cgwb_release_workfn() calls css_put(wb->blkcg_css) and then later accesses wb->blkcg_css again via blkcg_unpin_online(). If css_put() drops the last reference, the blkcg can be freed asynchronously (css_free_rwork_fn -> blkcg_css_free -> kfree) before blkcg_unpin_online() dereferences the pointer to access blkcg->online_pin, resulting in a use-after-free:
BUG: KASAN: slab-use-after-free in blkcg_unpin_online (./include/linux/instrumented.h:112 ./include/linux/atomic/atomic-instrumented.h:400 ./include/linux/refcount.h:389 ./include/linux/refcount.h:432 ./include/linux/refcount.h:450 block/blk-cgroup.c:1367) Write of size 4 at addr ff11000117aa6160 by task kworker/71:1/531 Workqueue: cgwb_release cgwb_release_workfn Call Trace: blkcg_unpin_online (./include/linux/instrumented.h:112 ./include/linux/atomic/atomic-instrumented.h:400 ./include/linux/refcount.h:389 ./include/linux/refcount.h:432 ./include/linux/refcount.h:450 block/blk-cgroup.c:1367) cgwb_release_workfn (mm/backing-dev.c:629) process_scheduled_works (kernel/workqueue.c:3278 kernel/workqueue.c:3385)
Freed by task 1016: kfree (./include/linux/kasan.h:235 mm/slub.c:2689 mm/slub.c:6246 mm/slub.c:6561) css_free_rwork_fn (kernel/cgroup/cgroup.c:5542) process_scheduled_works (kernel/workqueue.c:3302 kernel/workqueue.c:3385)
** Stack based on commit 66672af7a095 ("Add linux-next specific files for 20260410")
I am seeing this crash sporadically in Meta fleet across multiple kernel versions. A full reproducer is available at: https://github.com/leitao/debug/blob/main/reproducers/repro_blkcg_uaf.sh
(The race window is narrow. To make it easily reproducible, inject a msleep(100) between css_put() and blkcg_unpin_online() in cgwb_release_workfn(). With that delay and a KASAN-enabled kernel, the reproducer triggers the splat reliably in less than a second.)
Fix this by moving blkcg_unpin_online() before css_put(), so the cgwb's CSS reference keeps the blkcg alive while blkcg_unpin_online() accesses it.
{
"affected": [],
"aliases": [
"CVE-2026-31586"
],
"database_specific": {
"cwe_ids": [
"CWE-416"
],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-04-24T15:16:33Z",
"severity": "HIGH"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nmm: blk-cgroup: fix use-after-free in cgwb_release_workfn()\n\ncgwb_release_workfn() calls css_put(wb-\u003eblkcg_css) and then later accesses\nwb-\u003eblkcg_css again via blkcg_unpin_online(). If css_put() drops the last\nreference, the blkcg can be freed asynchronously (css_free_rwork_fn -\u003e\nblkcg_css_free -\u003e kfree) before blkcg_unpin_online() dereferences the\npointer to access blkcg-\u003eonline_pin, resulting in a use-after-free:\n\n BUG: KASAN: slab-use-after-free in blkcg_unpin_online (./include/linux/instrumented.h:112 ./include/linux/atomic/atomic-instrumented.h:400 ./include/linux/refcount.h:389 ./include/linux/refcount.h:432 ./include/linux/refcount.h:450 block/blk-cgroup.c:1367)\n Write of size 4 at addr ff11000117aa6160 by task kworker/71:1/531\n Workqueue: cgwb_release cgwb_release_workfn\n Call Trace:\n \u003cTASK\u003e\n blkcg_unpin_online (./include/linux/instrumented.h:112 ./include/linux/atomic/atomic-instrumented.h:400 ./include/linux/refcount.h:389 ./include/linux/refcount.h:432 ./include/linux/refcount.h:450 block/blk-cgroup.c:1367)\n cgwb_release_workfn (mm/backing-dev.c:629)\n process_scheduled_works (kernel/workqueue.c:3278 kernel/workqueue.c:3385)\n\n Freed by task 1016:\n kfree (./include/linux/kasan.h:235 mm/slub.c:2689 mm/slub.c:6246 mm/slub.c:6561)\n css_free_rwork_fn (kernel/cgroup/cgroup.c:5542)\n process_scheduled_works (kernel/workqueue.c:3302 kernel/workqueue.c:3385)\n\n** Stack based on commit 66672af7a095 (\"Add linux-next specific files\nfor 20260410\")\n\nI am seeing this crash sporadically in Meta fleet across multiple kernel\nversions. A full reproducer is available at:\nhttps://github.com/leitao/debug/blob/main/reproducers/repro_blkcg_uaf.sh\n\n(The race window is narrow. To make it easily reproducible, inject a\nmsleep(100) between css_put() and blkcg_unpin_online() in\ncgwb_release_workfn(). With that delay and a KASAN-enabled kernel, the\nreproducer triggers the splat reliably in less than a second.)\n\nFix this by moving blkcg_unpin_online() before css_put(), so the\ncgwb\u0027s CSS reference keeps the blkcg alive while blkcg_unpin_online()\naccesses it.",
"id": "GHSA-x7q8-xcw2-7mfg",
"modified": "2026-06-01T18:31:26Z",
"published": "2026-04-24T15:32:34Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-31586"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/115a5266749dcde7fe4127e8623d19c752088f69"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/1bd36e93b542d9dd020190c6607c6a3663405195"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/23acef4156c260e8598397a1a2e8b3a23e919893"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/50879a3c1faf06e661090015d59e2127255cff27"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/67cb119d32f35e32acd0393bbeb318b2bb1fdafe"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/740ba1ebb223f137ff088ab74d533a13f9167bd8"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/8f5857be99f1ed1fa80991c72449541f634626ee"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/dfc8292a1d6782c76b626315605e0585a5a18447"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/ea3af09eb87d8f8708c66747fcf1a2762902e839"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
"type": "CVSS_V3"
}
]
}
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