GHSA-M8HW-PQ6J-J4W4
Vulnerability from github – Published: 2026-09-11 21:31 – Updated: 2026-09-13 09:32In the Linux kernel, the following vulnerability has been resolved:
nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown
After a DESTROY_SESSION the per-session teardown path can free a session while rpciod still holds an inflight callback rpc_task that dereferences clp->cl_cb_session. nfsd4_probe_callback_sync() flushes cl_callback_wq, but once nfsd4_run_cb_work() has called rpc_call_async() the rpc_task lives on rpciod; flushing the workqueue does not wait for it. rpc_shutdown_client() does drain rpciod tasks, but uses a 1-second wait_event_timeout — tasks stuck in rpc_delay() (e.g. 2-second NFS4ERR_DELAY retries) can outlive the drain.
destroy path rpciod
------------ ------
unhash_session(ses)
nfsd4_probe_callback_sync(clp)
flush_workqueue(cl_callback_wq)
/* returns; rpc_task still live */
nfsd4_put_session_locked(ses)
free_session(ses) -> kfree(ses)
nfsd4_cb_sequence_done()
reads cb_clp->cl_cb_session
/* freed slab */
A second window exists in nfsd4_process_cb_update(). When __nfsd4_find_backchannel() returns NULL because unhash_session() has already removed the destroyed session from cl_sessions, setup_callback_client() takes the v4.1 early return so clp->cl_cb_session = ses never fires and the field retains a pointer to the about-to-be-freed session.
Fix both by converting cl_cb_session to an RCU-protected pointer:
-
Move the cl_cb_session = ses assignment in setup_callback_client() to after rpc_create() succeeds, so it is only published when a working backchannel exists. Clear cl_cb_session on the error return in nfsd4_process_cb_update(). Both stores use rcu_assign_pointer().
-
Annotate cl_cb_session with __rcu. All rpciod-side readers use rcu_read_lock()/rcu_dereference() and check for NULL, bailing to the appropriate error or requeue path: encode_cb_sequence4args(), decode_cb_sequence4resok(), nfsd41_cb_get_slot(), nfsd41_cb_release_slot(), nfsd4_cb_prepare(), and nfsd4_cb_sequence_done().
-
Switch __free_session() from kfree() to kfree_rcu() so the session slab is not reclaimed until after an RCU grace period, guaranteeing that rpciod readers inside rcu_read_lock() never dereference freed memory.
-
Pass the session pointer to the nfsd_cb_seq_status and nfsd_cb_free_slot tracepoints instead of having them re-read cl_cb_session.
-
nfsd4_cb_prepare() calls rpc_exit() when the session is NULL, routing through the done/release path to requeue the callback.
{
"affected": [],
"aliases": [
"CVE-2026-89708"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-09-11T20:19:58Z",
"severity": "CRITICAL"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nnfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown\n\nAfter a DESTROY_SESSION the per-session teardown path can free a\nsession while rpciod still holds an inflight callback rpc_task that\ndereferences clp-\u003ecl_cb_session. nfsd4_probe_callback_sync() flushes\ncl_callback_wq, but once nfsd4_run_cb_work() has called\nrpc_call_async() the rpc_task lives on rpciod; flushing the workqueue\ndoes not wait for it. rpc_shutdown_client() does drain rpciod tasks,\nbut uses a 1-second wait_event_timeout \u2014 tasks stuck in rpc_delay()\n(e.g. 2-second NFS4ERR_DELAY retries) can outlive the drain.\n\n destroy path rpciod\n ------------ ------\n unhash_session(ses)\n nfsd4_probe_callback_sync(clp)\n flush_workqueue(cl_callback_wq)\n /* returns; rpc_task still live */\n nfsd4_put_session_locked(ses)\n free_session(ses) -\u003e kfree(ses)\n nfsd4_cb_sequence_done()\n reads cb_clp-\u003ecl_cb_session\n /* freed slab */\n\nA second window exists in nfsd4_process_cb_update(). When\n__nfsd4_find_backchannel() returns NULL because unhash_session() has\nalready removed the destroyed session from cl_sessions,\nsetup_callback_client() takes the v4.1 early return so\nclp-\u003ecl_cb_session = ses never fires and the field retains a pointer\nto the about-to-be-freed session.\n\nFix both by converting cl_cb_session to an RCU-protected pointer:\n\n - Move the cl_cb_session = ses assignment in setup_callback_client()\n to after rpc_create() succeeds, so it is only published when a\n working backchannel exists. Clear cl_cb_session on the error\n return in nfsd4_process_cb_update(). Both stores use\n rcu_assign_pointer().\n\n - Annotate cl_cb_session with __rcu. All rpciod-side readers use\n rcu_read_lock()/rcu_dereference() and check for NULL, bailing to\n the appropriate error or requeue path:\n encode_cb_sequence4args(), decode_cb_sequence4resok(),\n nfsd41_cb_get_slot(), nfsd41_cb_release_slot(),\n nfsd4_cb_prepare(), and nfsd4_cb_sequence_done().\n\n - Switch __free_session() from kfree() to kfree_rcu() so the\n session slab is not reclaimed until after an RCU grace period,\n guaranteeing that rpciod readers inside rcu_read_lock() never\n dereference freed memory.\n\n - Pass the session pointer to the nfsd_cb_seq_status and\n nfsd_cb_free_slot tracepoints instead of having them re-read\n cl_cb_session.\n\n - nfsd4_cb_prepare() calls rpc_exit() when the session is NULL,\n routing through the done/release path to requeue the callback.",
"id": "GHSA-m8hw-pq6j-j4w4",
"modified": "2026-09-13T09:32:28Z",
"published": "2026-09-11T21:31:37Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-89708"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/01c5d5f58a5db9b0ee5afba2e49d3157788687b2"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/f164eb52b6f3cbf40f07fe379f9f421f88e02f76"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
"type": "CVSS_V3"
}
]
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
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- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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