CVE-2026-89708: Software Security Vulnerability Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-m8hw-pq6j-j4w4",
  "modified" : "2026-09-21T15:31:37Z",
  "published" : "2026-09-11T21:31:37Z",
  "aliases" : [ "CVE-2026-89708" ],
  "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->cl_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 — 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) -> kfree(ses)\n                                       nfsd4_cb_sequence_done()\n                                         reads cb_clp->cl_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->cl_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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H"
  } ],
  "affected" : [ ],
  "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/2775ec1d617e665eab71ce0f13ab1f7959713a2b"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/f164eb52b6f3cbf40f07fe379f9f421f88e02f76"
  } ],
  "database_specific" : {
    "cwe_ids" : [ ],
    "severity" : "CRITICAL",
    "github_reviewed" : false,
    "github_reviewed_at" : null,
    "nvd_published_at" : "2026-09-11T20:19:58Z"
  }
}