{ "dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": { "cveId": "CVE-2026-89708", "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "state": "PUBLISHED", "assignerShortName": "Linux", "dateReserved": "2026-09-11T19:38:34.751Z", "datePublished": "2026-09-11T19:46:24.061Z", "dateUpdated": "2026-10-19T08:51:21.242Z" }, "containers": { "cna": { "providerMetadata": { "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "shortName": "Linux", "dateUpdated": "2026-10-19T08:51:21.242Z" }, "descriptions": [ { "lang": "en", "value": "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." } ], "metrics": [ { "cvssV3_1": { "version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H", "baseScore": 9.8, "baseSeverity": "CRITICAL" }, "scenarios": [ { "lang": "en", "value": "AV:N - The UAF is in nfsd NFSv4.1 session teardown (nfsd4_destroy_session) and the rpciod callback path (nfsd4_cb_sequence_done). DESTROY_SESSION and the inflight CB_COMPOUND run over TCP/2049, so a remote NFS client reaches the bug with no local access.\nAC:L - The attacker controls both sides: they keep a callback rpc_task on rpciod (e.g. NFS4ERR_DELAY so rpc_delay is 2s) and send DESTROY_SESSION. flush_workqueue does not wait for rpciod, and rpc_shutdown_client's 1s drain can expire first, so the window is attacker-driven and retryable.\nPR:N - Typical nfsd exports use AUTH_SYS/AUTH_NULL, so RPC identities are self-asserted. EXCHANGE_ID, CREATE_SESSION with a backchannel, OPEN, and DESTROY_SESSION succeed for any host allowed by the export; nfsd4_mach_creds_match is a no-op under default SP4_NONE. No local account is required.\nUI:N - The attacking NFS client issues CREATE_SESSION, OPEN/CB_RECALL traffic, NFS4ERR_DELAY replies, and DESTROY_SESSION itself. No victim on the server must mount a filesystem, open a file, or take any other action.\nS:U - The use-after-free corrupts a kmalloc'd nfsd4_session in the host nfsd kernel. Impact stays in that kernel security authority and does not cross a VM, IOMMU, or sandbox boundary.\nC:H - After free_session kfree's the session, rpciod still reads se_sessionid and se_cb_seq_nr in encode_cb_sequence4args and decode_cb_sequence4resok. Reclaim of that slab lets the attacker disclose kernel memory via the callback they receive. Per kernel guidance a UAF is C:H.\nI:H - The same dangling cl_cb_session is written by update_cb_slot_table (attacker-controlled target_highest_slotid), grab_slot/nfsd41_cb_release_slot (se_cb_slot_avail and se_lock), and nfsd4_cb_sequence_done (se_cb_seq_nr++). That is a UAF write primitive for heap corruption and control-flow hijack.\nA:H - Use-after-free of struct nfsd4_session from an rpciod callback oopses or panics the NFS server even without full exploitation, and the attacker can repeat DESTROY_SESSION against inflight callbacks to deny service." } ] } ], "affected": [ { "product": "Linux", "vendor": "Linux", "defaultStatus": "unaffected", "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git", "programFiles": [ "fs/nfsd/nfs4callback.c", "fs/nfsd/nfs4state.c", "fs/nfsd/state.h", "fs/nfsd/trace.h" ], "versions": [ { "version": "dcbeaa68dbbdacbbb330a86c7fc95a28473fc209", "lessThan": "2775ec1d617e665eab71ce0f13ab1f7959713a2b", "status": "affected", "versionType": "git" }, { "version": "dcbeaa68dbbdacbbb330a86c7fc95a28473fc209", "lessThan": "f164eb52b6f3cbf40f07fe379f9f421f88e02f76", "status": "affected", "versionType": "git" }, { "version": "dcbeaa68dbbdacbbb330a86c7fc95a28473fc209", "lessThan": "13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25", "status": "affected", "versionType": "git" }, { "version": "dcbeaa68dbbdacbbb330a86c7fc95a28473fc209", "lessThan": "01c5d5f58a5db9b0ee5afba2e49d3157788687b2", "status": "affected", "versionType": "git" } ] }, { "product": "Linux", "vendor": "Linux", "defaultStatus": "affected", "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git", "programFiles": [ "fs/nfsd/nfs4callback.c", "fs/nfsd/nfs4state.c", "fs/nfsd/state.h", "fs/nfsd/trace.h" ], "versions": [ { "version": "2.6.38", "status": "affected" }, { "version": "0", "lessThan": "2.6.38", "status": "unaffected", "versionType": "semver" }, { "version": "6.12.111", "lessThanOrEqual": "6.12.*", "status": "unaffected", "versionType": "semver" }, { "version": "6.18.50", "lessThanOrEqual": "6.18.*", "status": "unaffected", "versionType": "semver" }, { "version": "7.2.4", "lessThanOrEqual": "7.2.*", "status": "unaffected", "versionType": "semver" }, { "version": "7.3", "lessThanOrEqual": "*", "status": "unaffected", "versionType": "original_commit_for_fix" } ] } ], "cpeApplicability": [ { "nodes": [ { "operator": "OR", "negate": false, "cpeMatch": [ { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "2.6.38", "versionEndExcluding": "6.12.111" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "2.6.38", "versionEndExcluding": "6.18.50" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "2.6.38", "versionEndExcluding": "7.2.4" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "2.6.38", "versionEndExcluding": "7.3" } ] } ] } ], "references": [ { "url": "https://git.kernel.org/stable/c/2775ec1d617e665eab71ce0f13ab1f7959713a2b" }, { "url": "https://git.kernel.org/stable/c/f164eb52b6f3cbf40f07fe379f9f421f88e02f76" }, { "url": "https://git.kernel.org/stable/c/13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25" }, { "url": "https://git.kernel.org/stable/c/01c5d5f58a5db9b0ee5afba2e49d3157788687b2" } ], "title": "nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown", "x_generator": { "engine": "bippy-1.2.0" } }, "adp": [ { "metrics": [ { "other": { "type": "ssvc", "content": { "id": "CVE-2026-89708", "role": "CISA Coordinator", "options": [ { "Exploitation": "none" }, { "Automatable": "yes" }, { "Technical Impact": "total" } ], "version": "2.0.3", "timestamp": "2026-10-16T19:26:20.755207Z" } } } ], "title": "CISA ADP Vulnrichment", "providerMetadata": { "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "shortName": "CISA-ADP", "dateUpdated": "2026-10-16T19:26:49.405Z" } } ] } }