CVE-2026-89689 (GCVE-0-2026-89689)
Vulnerability from cvelistv5
Published
2026-09-11 19:46
Modified
2026-10-19 08:51
Severity ?
VLAI Severity ?
EPSS score ?
Summary
In the Linux kernel, the following vulnerability has been resolved:
nfsd: don't free session slots that are still in use
nfsd4_sequence() can free the very slot it is currently processing.
When the session shrinker has reduced se_target_maxslots below
se_fchannel.maxreqs, the shrink path checks three conditions before
calling free_session_slots():
1. se_target_maxslots < maxreqs (shrink was advertised)
2. slot->sl_generation == se_slot_gen (slot is up-to-date)
3. seq->maxslots <= se_target_maxslots (client acknowledges)
However, seq->slotid is never checked against se_target_maxslots.
A client using a slot in the range [se_target_maxslots, maxreqs) can
satisfy all three conditions: its slot has the current generation
(set by a prior SEQUENCE), and it sends sa_highest_slotid <=
se_target_maxslots to acknowledge the reduction.
free_session_slots() then kfrees every slot at index >=
se_target_maxslots, including the caller's own slot. The function
continues to write sl_seqid, sl_flags, sl_generation, and stores the
dangling pointer in cstate->slot. Later, nfsd4_store_cache_entry()
copies up to maxresp_cached bytes of the compound reply into the freed
sl_data[] array, corrupting whatever slab object now occupies that
address.
Additionally, a concurrent thread processing SEQUENCE on a different
high-numbered slot can have its slot freed out from under it.
NFSD4_SLOT_INUSE is set under nn->client_lock before the lock is
released, so any concurrent thread past SEQUENCE will have its slot
marked. However, free_session_slots() does not check NFSD4_SLOT_INUSE
before freeing.
Fix both problems by:
1. Checking that the current request's slotid is below the shrink
boundary.
2. Scanning slots in the to-be-freed range for NFSD4_SLOT_INUSE and
deferring the shrink if any are active.
References
Impacted products
{
"containers": {
"cna": {
"affected": [
{
"defaultStatus": "unaffected",
"product": "Linux",
"programFiles": [
"fs/nfsd/nfs4state.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"versions": [
{
"lessThan": "72d40b103bb03f0124d3a3c8f71a30cbafaf6afa",
"status": "affected",
"version": "fc8738c68d0bbf5033dd98b4f63d277ecbd49fd7",
"versionType": "git"
},
{
"lessThan": "b449b134e7762ddf98653a4b1d3e76d2d212ea64",
"status": "affected",
"version": "fc8738c68d0bbf5033dd98b4f63d277ecbd49fd7",
"versionType": "git"
},
{
"lessThan": "f5d22e372f4ac3eb287037a488c29691d52a6330",
"status": "affected",
"version": "fc8738c68d0bbf5033dd98b4f63d277ecbd49fd7",
"versionType": "git"
}
]
},
{
"defaultStatus": "affected",
"product": "Linux",
"programFiles": [
"fs/nfsd/nfs4state.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"versions": [
{
"status": "affected",
"version": "6.14"
},
{
"lessThan": "6.14",
"status": "unaffected",
"version": "0",
"versionType": "semver"
},
{
"lessThanOrEqual": "6.18.*",
"status": "unaffected",
"version": "6.18.50",
"versionType": "semver"
},
{
"lessThanOrEqual": "7.2.*",
"status": "unaffected",
"version": "7.2.4",
"versionType": "semver"
},
{
"lessThanOrEqual": "*",
"status": "unaffected",
"version": "7.3",
"versionType": "original_commit_for_fix"
}
]
}
],
"cpeApplicability": [
{
"nodes": [
{
"cpeMatch": [
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "6.18.50",
"versionStartIncluding": "6.14",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "7.2.4",
"versionStartIncluding": "6.14",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "7.3",
"versionStartIncluding": "6.14",
"vulnerable": true
}
],
"negate": false,
"operator": "OR"
}
]
}
],
"descriptions": [
{
"lang": "en",
"value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnfsd: don\u0027t free session slots that are still in use\n\nnfsd4_sequence() can free the very slot it is currently processing.\nWhen the session shrinker has reduced se_target_maxslots below\nse_fchannel.maxreqs, the shrink path checks three conditions before\ncalling free_session_slots():\n\n 1. se_target_maxslots \u003c maxreqs (shrink was advertised)\n 2. slot-\u003esl_generation == se_slot_gen (slot is up-to-date)\n 3. seq-\u003emaxslots \u003c= se_target_maxslots (client acknowledges)\n\nHowever, seq-\u003eslotid is never checked against se_target_maxslots.\nA client using a slot in the range [se_target_maxslots, maxreqs) can\nsatisfy all three conditions: its slot has the current generation\n(set by a prior SEQUENCE), and it sends sa_highest_slotid \u003c=\nse_target_maxslots to acknowledge the reduction.\n\nfree_session_slots() then kfrees every slot at index \u003e=\nse_target_maxslots, including the caller\u0027s own slot. The function\ncontinues to write sl_seqid, sl_flags, sl_generation, and stores the\ndangling pointer in cstate-\u003eslot. Later, nfsd4_store_cache_entry()\ncopies up to maxresp_cached bytes of the compound reply into the freed\nsl_data[] array, corrupting whatever slab object now occupies that\naddress.\n\nAdditionally, a concurrent thread processing SEQUENCE on a different\nhigh-numbered slot can have its slot freed out from under it.\nNFSD4_SLOT_INUSE is set under nn-\u003eclient_lock before the lock is\nreleased, so any concurrent thread past SEQUENCE will have its slot\nmarked. However, free_session_slots() does not check NFSD4_SLOT_INUSE\nbefore freeing.\n\nFix both problems by:\n 1. Checking that the current request\u0027s slotid is below the shrink\n boundary.\n 2. Scanning slots in the to-be-freed range for NFSD4_SLOT_INUSE and\n deferring the shrink if any are active."
}
],
"metrics": [
{
"cvssV3_1": {
"baseScore": 9.8,
"baseSeverity": "CRITICAL",
"vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
"version": "3.1"
},
"scenarios": [
{
"lang": "en",
"value": "AV:N - The bug is in nfsd4_sequence() in the in-kernel NFSv4.1 server (fs/nfsd/nfs4state.c). A remote client reaches it by sending COMPOUND RPCs with OP_SEQUENCE over TCP/2049 after EXCHANGE_ID/CREATE_SESSION; no local access to the NFS server is required.\nAC:L - After CREATE_SESSION with multiple slots, the attacker can grow the slot table and consume DRC memory so the nfsd-DRC-slot shrinker lowers se_target_maxslots, which SEQUENCE replies report. A SEQUENCE on a high slotid with sa_highest_slotid acknowledging the shrink then kfree\u0027s that in-use slot. Slotid, maxslots, cachethis, and concurrent COMPOUNDs are attacker-controlled.\nPR:N - Typical nfsd deployments accept AUTH_SYS/AUTH_NULL without cryptographic authentication. Any host allowed by the export ACL can EXCHANGE_ID/CREATE_SESSION and then SEQUENCE (ALLOWED_WITHOUT_FH, no filehandle or OPEN required); no local account or capability on the NFS server is needed.\nUI:N - The attacking NFS client sends the EXCHANGE_ID, CREATE_SESSION, and crafted SEQUENCE compounds itself. No victim user or administrator action on the NFS server, such as mounting an export or opening a file, is required.\nS:U - The use-after-free corrupts nfsd session-slot heap objects inside the NFS server host kernel. Impact stays in that kernel security authority and does not cross a VM, IOMMU, or sandbox boundary.\nC:H - free_session_slots() kfree\u0027s the nfsd4_slot still referenced by cstate-\u003eslot. Later nfsd4_store_cache_entry() and SEQUENCE processing read the freed object, including sl_cred. Slab reuse of the DRC slot (up to 2048-byte sl_data) enables arbitrary kernel memory disclosure per UAF guidance.\nI:H - After the kfree, nfsd4_sequence() writes sl_seqid, sl_flags, and sl_generation through the dangling pointer, and nfsd4_store_cache_entry() copies up to maxresp_cached bytes of the compound reply into freed sl_data[] and runs free_svc_cred() on reused memory, yielding a kernel heap write/arbitrary-free primitive.\nA:H - Use-after-free of the in-use session slot oopses or panics nfsd when the dangling cstate-\u003eslot is dereferenced or when concurrent SEQUENCE threads hit a freed slot. The attacker can repeat the SEQUENCE compounds to deny NFS service and take down the host."
}
]
}
],
"providerMetadata": {
"dateUpdated": "2026-10-19T08:51:02.559Z",
"orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"shortName": "Linux"
},
"references": [
{
"url": "https://git.kernel.org/stable/c/72d40b103bb03f0124d3a3c8f71a30cbafaf6afa"
},
{
"url": "https://git.kernel.org/stable/c/b449b134e7762ddf98653a4b1d3e76d2d212ea64"
},
{
"url": "https://git.kernel.org/stable/c/f5d22e372f4ac3eb287037a488c29691d52a6330"
}
],
"title": "nfsd: don\u0027t free session slots that are still in use",
"x_generator": {
"engine": "bippy-1.2.0"
}
}
},
"cveMetadata": {
"assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"assignerShortName": "Linux",
"cveId": "CVE-2026-89689",
"datePublished": "2026-09-11T19:46:09.976Z",
"dateReserved": "2026-09-11T19:38:34.749Z",
"dateUpdated": "2026-10-19T08:51:02.559Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
Loading…
Loading…
Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or seen somewhere by the user.
- Confirmed: The vulnerability is confirmed from an analyst perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: This vulnerability was exploited and seen by the user reporting the sighting.
- Patched: This vulnerability was successfully patched by the user reporting the sighting.
- Not exploited: This vulnerability was not exploited or seen by the user reporting the sighting.
- 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.
Loading…
Loading…