* [CVE-2026-89686][MODERATE 7.0] nfsd: fix BUG_ON in nfsd4_alloc_layout_stateid on racing delegation revoke
@ 2026-09-12 6:36 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-12 6:36 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89686
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: fix BUG_ON in nfsd4_alloc_layout_stateid on racing delegation revoke
Commit: c517f27498757e616d2a8fe6d16caad1fee422e6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c517f27498757e616d2a8fe6d16caad1fee422e6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89686
Analysis date: Sat, 12 Sep 2026 02:36:47 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89686 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89686
The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.
This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.
The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.
Published priority for this report: MODERATE 7.0
Manual review required: YES
A detailed explanation of the methodology and priority rules is included
at the end of this message.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-89686 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89686 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-362;*CWE-416;CWE-617;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'A race in nfsd4_alloc_layout_stateid can occur when the SC_TYPE_DELEG path reads fp fi_deleg_file without holding fi_lock while the laundromat may revoke the delegation and clear the same pointer under fi_lock. A remote NFS client reachable to the export can hit this window during delegation recall and pNFS LAYOUTGET processing, causing nfsd_file_get to return NULL and triggering a BUG_ON that crashes the kernel. For the CVSS the PR:L value is used because the attacker normally needs access to the NFS export and valid NFS operations, but does not need local code execution on the server. The issue is network reachable through NFS server request handling, although reliable triggering requires a race involving delegation recall, revoke, and a concurrent LAYOUTGET. Impact is at least remote denial of service via kernel crash and in the paranoid case may include confidentiality or integrity impact because the unlocked pointer read is a plausible stale pointer or UAF candidate.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6) YES LOCK DANGER DISK SIMPLEFIX RACE NETWORK KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 5
* Applied conservative floor/cap: FLOOR 5 applied because this is a below-host-admin network-reachable active-object lifetime bug where the stale path can perform a lifetime/refcount operation through `nfsd_file_get()`.
* Raw paranoid score: 6
* Paranoid score: 6
* Applied paranoid floor/cap: none
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Conservative signals:
* +2 Remote/client-reachable trigger: supplied context states that two NFS clients can trigger the race through delegation recall, laundromat revoke, and concurrent `LAYOUTGET`.
* +2 Real lifetime/UAF primitive: `fp->fi_deleg_file` is read without `fi_lock` while another path can clear it under `fi_lock`. The stale path passes that pointer to `nfsd_file_get()`, which is a lifetime/refcount operation and requires a valid object.
* +1 Reliable kernel crash / strong DoS: the observed failure is `nfsd_file_get()` returning NULL followed by `BUG_ON(!ls->ls_file)`.
* -1 Hard race / timing: reliable triggering requires a delegation, forced recall, non-response until laundromat revoke, and a concurrent `LAYOUTGET` in the race window.
Paranoid additional signals:
* +1 Attacker-influenced repeatable lifetime manipulation: cooperating clients can influence delegation creation, recall pressure, revoke timing, and `LAYOUTGET` attempts.
* +1 Weak refcount/lifetime corruption leverage: if the unlocked read observes a non-NULL pointer while revoke is ending the object lifetime, `nfsd_file_get()` may mutate refcount state through stale storage. This is stronger than a pure NULL dereference but does not establish controlled reclaim.
Important non-awarded signals:
* Controlled reclaim not demonstrated.
* Arbitrary read/write not demonstrated.
* CLASS_A not awarded because there is no demonstrated controlled object replacement, callback control, directed write, or proven LPE.
* Confidentiality and integrity points are not awarded as established impacts. They remain paranoid review concerns due to the lifetime/refcount nature of the bug.
* Extra availability was not awarded because the same `BUG_ON` crash already accounts for DoS.
* Concrete repeated release/refcount signal was not awarded: this is not a visible first put followed by a second put/release of the exact same surviving reference. The relevant stale operation is a possible `get`/refcount mutation via an unlocked pointer.
Repeated-release/refcount checklist:
* First release can end lifetime: plausible via delegation revoke, but the exact put/free path is not visible in the supplied diff.
* Exact same reference survives first release: not established as a repeated-release control-flow pattern.
* Second operation is a real lifetime/refcount mutation: `nfsd_file_get()` is a lifetime/refcount operation on the read pointer.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: not under the repeated-release rule, but a separate weak lifetime/refcount corruption concern is counted.
* FLOOR 5 applied: yes, due to network-reachable active-object lifetime/refcount stale use.
3. Reachability analysis
The evaluated attacker is a remote NFS client with access to the relevant export and ability to issue valid NFSv4/pNFS operations. Host administrator privileges on the server are not required by the supplied scenario. This is not an ordinary unauthenticated Internet packet-path issue, because practical reachability depends on the NFS export, client access policy, delegations, and pNFS layout behavior.
The supplied trigger uses two NFS clients: one obtains or holds a delegation, another opens the same file to force recall, the first client does not respond, the laundromat revokes the delegation, and a concurrent `LAYOUTGET` using the delegation stateid reaches the race. This is network reachable through nfsd request handling, but the timing and protocol state requirements justify a race/timing penalty.
No namespace-scoped capability chain is involved. No special hardware is required. Prior NFS server/export configuration is a deployment prerequisite, not evidence that the attacker needs host-admin privileges to trigger the vulnerable operation once the service is exposed to them.
4. Primitive and severity interpretation
The exact primitive is a race-protected lifetime violation around `fp->fi_deleg_file`. The vulnerable path reads a lifetime-managed `nfsd_file` pointer without the lock used by the revoke path and then passes it to `nfsd_file_get()`. The directly demonstrated consequence is a NULL result followed by `BUG_ON`, causing kernel DoS.
This is an active structured object case, not fixed stale storage. The stale use occurs in active NFS request/layout stateid allocation, not merely in final cleanup. The patch both adds locking around the delegation-file pointer read and replaces the fatal assertion with graceful cleanup, which supports both a confirmed crash path and a meaningful lifetime-safety concern.
PrimitiveClass is CLASS_B because a real active-object lifetime/refcount primitive is present, but controlled reclaim, arbitrary kernel read/write, callback hijack, or demonstrated privilege escalation is not established. ImportantCandidate is YES because the bug is network-reachable and the stale path can involve a refcount operation on a lifetime-managed object. AutoDowngradeAllowed is NO because automatically closing it as ordinary crash-only DoS would discard unresolved active-object lifetime/refcount corruption leverage.
Likely operational severity is Actionable Moderate at minimum, with Strong Important review warranted. The conservative established impact is remote DoS against an NFS server. The speculative consequence is memory corruption from stale `nfsd_file` lifetime/refcount use, but not proven LPE.
5. One-sentence report phrase
A client-reachable race in `nfsd4_alloc_layout_stateid()` can read `fp->fi_deleg_file` without `fi_lock` while delegation revoke clears the pointer, allowing NFS clients to trigger a `BUG_ON` kernel crash and leaving a plausible active-object lifetime/refcount UAF concern that requires manual review.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: this is a network-reachable NFS server race with a demonstrated kernel crash and a plausible active-object lifetime/refcount stale-pointer primitive, but the available patch does not fully resolve whether practical exploitation is limited to DoS.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: nfsd: fix BUG_ON in nfsd4_alloc_layout_stateid on racing delegation revoke
Commit: c517f27498757e616d2a8fe6d16caad1fee422e6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c517f27498757e616d2a8fe6d16caad1fee422e6
Commit description:
nfsd4_alloc_layout_stateid reads fp->fi_deleg_file without holding
fi_lock when the parent stateid is a delegation. A concurrent delegation
revoke via the laundromat can clear fi_deleg_file under fi_lock, causing
nfsd_file_get() to return NULL and triggering the BUG_ON.
This race is client-reachable: two NFS clients can trigger it by having
one hold a delegation while another opens the same file to force a
recall. When the first client doesn't respond to the recall, the
laundromat revokes it. A concurrent LAYOUTGET from any client using the
delegation stateid hits the race window.
Fix this by taking fi_lock around the fi_deleg_file read in the
SC_TYPE_DELEG path, matching the locking discipline of the
find_any_file() arm, and replacing the BUG_ON with a graceful error
return that cleans up the partially-initialized layout stateid.
Fixes: c5c707f ("nfsd: implement pNFS layout recalls")
Cc: [email protected]
Assisted-by: kres:claude-opus-4-7
Reported-by: Chris Mason <[email protected]>
Signed-off-by: Jeff Layton <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Chuck Lever <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
fs/nfsd/nfs4layouts.c
Diff excerpt:
Not included in this email. See the upstream URL for the full patch.
Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c517f27498757e616d2a8fe6d16caad1fee422e6
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89686 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89686
The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.
The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.
This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.
The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.
Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:
Conservative CVSS vector: AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS vector: AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7.5
The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.
CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.
A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.
Conservative ActionableScore: 5
Paranoid ActionableScore: 6
The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.
For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:
ActionableScore below 5 -> MODERATE REGULAR
ActionableScore 5 or more -> MODERATE 7.0
The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.
There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).
KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.
AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0
These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).
Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
^ permalink raw reply [relevance 21%]
Results 1-1 of 1 | reverse | sort options + mbox downloads above
-- links below jump to the message on this page --
2026-09-12 6:36 21% [CVE-2026-89686][MODERATE 7.0] nfsd: fix BUG_ON in nfsd4_alloc_layout_stateid on racing delegation revoke AL-KERNEL
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox