* [CVE-2026-89642][MODERATE 7.0] cifs: call pagecache_isize_extended() in cifs_setsize() when extending
@ 2026-09-12 4:42 AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 4:42 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89642
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: cifs: call pagecache_isize_extended() in cifs_setsize() when extending
Commit: 12bafe32f09c0d24f5096154b59c347b9c6606e0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=12bafe32f09c0d24f5096154b59c347b9c6606e0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89642
Analysis date: Sat, 12 Sep 2026 00:42:52 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate
Manual review required: NO
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89642 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89642
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: NO
A detailed explanation of the methodology and priority rules is included
at the end of this message.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-89642 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89642 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N';*CWE-200;CWE-226;CWE-212;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '4.7';DESCR 'In the CIFS client, cifs_setsize did not call pagecache_isize_extended when a file was extended. The tail of the page crossing the old EOF could keep dirty stale bytes, and those bytes could later be written back to the SMB server as part of the newly extended file range. For the CVSS the PR:L is used because a local user or process must be able to operate on a CIFS mounted file and extend its size, but full administrative privileges are not normally required once the mount and file permissions allow writes. The issue is not a direct remote network attack against the client, but it can disclose local stale page cache data to a remote SMB server or to other readers of the extended file. Impact is confidentiality loss only, with no evidence of integrity loss, availability loss, memory corruption, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5) YES DISK SIMPLEFIX SERVERTOCLIENT LINUS KPANIC NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate**::
No floor or cap changed the score.
2. Signal breakdown
Positive signals:
* +1 Confidentiality plausible. The commit explicitly states that dirty bytes beyond the old EOF can be written back to the SMB server, exposing stale data in the newly extended range.
* +1 Local below-host-admin trigger. Once a CIFS mount exists and file permissions allow writes, a local user or process can trigger the affected path by extending a file. Host administrator privilege is not required for the file operation itself.
* +1 Important filesystem/storage path. The bug is in CIFS client file size handling, page cache EOF handling, and writeback semantics.
* +1 Practical disclosure chain strongly implied. The chain is file extension, missing old EOF tail zeroing, later writeback, and stale bytes becoming visible to the SMB server or readers of the extended file.
* +1 Page-cache size and zeroing invariant violation. The patch restores the standard truncate_setsize pattern by calling pagecache_isize_extended before truncate_pagecache when extending the file.
Non-awarded signals:
* RealUAF not awarded. No post-free object use is established.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Integrity impact not awarded. The patch supports stale data disclosure, not unauthorized modification of another object.
* Availability impact not awarded. No crash, panic, hang, or resource exhaustion is established.
* Authorization bypass not awarded. No missing permission check or ALLOW-vs-DENY decision change is shown.
* Dirty-Pipe-like COW ownership violation not awarded. The evidence shows disclosure of stale bytes, not attacker-authorized writes to A modifying independently permissioned B.
* Remote reachability against the client not awarded. The vulnerable operation is triggered by local file operations on a CIFS mount, although the disclosure sink can be a remote SMB server.
* LPE not awarded. No privilege escalation primitive is established.
Negative signals:
* No negative privilege penalty applied. Host-admin-equivalent privilege is not positively required for the triggering file operation once the mount and file permissions exist.
* No rare subsystem penalty applied. CIFS is optional but common enough that optionality alone is not a concrete reducing signal.
* No race penalty applied. This is not a timing race.
3. Reachability analysis
The immediate trigger is local filesystem activity on a CIFS mounted file. A local user or process that can write to and extend the affected file can reach cifs_setsize. The attacker does not need host root for the file operation itself, although a CIFS mount must already exist or be provisioned by some prior configuration.
The issue is not directly network-triggerable against the client by arbitrary inbound packets. The network aspect is that stale local page-cache bytes may be written back to the remote SMB server. If the SMB server is malicious or less trusted than the client, the confidentiality impact is more relevant.
No namespace-scoped capability requirement is established. No special hardware, boot option, or race timing is required. The vulnerable behavior depends on CIFS client use and file extension semantics.
4. Primitive and severity interpretation
The exact primitive is stale page-cache data disclosure caused by missing zeroing of the page tail that crosses the old EOF during file extension. This is not memory corruption, not UAF, not double free, and not a refcount/lifetime mutation through a dangling object.
The bug is best treated as a concrete filesystem/page-cache confidentiality flaw. The demonstrated consequence is disclosure of stale dirty bytes to the SMB server or to readers of the newly extended file range. The speculative consequence is that the leaked bytes may contain sensitive local data, but the patch does not establish arbitrary memory disclosure or privilege escalation.
PrimitiveClass is CLASS_B because a real stale-data storage/page-cache security primitive exists and the sensitivity of exposed data needs triage. ImportantCandidate is NO because the primitive is bounded and no LPE, authorization bypass, arbitrary read/write, or ownership/COW overwrite is established. AutoDowngradeAllowed is NO because this is a concrete confidentiality leak in a network filesystem client and should not be silently closed as a harmless cleanup fix.
Likely operational severity is Actionable Moderate.
5. One-sentence report phrase
A CIFS client file extension can miss page-cache EOF tail zeroing, allowing stale dirty bytes to be written back to the SMB server and exposing local data without requiring host administrator privileges for the triggering file operation.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
The issue has a concrete confidentiality disclosure path, but the amount and sensitivity of leaked data depend on workload, file permissions, mount trust model, and page-cache state.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: cifs: call pagecache_isize_extended() in cifs_setsize() when extending
Commit: 12bafe32f09c0d24f5096154b59c347b9c6606e0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=12bafe32f09c0d24f5096154b59c347b9c6606e0
Commit description:
cifs_setsize() calls truncate_pagecache() but skips
pagecache_isize_extended() on extension. truncate_setsize() shows
the correct pattern:
i_size_write(inode, newsize);
if (newsize > oldsize)
pagecache_isize_extended(inode, oldsize, newsize);
truncate_pagecache(inode, newsize);
pagecache_isize_extended() zeroes the tail of the page straddling old
EOF. Without it, dirty bytes in that region can be written back to
the server, exposing stale data in the newly extended range.
Cc: [email protected]
Cc: David Howells <[email protected]>
Signed-off-by: Frank Sorenson <[email protected]>
Acked-by: Namjae Jeon <[email protected]>
Signed-off-by: Paulo Alcantara <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
fs/smb/client/inode.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=12bafe32f09c0d24f5096154b59c347b9c6606e0
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89642 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89642
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:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N
The Best / paranoid CVSS score: 4.7
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: unknown
Paranoid ActionableScore: 5
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).
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