* [CVE-2026-89639][MODERATE REGULAR] cifs: use cifs_invalidate_cache() in cifs_do_truncate() for O_TRUNC
@ 2026-09-12 2:55 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 2:55 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89639
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: cifs: use cifs_invalidate_cache() in cifs_do_truncate() for O_TRUNC
Commit: 8b9b10fe5b8b492b27b9f4546742ea541a650213
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8b9b10fe5b8b492b27b9f4546742ea541a650213
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89639
Analysis date: Fri, 11 Sep 2026 22:55:14 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89639 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89639
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 REGULAR
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-2026-89639 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89639 CHECK WITH IMPACT FROM ORIG NN IMPORTANT Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:N';CWE-524;*CWE-672;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '5.3';DESCR 'A CIFS/SMB client cache coherency flaw can allow stale cached file data to be served after an O_TRUNC operation. The truncate path updates the local inode size but may leave the fscache cookie without proper invalidation, so old cached contents can later be treated as readable data after the cookie is activated. For the CVSS the PR:L is used because a local user or process on the client must be able to access the CIFS mount and trigger O_TRUNC on a file. The issue is not directly network reachable from arbitrary packet traffic, but it affects a network filesystem client and requires a mounted SMB share with fscache behavior in scope. Impact is mainly a storage consistency and policy bypass issue, with possible limited confidentiality exposure of stale data and possible integrity impact from serving data that should have been discarded.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4) SKIP CVE-2026-89639 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE DISK SIMPLEFIX IMPROVEONLY SERVERTOCLIENT LINUS LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 3. No floor or cap applied.
* Raw paranoid score: 4
* Paranoid score: 4. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::
2. Signal breakdown
Conservative positive signals:
* +1 Local unprivileged trigger. A local user or process with normal access to the mounted CIFS file can trigger the O_TRUNC path through open.
* +1 Confidentiality plausible. Stale cached file contents may be served after truncation, so data that should no longer be readable from that file can be exposed through the client cache.
* +1 Important filesystem/storage path. The bug is in CIFS client truncate and fscache coherency, which affects file data visibility and storage semantics.
Paranoid additional signal:
* +1 Integrity plausible. Serving stale data after O_TRUNC can violate expected file contents and application level data semantics, although the patch does not show unauthorized server side file modification.
Negative signals:
* No host-admin penalty applied. Host-admin privilege is not required for the file open and O_TRUNC operation once the CIFS mount exists and permissions allow access.
* No race penalty applied. The issue is a missing cache invalidation in an ordered truncate path, not a narrow timing race.
* No rare hardware or physical access penalty applied. No special hardware is involved.
Material non-awarded signals:
* RealUAF not awarded. No use-after-free or stale kernel object lifetime violation is established.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Kernel memory corruption not demonstrated.
* Reliable kernel crash or host-wide DoS not demonstrated.
* Remote reachability not awarded. This is not triggered by arbitrary SMB network packets against the client.
* Dirty-Pipe-like ownership/COW violation not awarded. The patch does not show that a write authorized for one file modifies another independently permissioned file.
* Security-boundary authorization bypass not awarded. No missing capability or permission check is shown.
* Privileged FD/resource theft not awarded.
* LPE not awarded.
3. Reachability analysis
The practical trigger is local on the CIFS client. A process that can access a CIFS-mounted file and has permission to open it with O_TRUNC can reach cifs_do_truncate. The issue depends on CIFS fscache behavior being in scope, so an existing CIFS mount and relevant cache configuration are deployment prerequisites.
The vulnerability is not directly network reachable from arbitrary packet traffic. A remote SMB server is part of the filesystem environment, but the supplied patch establishes the problematic operation as the local client O_TRUNC path and missing fscache invalidation, not a packet parser or remotely forced kernel crash.
HostAdminRequired is NO for the vulnerable operation itself because normal file access can trigger O_TRUNC after the mount exists. NamespaceScopedPrivilege is NO because no namespace-scoped capability is needed for the file operation and no namespace-specific privilege chain is relevant. Prior privileged setup may exist for creating the mount or enabling fscache, but that is a deployment condition and not evidence that only host admin can trigger the bug once exposed.
No special hardware exists. No timing race is required.
4. Primitive and severity interpretation
The exact primitive is a filesystem cache coherency failure. After cifs_setsize(inode, 0), the fscache cookie may still allow stale cached data to be served later unless the cache is invalidated. This is not a kernel memory corruption primitive and not a UAF.
The affected state is storage/cache state rather than active heap object lifetime. There is no concrete repeated release/refcount operation. There is no evidence that the first lifetime operation frees an object and a second operation mutates a dangling reference.
PrimitiveClass is CLASS_C because the demonstrated behavior is invalid stale-data state in a filesystem cache, without an independently useful kernel corruption primitive. The demonstrated consequence is stale data exposure or incorrect file contents after truncate. The speculative consequence is stronger application-level integrity impact in deployments where truncation is used as a security or data lifecycle boundary, but the patch does not establish privilege escalation, arbitrary write, or cross-file ownership bypass.
ImportantCandidate is NO because no Class A or strong Class B primitive is present. ManualReview is RECOMMENDED because stale file data after O_TRUNC can be security relevant depending on CIFS mount options, fscache configuration, user sharing model, and application assumptions. AutoDowngradeAllowed is YES because there is no unresolved memory corruption, authorization bypass, COW ownership violation, or concrete repeated lifetime mutation.
Likely operational severity is low-to-moderate, with practical impact depending heavily on whether CIFS fscache is enabled and whether stale data visibility crosses user or policy boundaries.
5. One-sentence report phrase
A CIFS client fscache coherency flaw can serve stale cached file data after O_TRUNC, causing possible limited data exposure or integrity semantic violations for local users with access to the affected mounted file.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Reason: no kernel corruption or LPE primitive is established, but stale file data after truncate can be security relevant in multi-user or policy-sensitive CIFS cache deployments.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: cifs: use cifs_invalidate_cache() in cifs_do_truncate() for O_TRUNC
Commit: 8b9b10fe5b8b492b27b9f4546742ea541a650213
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8b9b10fe5b8b492b27b9f4546742ea541a650213
Commit description:
cifs_do_truncate() is invoked from cifs_open() without i_rwsem, so it
cannot use cifs_resize_file_locked() to perform a proper fscache cookie
resize. Instead, add cifs_invalidate_cache() after cifs_setsize().
cifs_invalidate_cache() calls fscache_invalidate(), which works without
holding i_rwsem: it unconditionally increments inval_counter and sets
FSCACHE_COOKIE_NO_DATA_TO_READ, ensuring that stale cached data is not
served once the cookie is later activated by fscache_use_cookie().
Truncation to zero leaves no valid cached data, making invalidation the
correct semantic here.
Fixes: fa724e2 ("cifs: add fscache_resize_cookie() to cifs_setsize()")
Cc: [email protected]
Cc: David Howells <[email protected]>
Cc: Paulo Alcantara <[email protected]>
Signed-off-by: Frank Sorenson <[email protected]>
Signed-off-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/file.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=8b9b10fe5b8b492b27b9f4546742ea541a650213
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89639 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89639
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:L/A:N
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:N
The Best / paranoid CVSS score: 5.3
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: 3
Paranoid ActionableScore: 4
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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR
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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