[CVE-2026-99115][MODERATE REGULAR] ksmbd: fill in FileSysIdentifier in FS_POSIX_INFORMATION

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-99115
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ksmbd: fill in FileSysIdentifier in FS_POSIX_INFORMATION
Commit: beba2ef6f66a71f20531ac70a1acb0b221e362e9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=beba2ef6f66a71f20531ac70a1acb0b221e362e9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99115
Analysis date: Fri, 30 Oct 2026 12:02:25 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A crafted ksmbd compound request can disclose eight stale bytes from an earlier response because `FileSysIdentifier` was returned uninitialized.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-99115 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99115

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-99115	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99115 CHECK WITH IMPACT FROM ORIG NN NONE | AEGIS IMPACT MODERATE AEGIS_AS=3 LEGACY=MODERATE RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N';*CWE-200;CWE-908;CWE-665;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.3';DESCR 'ksmbd reports a 56 byte FILE_SYSTEM_POSIX_INFO response but did not initialize the eight byte FileSysIdentifier field. A crafted SMB compound request can leave bytes from a previously generated and then replaced response in the shared response buffer, after which the next filesystem information reply exposes those stale bytes to the client. This is a confirmed limited information disclosure rather than an OOB read or memory corruption primitive. The disclosed value is limited to eight bytes from an earlier response in the same compound buffer, so confidentiality impact is Low while integrity and availability are unaffected. For the CVSS the PR:L base value reflects the need for an authenticated SMB session and share access in typical deployments. For the CVSS the PR:N paranoid value covers guest or anonymous shares where an unauthenticated network client may reach the affected request path. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.3 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the manual review requirement was applied. ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4. The independent weaker Aegis shadow evaluated CVSS 4.3 and ActionableScore 3 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	MAYBE	REMOTE DISK SIMPLEFIX LEAK SKIP LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 3.
* Conservative score: 3; no floor or cap applied.
* Raw paranoid score: 3.
* Paranoid score: 3; anonymous-share reachability does not add beyond the already-scored network exposure.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 Remote/network reachability: an SMB client can invoke the compound request sequence against an exposed ksmbd service.
* +1 Confidentiality plausible: eight bytes retained from an earlier response in the same compound response buffer can be returned to the client.
* No OOB read/write, UAF, arbitrary disclosure, integrity impact, crash, or authorization bypass is shown.
* No additional broad-exposure point: ksmbd deployment and the vulnerable POSIX filesystem-information path are not established as broadly/default exercised.
* No constrained-corruption penalty: this is a limited disclosure, not a corruption primitive.
* NEW52/B5-R does not apply: there is no retry-accumulated heap OOB write.
* NEW53/B3-PENDING does not apply: no shared-backing write exists.
* B4-PENDING does not apply: the patch initializes an output field and restores no DENY-to-ALLOW authorization guard.

3. Reachability analysis

The complete trigger is remotely reachable by an SMB client with access to the relevant request path. Typical deployments may require authentication and share access; guest or anonymous shares may permit unauthenticated access. Neither path requires host UID 0 or host-administrator powers. Namespace-scoped privilege is irrelevant and not established. No race, special hardware, boot option, or prior privileged setup beyond operating the SMB service is shown.

4. Primitive and severity reading

The primitive is a confirmed, bounded eight-byte stale-response disclosure from a reused compound-response buffer. The allocation is initially zeroed, but a generated response later replaced by a short error can leave bytes that the following response overlays without initializing `FileSysIdentifier`. This is CLASS_B as a real but constrained information-disclosure primitive, not memory corruption. No protected secret, cross-request disclosure, arbitrary offset, or high-impact confidentiality effect is demonstrated. ImportantCandidate is therefore NO, and automatic Important-preservation is unnecessary.

5. One-sentence report phrase

A crafted ksmbd compound request can disclose eight stale bytes from an earlier response because `FileSysIdentifier` was returned uninitialized.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm authentication/share requirements and whether the retained response bytes can contain sensitive data in affected deployments.

7. Call-site confidence

Call-site confidence: high

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UPSTREAM PATCH SUMMARY
======================================================================

Patch: ksmbd: fill in FileSysIdentifier in FS_POSIX_INFORMATION
Commit: beba2ef6f66a71f20531ac70a1acb0b221e362e9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=beba2ef6f66a71f20531ac70a1acb0b221e362e9

Commit description:

smb2_get_info_filesystem() reports 56 bytes for FS_POSIX_INFORMATION,
that is the whole of FILE_SYSTEM_POSIX_INFO, but never assigns
FileSysIdentifier.  Those eight bytes go to the client as they are found
in the response buffer.

The buffer is zeroed on allocation, so a standalone request leaks
nothing.  A compound request can leak: the offset of the next response
is advanced by the length pinned for the previous one, so a reply that
was written into the buffer and then dropped in favour of the short
error response of smb2_set_err_rsp() stays there, and the next reply is
laid over it with only the header cleared.

Report the file system id statfs() returned, which is what the field is
for.  FileSysIdentifier is __le64 and f_fsid is a pair of ints, so
assemble the value first, val[0] as the low half, and convert it on the
way out.

Fixes: e2f3448 ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Signed-off-by: Aleksandr Khromov <haa@amicon.ru>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/smb/server/smb2pdu.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=beba2ef6f66a71f20531ac70a1acb0b221e362e9

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DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-99115 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99115

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:L/PR:L/UI:N/S:U/C:L/I:N/A:N
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/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: unknown
  Paranoid ActionableScore: 3

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).