[CVE-2026-68099][IMPORTANT] ksmbd: restore DACL size on check_add_overflow() to avoid malformed ACL

[CVE-2026-68099][IMPORTANT] ksmbd: restore DACL size on check_add_overflow() to avoid malformed ACL

[AL-KERNEL]

CVE: CVE-2026-68099
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ksmbd: restore DACL size on check_add_overflow() to avoid malformed ACL
Commit: f4fcd0c1a243d449307b887fafee23921e9db5ab
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f4fcd0c1a243d449307b887fafee23921e9db5ab
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68099
Analysis date: Mon, 10 Aug 2026 10:50:46 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A ksmbd DACL size accounting overflow can produce a malformed NT ACL whose size under-reports already written ACE data, allowing a remote authenticated SMB user with ACL modification rights to trigger OOB reads and likely DoS, with limited confidentiality or integrity concern in paranoid triage.

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

The original Linux kernel CVE announcement for CVE-2026-68099 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68099

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: IMPORTANT
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-2026-68099	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN NONE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-190;CWE-125;CWE-754;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '7.6';DESCR 'An integer overflow handling bug in ksmbd DACL construction can leave the committed NT ACL with a size field that is smaller than the ACE bytes already written. A remote SMB user who can create or modify sufficiently large ACLs may cause a malformed ACL that can trigger an out of bounds read when reparsed by ksmbd or by clients. For the CVSS the PR:L value is used because the attacker normally needs authenticated SMB access and permission to set or influence ACLs on a share or file. The issue is network reachable through SMB when ksmbd is exposed to the attacker. Impact is at least denial of service through kernel crash and in the paranoid case may include limited confidentiality or integrity impact from the out of bounds read or malformed ACL interpretation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-68099 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	REMOTE OOB DISK SKIP  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6	-	-	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScoreLower=6
ActionableScore=7

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  ksmbd is an SMB server path, and malformed ACL construction can be reached through network SMB operations when the attacker has access to a share and can influence ACLs.

* Memory corruption, Weak/indirect corruption candidate: +1
  The described primitive is an out-of-bounds read caused by malformed ACL size accounting, not an OOB write or attacker-controlled overwrite.

* Memory layout invariant restoration: +1
  The patch restores consistency between bytes already written and the DACL size later committed into the NT ACL.

* Reliable kernel crash / strong DoS: +1
  The commit explicitly says the malformed ACL can trigger OOB reads when reparsed by ksmbd itself, which is a plausible kernel crash path.

* Confidentiality impact plausible: +1
  OOB read is present, but disclosure is not clearly demonstrated. Conservative impact is limited confidentiality concern.

Paranoid additional signal:

* Integrity impact plausible: +1
  A malformed ACL can potentially be interpreted inconsistently by ksmbd or clients. This is weaker than arbitrary write, but enough for paranoid triage because ACL semantics affect access control.

Not awarded:

* Strong LPE plausibility: +0
  No reclaim, controlled overwrite, callback control, arbitrary write, or privilege escalation primitive is shown.

* Strong corruption primitive: +0
  The patch supports OOB read and malformed serialized ACL state, not OOB write or UAF.

* Broad/default/common subsystem: +0
  ksmbd is network-facing when enabled, but it is not a universally default core networking path.

## 3. Reachability analysis

The likely attacker is a remote authenticated SMB user who can create, modify, or otherwise influence ACLs on a ksmbd-exported share. PR is effectively low, not high, because the attacker does not need host root privileges, only SMB-level access and sufficient share or file permissions.

Namespaces and containers do not materially reduce the network exposure once ksmbd is serving the affected share. The path is not Internet-default in most deployments, but ksmbd is a real network service and may be exposed inside enterprise, appliance, NAS, or internal file-sharing networks.

The trigger likely requires constructing a sufficiently large or edge-case ACL so that u16 DACL size accounting overflows. That makes exploitation less trivial than a single malformed packet, but the bug is still actionable because the affected code serializes security metadata that is later reparsed.

Call-site confidence: high. The patch and commit message show both the corrupted size value and the downstream consumption in pndacl->size.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is network reachable through SMB and involves a concrete OOB read condition in kernel server code. It does not currently justify Critical or definite Important based on LPE or arbitrary code execution, because the patch does not show an attacker-controlled write, UAF reclaim, type confusion, or object replacement primitive.

Conservatively, this is an Actionable Moderate. Paranoid triage can treat it as a Strong Important candidate because it combines network reachability, ACL/security-metadata parsing, malformed kernel-generated wire state, and OOB read risk.

## 5. One-sentence report phrase

A ksmbd DACL size accounting overflow can produce a malformed NT ACL whose size under-reports already written ACE data, allowing a remote authenticated SMB user with ACL modification rights to trigger OOB reads and likely DoS, with limited confidentiality or integrity concern in paranoid triage.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: network-reachable ksmbd server code plus a concrete OOB read candidate in ACL construction/parsing is too actionable to auto-close, even though the demonstrated primitive is weaker than arbitrary write or proven privilege escalation.

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

Patch: ksmbd: restore DACL size on check_add_overflow() to avoid malformed ACL
Commit: f4fcd0c1a243d449307b887fafee23921e9db5ab
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f4fcd0c1a243d449307b887fafee23921e9db5ab

Commit description:

check_add_overflow() unconditionally writes the truncated sum into *d
even on overflow, per its contract in include/linux/overflow.h.
The four check_add_overflow() guards in set_posix_acl_entries_dacl()
and set_ntacl_dacl() break out of the ACE-building loops on overflow,
but the truncated *size is then consumed downstream at the end of
set_ntacl_dacl():

    pndacl->size = cpu_to_le16(le16_to_cpu(pndacl->size) + size);

This produces an on-wire NT ACL whose pndacl->size under-reports the
bytes actually written by the preceding fill_ace_for_sid()/memcpy()
calls, yielding a malformed ACL that can trigger out-of-bounds reads
when re-parsed by clients or ksmbd itself.

Restore *size to its pre-addition value on each overflow branch (via
`*size -= ace_sz` / `size -= nt_ace_size`) so that after the break,
*size once again holds the cumulative size of the successfully-written
ACEs. The committed ACL is then truncated-but-self-consistent rather
than malformed.

The ksmbd DACL builders are the only check_add_overflow() sites found
where an overflow path breaks out of a loop and the destination value
is consumed afterward. The other nearby break-style cases either
return -EINVAL on overflow (transport_ipc.c) or break without
consuming the overflowed destination value afterward (buildid.c).

Fixes: 299f962 ("ksmbd: use check_add_overflow() to prevent u16 DACL size overflow")
Assisted-by: atomcode:glm-5.2
Assisted-by: Codex:gpt-5.5
Cc: stable@vger.kernel.org
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  include/linux/overflow.h
  fs/smb/server/smbacl.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=f4fcd0c1a243d449307b887fafee23921e9db5ab

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

The original Linux kernel CVE announcement for CVE-2026-68099 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68099

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:L/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 7.6

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: 6
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).