[CVE-2026-99276][LOW] ksmbd: defer connection scavenging while session locks are busy

[CVE-2026-99276][LOW] ksmbd: defer connection scavenging while session locks are busy

[AL-KERNEL]

CVE: CVE-2026-99276
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: ksmbd: defer connection scavenging while session locks are busy
Commit: 4e4e8b38994a0b0fc23f00ca6698b469a41abe70
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4e4e8b38994a0b0fc23f00ca6698b469a41abe70
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99276
Analysis date: Sat, 07 Nov 2026 22:12:38 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
An authenticated SMB client can potentially trigger a timing-dependent KSMBD lock-cycle deadlock, causing service availability loss without shown memory corruption or authorization bypass.

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

The original Linux kernel CVE announcement for CVE-2026-99276 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99276

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: LOW
Manual review required: NO

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-2026-99276	LOW	https://www.kernelcve.org/list/?q=CVE-2026-99276 CHECK WITH IMPACT FROM ORIG NN NONE | AEGIS IMPACT MODERATE AEGIS_AS=2 LEGACY=LOW RESULT=DIFF (final Legacy and Aegis differ; FINAL: LOW, Aegis: MODERATE)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-833;CWE-667;CWE-362;Other CVSS 'AV:A/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.8';DESCR 'A lock ordering deadlock can occur when the KSMBD expiration worker holds conn_list_lock and waits for conn session_lock or sessions_table_lock while session teardown holds the session locks and waits to reacquire conn_list_lock behind a queued writer. The required overlap makes attack complexity high. For the CVSS the PR:L value is selected because reliable triggering normally requires control of an authenticated SMB session that can enter previous session teardown or expiration handling. The issue is reachable through SMB session lifecycle activity on an exposed KSMBD server but it is not a direct unauthenticated packet parsing flaw. Impact is limited to availability because the patch context shows a lock cycle and service stall without a memory corruption or information disclosure primitive. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.8 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage LOW to LOW because the cross model evidence reconciliation 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 V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED. The independent weaker Aegis shadow evaluated CVSS 3.4 and ActionableScore 2 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE. The models disagreed but the aggregation rules retained the production result. The final reported impact is LOW.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	MAYBE	REMOTE DISK SIMPLEFIX LEAK ERRORPATH SKIP LINUS  V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
PrimitiveClass=CLASS_C
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: 2.
* Conservative score: 2; no floor or cap applied.
* Raw paranoid score: 2.
* Paranoid score: 2; no distinct corruption or boundary effect supports escalation.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +2 remote/network reachability: an authenticated SMB client can exercise session lifecycle activity on an exposed KSMBD server.
* +1 strong DoS: the documented lock cycle can deadlock connection expiration and session teardown, stalling KSMBD processing.
* -1 hard timing/overlap: triggering requires the worker, teardown path, and queued `conn_list_lock` writer to overlap in the stated order.
* No memory corruption, UAF, write primitive, disclosure, authorization bypass, or protected-resource impact is shown.
* No extra availability point: service deadlock is the same DoS already counted.
* NEW52/B5-R does not apply: there is no heap OOB write, retry-accumulated index, or later consumption of corrupted state.
* NEW53/B3-PENDING does not apply: no shared-backing write exists.
* NEW54/B4-PENDING does not apply: no mandatory authorization guard changes from DENY to ALLOW.

3. Reachability analysis

The supported attacker is a remote authenticated SMB user, not host UID 0 or host administrator. Session teardown/expiration activity is network-invocable, although reliable deadlock creation requires a specific concurrent lock ordering. No namespace capability is implicated. KSMBD must already be enabled and exposed, but no special hardware or attacker-controlled host configuration is shown.

4. Primitive and severity reading

The exact primitive is a lock-ordering deadlock: the expiration worker holds `conn_list_lock` while waiting for a session lock, while teardown holds session locks and attempts a read acquisition blocked behind a queued writer. The patch avoids waiting by using trylocks and deferring scavenging. This is CLASS_C availability behavior, with no leverage past service DoS. Therefore it is not an Important candidate and may be downgraded after concurrency review.

5. One-sentence report phrase

An authenticated SMB client can potentially trigger a timing-dependent KSMBD lock-cycle deadlock, causing service availability loss without shown memory corruption or authorization bypass.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm whether the deadlock stalls only KSMBD or can cause broader host-wide task blockage.

7. Call-site confidence

Call-site confidence: high

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

Patch: ksmbd: defer connection scavenging while session locks are busy
Commit: 4e4e8b38994a0b0fc23f00ca6698b469a41abe70
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4e4e8b38994a0b0fc23f00ca6698b469a41abe70

Commit description:

The expiration worker holds conn_list_lock for reading while checking
for authenticated sessions. Without this change, the check can wait for
conn->session_lock or sessions_table_lock.

In destroy_previous_session(), the status updates after
ksmbd_conn_wait_idle_sess() run with both session locks held for
writing. Both the KSMBD_SESS_GOOD error path and the
KSMBD_SESS_NEED_SETUP success path call ksmbd_all_conn_set_status(),
which takes conn_list_lock for reading.

A writer queued on conn_list_lock can block these new read acquisitions.
The worker then waits for a session lock held by
destroy_previous_session(), which waits behind the writer. The writer
cannot proceed until the worker releases conn_list_lock.

Try both session locks instead of waiting in the scavenging check. Keep
the connection when either lock is busy and let the periodic worker
retry later.

Fixes: d7fd1f9 ("ksmbd: follow SMB2 session expiration semantics")
Cc: stable@vger.kernel.org
Reported-by: Abdifatah Suruur <suruurism@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/smb/server/mgmt/user_session.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=4e4e8b38994a0b0fc23f00ca6698b469a41abe70

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

The original Linux kernel CVE announcement for CVE-2026-99276 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99276

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:A/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.8

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: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).