[CVE-2026-72315][LOW] smb: client: fix busy dentry warning on unmount after DIO [ Upstream

[CVE-2026-72315][LOW] smb: client: fix busy dentry warning on unmount after DIO [ Upstream

[AL-KERNEL]

CVE: CVE-2026-72315
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: smb: client: fix busy dentry warning on unmount after DIO [ Upstream
Commit: f0eac9c3c3711f24efc2aaf12b8ec3e54a38c214
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f0eac9c3c3711f24efc2aaf12b8ec3e54a38c214
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72315
Analysis date: Sat, 15 Aug 2026 04:35:56 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
The CIFS DIO path can leave delayed file and dentry reference cleanup pending during unmount, causing a busy dentry warning and possible local DoS in panic_on_warn configurations, but the patch does not show a memory corruption or privilege escalation primitive.

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

The original Linux kernel CVE announcement for CVE-2026-72315 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72315

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-72315	LOW	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:N/I:N/A:H';CWE-362;*CWE-772;CWE-404;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'The CIFS DIO path can leave delayed cifsFileInfo and dentry reference release work pending while the superblock is being unmounted. If unmount reaches shrink_dcache_for_umount before the delayed netfs request cleanup and fileinfo put work have completed, the kernel can report a busy dentry warning and the system may suffer a local denial of service in configurations such as panic_on_warn. For the CVSS the PR:L is used for the paranoid score because an untrusted local user with write access to an existing CIFS mount may be able to keep DIO requests in flight while a privileged service or delegated environment performs unmount or recovery. The issue is not directly network reachable because the trigger is the local CIFS client unmount path, although a remote SMB server may influence I/O timing. Impact is availability only, with no supported confidentiality or integrity impact because the patch shows delayed reference cleanup rather than a UAF or write corruption primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)  SKIP CVE-2026-72315 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 340cea84f691c5206561bb2e0147158fe02070be YES  NO NO unknown 	MAYBE	DANGER DISK INIT SERVERTOCLIENT  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* Real lifetime / async cleanup mismatch: +1
  The bug is caused by delayed CIFS DIO request cleanup and delayed dentry reference release during unmount.
* Availability impact realistic: +1
  The visible impact is a busy dentry warning during unmount. It may become more serious on systems configured with panic_on_warn.
* Hard or timing-dependent condition: -1
  The issue requires a specific ordering between DIO completion cleanup, close, and unmount.
* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2
  Mount and unmount of CIFS filesystems are normally privileged operations.

Paranoid adjustment:

* Local unprivileged participation: +1
  A local user with write access to an already mounted CIFS share may be able to keep DIO requests active while a privileged service or delegated environment performs unmount or recovery.
* Availability impact kept: +1
  The practical worst case remains local DoS or panic_on_warn-triggered crash.
* No memory corruption, no C/I impact, no LPE signal added.

## 3. Reachability analysis

The main trigger is local CIFS client activity combined with unmount. In normal deployments, the unmount side requires administrative privilege, so conservative reachability is privileged local. A weaker paranoid interpretation exists if an unprivileged user can perform DIO writes on an existing CIFS mount while another privileged actor performs unmount.

This is not directly network reachable. A remote SMB server may influence I/O timing, but the vulnerable condition depends on the local client unmount path. CIFS is common enough to avoid treating it as rare, but this specific DIO plus unmount race is not a default packet-processing or remotely triggerable path.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like kernel issue, not an Important-class vulnerability. The patch shows delayed reference cleanup and missing synchronization before unmount, not UAF, OOB write, double free, arbitrary write, type confusion, or page-cache corruption.

Theoretical impact is local availability loss, especially with panic_on_warn. Realistic exploitation evidence supports DoS only and does not support privilege escalation, confidentiality loss, or integrity impact.

## 5. One-sentence report phrase

The CIFS DIO path can leave delayed file and dentry reference cleanup pending during unmount, causing a busy dentry warning and possible local DoS in panic_on_warn configurations, but the patch does not show a memory corruption or privilege escalation primitive.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

The issue is availability-only, timing-dependent, and normally requires privileged unmount involvement. It can be handled as non-urgent unless the affected product enables panic_on_warn broadly or delegates CIFS mount lifecycle operations to untrusted users.

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

Patch: smb: client: fix busy dentry warning on unmount after DIO [ Upstream
Commit: f0eac9c3c3711f24efc2aaf12b8ec3e54a38c214
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f0eac9c3c3711f24efc2aaf12b8ec3e54a38c214

Changed files:
  fs/smb/client/cifs_fs_sb.h
  fs/smb/client/cifsfs.c
  fs/smb/client/connect.c
  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=f0eac9c3c3711f24efc2aaf12b8ec3e54a38c214

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

The original Linux kernel CVE announcement for CVE-2026-72315 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72315

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:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  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: 0
  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).