[CVE-2026-46040][LOW] inotify: fix watch count leak when fsnotify_add_inode_mark_locked() fails

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

CVE: CVE-2026-46040
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: inotify: fix watch count leak when fsnotify_add_inode_mark_locked() fails
Commit: 8bcc1cd237ab5ccfdd102869fa031c541943cf40
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8bcc1cd237ab5ccfdd102869fa031c541943cf40
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46040
Analysis date: Wed, 27 May 2026 10:19:03 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A local inotify error path can leak the per-namespace watch count when mark insertion fails, allowing repeated failures to exhaust `max_user_watches` and cause inotify DoS without memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-46040 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46040

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-2026-46040	LOW	CHECK	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-772;**CWE-400;CWE-770;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A resource accounting leak can occur in inotify_new_watch() when inc_inotify_watches() succeeds but fsnotify_add_inode_mark_locked() later fails. The error path removes the temporary mark from the idr but does not decrement the per namespace watch counter, so repeated failures can exhaust max_user_watches and make future inotify_add_watch() calls fail with ENOSPC even when no watches are active. For the CVSS the PR:L is used because a local user can normally invoke inotify watch creation, but reliable triggering requires making the mark insertion fail repeatedly. The issue is not network reachable and does not involve UAF, out of bounds access, arbitrary write, information disclosure, or privilege escalation. Impact is denial of service against inotify watch quota only.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	DISK SIMPLEFIX LEAK ERRORPATH LINUS 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2

## 1. ActionableScore

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

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  `inotify_add_watch()` is normally reachable by local unprivileged users.

* Broad/default/common subsystem exposure: +1
  inotify/fsnotify is a common Linux userspace API.

* Availability impact realistic: +1
  Repeated failures can leak the per-namespace watch count and exhaust `max_user_watches`, causing later watch creation to fail with `ENOSPC`.

Subtracting signals:

* Hard or unreliable trigger condition: -1
  The attacker must repeatedly cause `fsnotify_add_inode_mark_locked()` to fail after `inc_inotify_watches()` succeeds, which is not the normal successful watch-add path.

No signals apply for memory corruption, UAF, OOB access, arbitrary write, privilege escalation, sensitive-object access, remote reachability, or security-boundary bypass.

## 3. Reachability analysis

A local user can reach the inotify watch creation path, but reliable exploitation requires repeatedly hitting the specific error path where mark insertion fails after the watch counter has already been incremented. Namespaces matter because the affected accounting is per namespace after the per-namespace limits conversion, but the bug does not appear to bypass namespace isolation. It mainly consumes the affected user or namespace watch quota.

The path is not network reachable. It is default-available on normal Linux systems, but the practical DoS is limited to inotify watch quota exhaustion rather than system-wide compromise.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like local resource accounting bug. It can cause denial of service for inotify users by exhausting `max_user_watches`, but it does not create a real object lifetime corruption primitive.

Theoretical impact is quota exhaustion. Realistic evidence does not support memory corruption, information disclosure, integrity impact, or privilege escalation.

## 5. One-sentence report phrase

A local inotify error path can leak the per-namespace watch count when mark insertion fails, allowing repeated failures to exhaust `max_user_watches` and cause inotify DoS without memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a local resource accounting leak with constrained DoS impact only and no supported memory corruption, remote exposure, or privilege-escalation primitive.

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

Patch: inotify: fix watch count leak when fsnotify_add_inode_mark_locked() fails
Commit: 8bcc1cd237ab5ccfdd102869fa031c541943cf40
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8bcc1cd237ab5ccfdd102869fa031c541943cf40

Commit description:

When fsnotify_add_inode_mark_locked() fails in inotify_new_watch(),
the error path calls inotify_remove_from_idr() but does not call
dec_inotify_watches() to undo the preceding inc_inotify_watches().
This leaks a watch count, and repeated failures can exhaust the
max_user_watches limit with -ENOSPC even when no watches are active.

Prior to commit 1cce1ee ("inotify: Convert to using per-namespace
limits"), the watch count was incremented after fsnotify_add_mark_locked()
succeeded, so this path was not affected. The conversion moved
inc_inotify_watches() before the mark insertion without adding the
corresponding rollback.

Add the missing dec_inotify_watches() call in the error path.

Fixes: 1cce1ee ("inotify: Convert to using per-namespace limits")
Cc: stable@vger.kernel.org
Signed-off-by: Chia-Ming Chang <chiamingc@synology.com>
Signed-off-by: robbieko <robbieko@synology.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://patch.msgid.link/20260224093442.3076294-1-chiamingc@synology.com
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/notify/inotify/inotify_user.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=8bcc1cd237ab5ccfdd102869fa031c541943cf40

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

The original Linux kernel CVE announcement for CVE-2026-46040 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46040

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: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: 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).