[CVE-2026-72034][MODERATE REGULAR] fhandle: reject detached mounts in capable_wrt_mount()

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

CVE: CVE-2026-72034
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fhandle: reject detached mounts in capable_wrt_mount()
Commit: 15104234c267ebe04b9f9a73e5c2179cc265ff60
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=15104234c267ebe04b9f9a73e5c2179cc265ff60
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72034
Analysis date: Sun, 16 Aug 2026 06:10:27 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline manual review recommended
Manual review required: YES

Summary:
A race in capable_wrt_mount() can allow open_by_handle_at() to observe a detached mount with mount mnt_ns cleared, causing a NULL pointer dereference and local kernel crash when mount and file handle APIs are reachable.

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

The original Linux kernel CVE announcement for CVE-2026-72034 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72034

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-2026-72034	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;CWE-367;**CWE-476;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'capable_wrt_mount can dereference a NULL mnt_ns after open_by_handle_at passes an earlier mounted check, because a cloned detached mount from open_tree(OPEN_TREE_CLONE) may be dissolved concurrently and umount_tree can clear mount mnt_ns. This is a local race condition and can lead to a kernel panic through a NULL pointer dereference. For the CVSS the PR:L is used because a local process with namespace or mount related capabilities may be enough in deployments that allow user namespaces, while fully locked down systems may require administrative privileges. The issue is not network reachable and requires local syscall interaction with file handle and mount APIs. Impact is denial of service via kernel crash. No concrete UAF, OOB access, information leak, or write primitive is visible from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended (with actual score 3)	YES	DANGER DISK SIMPLEFIX LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline manual review recommended**::

## 2. Signal breakdown

* Local unprivileged or low-privileged trigger: +1
  The relevant path is local syscall driven through open_by_handle_at(), open_tree(OPEN_TREE_CLONE), fput(), and mount teardown behavior. In hardened deployments this may require mount or namespace related capabilities, but user namespaces or delegated mount capabilities can make PR:L plausible.

* Reliable kernel crash / strong DoS: +1
  If capable_wrt_mount() observes mount->mnt_ns as NULL, the old code dereferences mnt_ns->user_ns and can panic the kernel.

* Broad/default/common subsystem: +1 in paranoid score only
  This is VFS and mount namespace code, not a rare driver. However, the exact file-handle plus detached-mount sequence is not a common everyday path, so I do not count this in the conservative score.

* Hard or unreliable race / special timing required: -1
  The bug depends on a race between the earlier mounted check and later READ_ONCE(mount->mnt_ns), with concurrent detached mount dissolution or umount_tree() clearing mnt_ns.

* No generic memory corruption: +0
  The visible primitive is a NULL pointer dereference. The patch does not show UAF, stale object reuse, OOB access, arbitrary write, callback control, refcount takeover, or attacker-controlled reclaim.

* No privilege escalation plausible signal: +0
  The affected helper is security-sensitive, but the demonstrated failure mode is panic before authorization succeeds. No practical fd theft, credential bypass, or namespace escape chain is shown.

## 3. Reachability analysis

A local process must interact with file-handle and mount APIs and win a race involving a detached cloned mount from open_tree(OPEN_TREE_CLONE) while open_by_handle_at() is between permission checks. Namespaces matter because CAP_SYS_ADMIN or related checks may be satisfied inside a user or mount namespace in some deployments, making PR:L plausible rather than full host-root. The issue is not network reachable. The code path is in core VFS and mount handling, but the trigger sequence is specialized and timing-dependent.

## 4. Severity interpretation

This behaves like a local DoS race rather than an Important-class vulnerability. The theoretical concern is that the bug is in a permission helper, but the concrete primitive shown by the patch is only NULL dereference and kernel panic. There is no supported evidence of memory corruption beyond invalid NULL dereference, no confidentiality impact, no integrity impact, and no demonstrated privilege escalation path. Conservative handling is ordinary Moderate / Low-like. Paranoid handling is borderline because the race is in a security-sensitive VFS permission path and namespace behavior may lower practical trigger privileges.

## 5. One-sentence report phrase

A race in capable_wrt_mount() can allow open_by_handle_at() to observe a detached mount with mount mnt_ns cleared, causing a NULL pointer dereference and local kernel crash when mount and file handle APIs are reachable.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the visible primitive is DoS-only, but the bug is a race in fhandle permission logic and namespace-dependent capability behavior can affect practical reachability.

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

Patch: fhandle: reject detached mounts in capable_wrt_mount()
Commit: 15104234c267ebe04b9f9a73e5c2179cc265ff60
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=15104234c267ebe04b9f9a73e5c2179cc265ff60

Commit description:

The recent fhandle RCU fix moved the mount namespace capability check
into capable_wrt_mount(), so a non-NULL mnt_namespace survives the
ns_capable() dereference. The helper still assumes the later
READ_ONCE(mount->mnt_ns) must be non-NULL because may_decode_fh()
checked is_mounted() first.

That assumption is not stable. A detached mount from
open_tree(..., OPEN_TREE_CLONE) can be dissolved on fput while
open_by_handle_at() is between those checks, and umount_tree() can
clear mount->mnt_ns. If the helper observes NULL, it dereferences
mnt_ns->user_ns and panics.

Return false when the RCU read observes a detached mount. This keeps
the relaxed permission path conservative: a mount no longer attached
to a namespace cannot authorize open_by_handle_at() access.

Fixes: 620c266 ("fhandle: relax open_by_handle_at() permission checks")
Cc: stable@vger.kernel.org
Signed-off-by: David Lee <david.lee@trailofbits.com>
Assisted-by: LLM
Link: https://patch.msgid.link/20260701114438.24431-1-david.lee@trailofbits.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/fhandle.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=15104234c267ebe04b9f9a73e5c2179cc265ff60

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

The original Linux kernel CVE announcement for CVE-2026-72034 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72034

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:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 2
  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).