[CVE-2026-90139][LOW] fuse: check for NULL root inode in fuse_fill_super_submount [ Upstream

[CVE-2026-90139][LOW] fuse: check for NULL root inode in fuse_fill_super_submount [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90139
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: fuse: check for NULL root inode in fuse_fill_super_submount [ Upstream
Commit: 17120c5bfb16808509ea18d7fab6027802d93eba
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=17120c5bfb16808509ea18d7fab6027802d93eba
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90139
Analysis date: Fri, 18 Sep 2026 08:08:26 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90139 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90139

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-2026-90139	LOW	https://www.kernelcve.org/list/?q=CVE-2026-90139 CHECK WITH IMPACT FROM ORIG NN LOW	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-476;CWE-252;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference in the FUSE submount path occurs because fuse_fill_super_submount can use the result of fuse_iget without checking whether inode allocation failed. Under memory pressure fuse_iget can return NULL, after which get_fuse_inode is evaluated on a NULL root inode and the nlookup decrement can fault on a bogus low address. A local user may trigger this by accessing a FUSE automount or submount path while the system is under memory pressure, causing the mount path to oops and potentially crash the kernel. For the CVSS the PR:L is used because triggering requires a local user or process context able to reach the FUSE mount path, but does not necessarily require full administrative privileges. The issue is not network reachable. Impact is denial of service via kernel crash or mount path oops, with no supported information disclosure or privilege escalation primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	READ KASAN NULLPTR DISK NOMEMCHK HARDWARE  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1. No floor or cap applied.
* Raw paranoid score: 2
* Paranoid score: 2. No floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Conservative signals:

* +1 Local unprivileged trigger. The visible trace shows a normal local path lookup/statx style operation reaching fuse_dentry_automount and fuse_get_tree_submount.
* +1 Reliable kernel crash / strong DoS. The supplied KASAN trace shows a NULL pointer dereference in the FUSE submount path, and the text states that this oopses the mount.
* -1 Special trigger condition. The fault requires inode allocation failure in fuse_iget, which depends on memory pressure at the submount root allocation point.

Paranoid signals:

* +1 Local unprivileged trigger. A local process can access the automount/submount path once such a FUSE setup exists.
* +1 Reliable kernel crash / strong DoS. NULL root inode handling can lead to a kernel oops or crash.
* 0 Special trigger condition not subtracted in the paranoid score. A local user may be able to create memory pressure and repeatedly access the path, so the condition is not treated as a strong reducing factor for the upper score.

Non-awarded signals:

* RealUAF not awarded. The bug is a NULL return value used as an inode pointer, not post-lifetime use of a freed object.
* Concrete repeated release/refcount not awarded. There is no first put/release followed by a second lifetime mutation on the same dangling reference.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity not awarded. The low-address nlookup decrement is a faulting bogus-address access, not an attacker-directed write primitive.
* Extra availability not awarded. The same oops/crash is already counted once.
* Remote reachability not established.
* ImportantCandidate not awarded because this is a NULL dereference DoS without a meaningful corruption, ownership, or authorization primitive.

3. Reachability analysis

The evaluated attacker is a local user or local process that can reach a FUSE automount or submount path. The call trace shows access through path lookup and statx, which supports below-host-admin triggering for the lookup side. The complete scenario also requires an existing FUSE submount or automount configuration and allocation failure under memory pressure.

Host-admin-equivalent privilege is not required for the demonstrated lookup trigger itself, so HostAdminRequired=NO. Namespace-scoped capability is not the relevant access model here, so NamespaceScopedPrivilege=NO. There is no network trigger. There is no special hardware requirement. This is not a race. The main reliability limitation is the need to hit the allocation failure path, which is reflected only in the conservative lower score.

4. Primitive and severity interpretation

The exact primitive is unchecked NULL return from fuse_iget in fuse_fill_super_submount. The NULL root inode is passed to get_fuse_inode and then used for fi->nlookup--, causing a NULL pointer dereference or a bogus low-address access.

This is not an active structured UAF and not fixed stale storage. It is a CLASS_C invalid dereference / DoS primitive. The demonstrated consequence is mount-path oops or possible kernel crash. The speculative consequence would be stronger memory corruption, but the patch and context do not support a useful attacker-controlled write, reclaim, disclosure, or privilege escalation path.

ImportantCandidate=NO because no Important-preserving primitive is established. AutoDowngradeAllowed=YES because the issue is a local memory-pressure-dependent NULL pointer dereference with DoS-only impact and no unresolved corruption or boundary-bypass leverage.

Likely operational severity is ordinary Moderate or Low-like depending on product exposure and whether untrusted local users can reach FUSE submounts while applying memory pressure.

5. One-sentence report phrase

A NULL pointer dereference in the FUSE submount path can occur when fuse_iget fails under memory pressure and fuse_fill_super_submount uses the NULL root inode, allowing a local user that can reach the FUSE automount path to cause a mount-path oops or kernel DoS.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the established primitive is NULL dereference only, with no UAF, double release, controlled overwrite, information leak, authorization bypass, or privilege escalation evidence.

7. Call-site confidence

Call-site confidence: high

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

Patch: fuse: check for NULL root inode in fuse_fill_super_submount [ Upstream
Commit: 17120c5bfb16808509ea18d7fab6027802d93eba
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=17120c5bfb16808509ea18d7fab6027802d93eba

Changed files:
  fs/fuse/inode.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=17120c5bfb16808509ea18d7fab6027802d93eba

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

The original Linux kernel CVE announcement for CVE-2026-90139 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90139

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: 1
  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).