[CVE-2026-64596][MODERATE REGULAR] libfs: set SB_I_NOEXEC and SB_I_NODEV by default in init_pseudo()

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

CVE: CVE-2026-64596
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: libfs: set SB_I_NOEXEC and SB_I_NODEV by default in init_pseudo()
Commit: b9d45d328fcda4f0d3d281b7d6f12d3f181d9381
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b9d45d328fcda4f0d3d281b7d6f12d3f181d9381
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64596
Analysis date: Thu, 06 Aug 2026 03:45:29 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / ordinary Moderate, not an Important candidate
Manual review required: NO

Summary:
Pseudo filesystems created by init_pseudo() lacked SB_I_NOEXEC and SB_I_NODEV, allowing local executable mmap attempts on pseudo-fs backed fds such as dma-buf and causing DEBUG_VFS warnings, but with no evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-64596 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64596

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: 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-2026-64596	MODERATE	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:L/A:H';CWE-693;CWE-284;CWE-732;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.3';DESCR 'Pseudo filesystem mounts created by init_pseudo() did not get SB_I_NOEXEC and SB_I_NODEV by default, so pseudo-fs file descriptors such as dma-buf backed files could violate the expected noexec invariant. A local process with access to such an fd could attempt an executable mmap that should not be allowed for this internal filesystem class. On CONFIG_DEBUG_VFS enabled kernels, ordinary dma-buf mmap activity can also trigger a path_noexec() warning, and availability impact becomes more relevant if warning policy is configured to panic. For the CVSS the PR:L is used because triggering requires local code execution or access to a local pseudo-fs backed file descriptor, not only remote network reachability. The issue is not network reachable. Impact is mainly a local integrity policy bypass and possible denial of service on debug or panic_on_warn systems, with no evidence of memory corruption or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Borderline / ordinary Moderate, not an Important candidate (with actual score 3)  SKIP CVE-2026-64596 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 1e7ab6f67824343ee3e96f100f0937c393749a8a YES  NO NO unknown 	MAYBE	READ DISK HARDWARE LINUS MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	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 / ordinary Moderate, not an Important candidate**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process with access to a pseudo-fs backed fd such as dma-buf can attempt mmap with executable permissions.
* Integrity impact plausible: +1. The missing SB_I_NOEXEC allows an executable mapping that should be rejected for this internal pseudo filesystem class.

Paranoid additional signal:

* Reliable kernel crash / strong DoS: +1. Only in debug or panic_on_warn style configurations, path_noexec warning can become availability relevant.

Discounts / non-applied signals:

* No generic memory corruption. The patch only sets superblock internal flags and does not indicate UAF, OOB, double free, refcount misuse, type confusion, or arbitrary write.
* No privilege escalation plausible. The issue is a noexec invariant violation, not a demonstrated LPE primitive.
* No remote reachability. The trigger is local mmap behavior.
* CONFIG_DEBUG_VFS warning impact is configuration dependent, so it does not justify Important severity by itself.
* Call-site confidence: high. The changed init_pseudo() behavior and described mmap path are visible in the commit context.

## 3. Reachability analysis

A local process that can obtain or receive a pseudo-fs backed file descriptor, for example a dma-buf fd, can attempt an executable mmap. This does not require network access and is not remotely triggerable. Namespace or container behavior may matter only if the containerized workload can access such device or dma-buf interfaces, but the bug does not itself break a namespace boundary. The warning-based DoS aspect mainly applies to CONFIG_DEBUG_VFS kernels or systems configured to panic on warnings, which is not a normal production default.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like policy/invariant bug, not an Important-class kernel vulnerability. The realistic impact is a local noexec policy bypass for pseudo-fs backed file descriptors and possible warning-triggered DoS in debug or panic_on_warn configurations. Theoretical impact does not extend to memory corruption or privilege escalation based on the patch.

## 5. One-sentence report phrase

Pseudo filesystems created by init_pseudo() lacked SB_I_NOEXEC and SB_I_NODEV, allowing local executable mmap attempts on pseudo-fs backed fds such as dma-buf and causing DEBUG_VFS warnings, but with no evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a local policy/invariant fix with no concrete memory corruption, no remote reachability, and no demonstrated privilege-escalation path.

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

Patch: libfs: set SB_I_NOEXEC and SB_I_NODEV by default in init_pseudo()
Commit: b9d45d328fcda4f0d3d281b7d6f12d3f181d9381
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b9d45d328fcda4f0d3d281b7d6f12d3f181d9381

Commit description:

Since commit 1e7ab6f ("anon_inode: rework assertions"),
path_noexec() warns when an anonymous-inode file is mmap'd from a
superblock that has not set SB_I_NOEXEC. dma-buf backs its files this
way and never set the flag, so mmap of any exported buffer trips the
warning on a CONFIG_DEBUG_VFS=y kernel:

  WARNING: CPU: 11 PID: 121813 at fs/exec.c:118 path_noexec+0x47/0x50
   do_mmap+0x2b5/0x680
   vm_mmap_pgoff+0x129/0x210
   ksys_mmap_pgoff+0x177/0x240
   __x64_sys_mmap+0x33/0x70

init_pseudo() sets up internal SB_NOUSER mounts that are never
path-reachable. Set both flags here so every pseudo filesystem gets
them by default instead of each caller setting them.

SB_I_NODEV is inert for unreachable mounts. SB_I_NOEXEC has one
visible effect: an executable mapping of a pseudo-fs fd, such as a
dma-buf, now fails with -EPERM, which is the invariant the assertion
enforces. No in-tree caller maps these executable.

Reproduce on CONFIG_DEBUG_VFS=y:

  make -C tools/testing/selftests/dmabuf-heaps
  sudo ./tools/testing/selftests/dmabuf-heaps/dmabuf-heap -t system

Fixes: 1e7ab6f ("anon_inode: rework assertions")
Suggested-by: Christoph Hellwig <hch@infradead.org>
Cc: stable@vger.kernel.org
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Link: https://patch.msgid.link/20260604025315.245910-2-jhubbard@nvidia.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/exec.c
  fs/libfs.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=b9d45d328fcda4f0d3d281b7d6f12d3f181d9381

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

The original Linux kernel CVE announcement for CVE-2026-64596 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64596

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:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:H
  The Best / paranoid CVSS score: 5.3

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).