[CVE-2026-64324][MODERATE 7.0] udf: validate free block extents against the partition length

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

CVE: CVE-2026-64324
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: udf: validate free block extents against the partition length
Commit: fdd6229d2ae9914c1f25d1041db0f4f312a4fa76
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fdd6229d2ae9914c1f25d1041db0f4f312a4fa76
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64324
Analysis date: Sat, 25 Jul 2026 10:13:52 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A crafted UDF filesystem can bypass a free block extent boundary check and cause an out of bounds bitmap pointer access during ftruncate, allowing a local user with removable media mount access to reliably panic the kernel.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64324 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64324

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 7.0
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-2026-64324	MODERATE	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:L/I:L/A:H';CWE-125;CWE-129;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.6';DESCR 'A crafted UDF filesystem can bypass the partition length check in udf_free_blocks because the final bound check used logicalBlockNum plus count and dropped the extent offset. The resulting block group can point past the space bitmap array and load_block_bitmap can index s_block_bitmap out of range, producing an out of bounds pointer slot read followed by a kernel panic. For the CVSS the PR:L is selected for the paranoid score because desktop systems may allow an active local user to mount removable UDF media through UDisks or polkit without CAP_SYS_ADMIN, and the user can then trigger the crash with ftruncate on a writable file. The issue is not network reachable. Impact is at least local denial of service via kernel crash. The reproduced primitive is an out of bounds read and invalid pointer dereference, so confidentiality and integrity are kept limited in the paranoid score rather than treated as proven privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 6)  SKIP CVE-2026-64324 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 56e69e59751d20993f243fb7dd6991c4e522424c YES  NO NO unknown 	MAYBE	OOB DANGER DISK IMPROVEONLY LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. On desktop systems UDisks and polkit may allow the active local user to mount removable UDF media without CAP_SYS_ADMIN.
* Memory corruption, weak or indirect corruption candidate: +1. The concrete primitive is an out of bounds read of a bitmap pointer slot and invalid pointer use, not an OOB write or UAF.
* Reliable kernel crash / strong DoS: +1. The commit states that a single ftruncate on a file backed by the crafted extent reliably panics the kernel.
* Filesystem/storage metadata corruption reachable by an unprivileged local user: +1. Crafted UDF filesystem metadata drives the invalid free block extent path.
* Availability impact realistic: +1. The impact is a reliable host kernel panic from a simple local file operation after mounting the crafted filesystem.

Paranoid additional signal:

* Important filesystem/storage path: +1. The bug is in UDF block allocation/free bitmap handling, and the malformed metadata reaches filesystem allocator bookkeeping after mount.

Not counted:

* Remote reachable: +0. No network path.
* Strong LPE plausibility: +0. No UAF reclaim, write primitive, type confusion, callback control, or refcount takeover is shown.
* Confidentiality / integrity impact: +0 for ActionableScore. The CVSS paranoid C:L/I:L is defensible for triage sensitivity, but the demonstrated primitive is read-only OOB followed by panic with no disclosure or writeback path.
* Admin penalty: not applied for the main score because common desktop automount policy can make this reachable by an active local user. On systems requiring CAP_SYS_ADMIN to mount UDF images, the practical score would be lower.

## 3. Reachability analysis

A local user can trigger the bug by supplying a crafted UDF filesystem and truncating a writable file whose extent causes logicalBlockNum plus offset plus count to exceed the partition length. Desktop automount configurations are important because UDisks and polkit may allow an active user to mount removable UDF media without direct CAP_SYS_ADMIN. In stricter server configurations where only root can mount the image, this becomes a privileged local DoS and is less actionable.

Namespaces and containers do not generally make this unprivileged unless mount capability or a device/media mounting service is delegated. The path is not network reachable. The affected code is not a packet parser or remote protocol handler, but UDF support and removable-media workflows are realistic enough that this should not be treated as an obscure kernel-only correctness issue.

Call-site confidence: high. The patch and commit identify the failing path through udf_free_blocks and load_block_bitmap, and the described reproducer gives a direct local trigger.

## 4. Severity interpretation

This behaves like an actionable Moderate issue rather than ordinary Low or clear Important. The realistic demonstrated impact is local availability loss through reliable kernel panic. The theoretical memory-safety concern is limited because the primitive is an out of bounds pointer slot read and invalid dereference, not a controlled write, UAF reuse, or object replacement primitive. Manual review is still warranted because crafted filesystem metadata reaches kernel allocator bitmap logic and the unprivileged desktop automount scenario makes the trigger practical.

## 5. One-sentence report phrase

A crafted UDF filesystem can bypass a free block extent boundary check and cause an out of bounds bitmap pointer access during ftruncate, allowing a local user with removable media mount access to reliably panic the kernel.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is a crafted filesystem parser and metadata handling bug with an OOB access and reliable kernel panic, and desktop automount policy can make it reachable by an unprivileged local user.

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

Patch: udf: validate free block extents against the partition length
Commit: fdd6229d2ae9914c1f25d1041db0f4f312a4fa76
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fdd6229d2ae9914c1f25d1041db0f4f312a4fa76

Commit description:

udf_free_blocks() checks the logical block number and count against the
partition length, but drops the extent offset from that final bound.  A
crafted extent can pass the guard while logicalBlockNum + offset + count
points past the partition, which later indexes past the space bitmap
array.

A single ftruncate(2) on a file backed by such an extent reliably
panics the kernel.  This is a local availability issue.  On desktop
systems where UDisks/polkit allows the active user to mount removable
UDF media without CAP_SYS_ADMIN, an unprivileged local user can supply
the crafted filesystem and trigger the panic by truncating a writable
file on it.  Systems that require root or CAP_SYS_ADMIN to mount the
image have a higher prerequisite.

No confidentiality or integrity impact is claimed: the reproduced
primitive is an out-of-bounds read of a bitmap pointer slot followed by
a kernel panic.

Use the already computed logicalBlockNum + offset + count value for the
partition length check.  Also make load_block_bitmap() reject an
out-of-range block group before indexing s_block_bitmap[], so corrupted
callers cannot walk past the flexible array.

Fixes: 56e69e5 ("udf: prevent integer overflow in udf_bitmap_free_blocks()")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Link: https://patch.msgid.link/20260515142327.1120767-1-michael.bommarito@gmail.com
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/udf/balloc.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=fdd6229d2ae9914c1f25d1041db0f4f312a4fa76

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

The original Linux kernel CVE announcement for CVE-2026-64324 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64324

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:L/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:L/I:L/A:H
  The Best / paranoid CVSS score: 6.6

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: 5
  Paranoid ActionableScore: 6

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).