[CVE-2026-64360][MODERATE REGULAR] hfs/hfsplus: zero-initialize buffer in hfs_bnode_read

[CVE-2026-64360][MODERATE REGULAR] hfs/hfsplus: zero-initialize buffer in hfs_bnode_read

[AL-KERNEL]

CVE: CVE-2026-64360
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: hfs/hfsplus: zero-initialize buffer in hfs_bnode_read
Commit: 34684a04777358b2b40ac729e54c8e45359e46b3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=34684a04777358b2b40ac729e54c8e45359e46b3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64360
Analysis date: Sat, 25 Jul 2026 14:35:36 -0400
ActionableScore: 4
ActionableScore lower bound: 1
Actionable bucket: Borderline manual review recommended
Manual review required: YES

Summary:
HFS and HFSplus btree node reads could return without initializing caller stack buffers, allowing a crafted local filesystem image to trigger uninitialized kernel value use during metadata parsing and potentially cause local DoS or undefined parser behavior.

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

The original Linux kernel CVE announcement for CVE-2026-64360 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64360

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
======================================================================

CVE-2026-64360	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-457;CWE-908;*CWE-20;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '6.6';DESCR 'hfs_bnode_read() in HFS and HFSplus can return before writing to the caller supplied output buffer when the bnode offset is invalid or the requested length is corrected to zero. Callers such as hfs_bnode_read_u16() and hfs_bnode_read_u8() may pass stack buffers and use the returned value unconditionally, causing use of uninitialized kernel stack data while parsing attacker controlled filesystem metadata. For the CVSS the PR:L is used in the paranoid score because some desktop, appliance, or delegated mount deployments may allow a local user or mount helper to trigger parsing of a crafted HFS image without full administrator rights. The issue is not network reachable and requires local filesystem parsing. Impact is at least local denial of service or undefined parser behavior, and in worst case may allow limited confidentiality or integrity impact due to uninitialized value use in filesystem metadata handling.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended (with actual score 3)  SKIP CVE-2026-64360 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	WRITE KASAN OOB DISK SIMPLEFIX LINUS SYZBOT  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=4
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended**

## 2. Signal breakdown

Paranoid score signals:

* Local unprivileged trigger: +1
  Applies only for desktop, appliance, automount, removable media, or delegated mount-helper scenarios where a local user can cause parsing of a crafted HFS/HFSplus image.

* Memory corruption, weak/indirect corruption candidate: +1
  The issue is uninitialized kernel stack data used by callers such as `hfs_bnode_read_u16()` and `hfs_bnode_read_u8()`. This is not a proven UAF, OOB write, or arbitrary write primitive, but it can influence filesystem metadata parsing.

* Availability impact realistic: +1
  syzbot/KMSAN reports show reachable uninitialized-value use. In production this may manifest as incorrect parser behavior, mount failure, warning, or crash depending on the affected call path.

* Important filesystem/storage path: +1
  The bug is in HFS/HFSplus btree node metadata parsing, which processes attacker-controlled filesystem images.

* Legacy / non-default subsystem: -1
  HFS/HFSplus is not a common default Linux filesystem and usually requires explicit mounting or automount of a crafted image.

Conservative interpretation:

* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2
  Direct mounting of HFS/HFSplus images normally requires administrative privileges.

* No strong memory corruption primitive counted.
  The patch shows zero-initialization to avoid uninitialized stack value use, not UAF, double-free, OOB write, controlled overwrite, or page-cache corruption.

* No confidentiality or integrity score is added in the conservative score.
  There is no concrete read-back path for leaking stack data and no demonstrated write or corruption sink.

## 3. Reachability analysis

A typical direct trigger requires mounting or otherwise parsing a crafted HFS/HFSplus filesystem image, which normally requires `CAP_SYS_ADMIN`. However, practical deployments may include desktop automount, removable-media handlers, container or appliance helpers, or delegated mount services that reduce the required attacker privilege to local user interaction with a crafted image.

The path is not network reachable. It is not a default hot path for most Linux servers, but it is reachable through ordinary filesystem parsing when HFS/HFSplus support is enabled.

Call-site confidence: medium. The patch body and commit message identify the affected helper and representative callers, but the full downstream use of each returned value is not included in the provided diff.

## 4. Severity interpretation

This behaves more like a borderline actionable Moderate than an Important-class vulnerability. The theoretical concern is kernel stack uninitialized-value use in attacker-controlled filesystem metadata parsing. The realistic evidence currently supports undefined parser behavior and possible local DoS, not a demonstrated privilege escalation primitive.

The paranoid score keeps the issue for manual review because uninitialized metadata values can sometimes become indices, lengths, flags, or object selectors in filesystem code. However, the patch does not show a concrete info leak, arbitrary write, UAF, or page-cache corruption primitive.

## 5. One-sentence report phrase

HFS and HFSplus btree node reads could return without initializing caller stack buffers, allowing a crafted local filesystem image to trigger uninitialized kernel value use during metadata parsing and potentially cause local DoS or undefined parser behavior.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is not strong enough for automatic Important classification, but it is kernel stack uninitialized-value use in filesystem metadata parsing with attacker-controlled input, so it should not be blindly auto-closed without checking downstream callers for index, length, or pointer-sensitive use.

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

Patch: hfs/hfsplus: zero-initialize buffer in hfs_bnode_read
Commit: 34684a04777358b2b40ac729e54c8e45359e46b3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=34684a04777358b2b40ac729e54c8e45359e46b3

Commit description:

hfs_bnode_read() can return early without writing to the output buffer
when is_bnode_offset_valid() fails or when check_and_correct_requested_
length() corrects the length to zero.  Callers such as hfs_bnode_read_
u16() and hfs_bnode_read_u8() pass stack-allocated buffers and use the
result unconditionally, leading to KMSAN uninit-value reports.

Rather than initializing at each individual call site, zero the buffer
at the start of hfs_bnode_read() before any validation checks.  This
ensures all callers in both hfs and hfsplus get a deterministic zero
value regardless of which early-return path is taken.

Reported-by: syzbot+217eb327242d08197efb@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=217eb327242d08197efb
Tested-by: syzbot+217eb327242d08197efb@syzkaller.appspotmail.com
Fixes: a431930 ("hfs: fix slab-out-of-bounds in hfs_bnode_read()")
Cc: stable@vger.kernel.org
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
Reviewed-by: Viacheslav Dubeyko <slava@dubeyko.com>
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>
Link: https://lore.kernel.org/r/20260505111300.3592757-3-tristmd@gmail.com
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/hfs/bnode.c
  fs/hfsplus/bnode.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=34684a04777358b2b40ac729e54c8e45359e46b3

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

The original Linux kernel CVE announcement for CVE-2026-64360 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64360

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:H/UI:N/S:U/C:N/I:N/A:L
  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: 1
  Paranoid ActionableScore: 4

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).