[CVE-2026-89557][MODERATE REGULAR] md: do overflow check for sb->bblog_shift in super_1_load()

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

CVE: CVE-2026-89557
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: md: do overflow check for sb->bblog_shift in super_1_load()
Commit: 75d15738fd33a782606d0dc80cfeff47edf2ddd8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=75d15738fd33a782606d0dc80cfeff47edf2ddd8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89557
Analysis date: Fri, 11 Sep 2026 21:02:16 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89557 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89557

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89557	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89557 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:H/UI:N/S:U/C:L/I:L/A:H';CWE-190;*CWE-20;CWE-1284;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.6';DESCR 'An integer overflow in md v1 metadata loading can occur because sb bblog_shift is read from the on disk superblock as an 8 bit value and was used as a shift count without validating it against the width of sector_t. A crafted md member device with a bogus bblog_shift can make sector and count calculations overflow while the kernel imports the bad block log through the badblocks API, which may lead to array load failure, kernel warnings, or denial of service. For the CVSS the PR:H because reliable triggering normally requires administrative control over md assembly or the ability to present a crafted block device to the host. The issue is not network reachable. Impact is primarily local denial of service, with a conservative worst case of limited confidentiality or integrity impact plus high availability impact if the overflow corrupts badblocks state in a way that affects later kernel processing.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	DISK LOG IMPROVEONLY LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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

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

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap changed the score.

2. Signal breakdown

Conservative positive signals:

* +1 Weak or indirect corruption candidate. `sb->bblog_shift` is attacker-influenced on-disk md metadata and is used as an unchecked shift count before being passed into badblocks handling.
* +1 Important storage path. The bug is in md v1 metadata loading and badblocks import, which is a kernel storage metadata path.
* +1 Local storage availability concern. A crafted or corrupt md member may cause array load failure, warning, or badblocks processing failure.

Conservative negative signals:

* -2 Host-admin-equivalent trigger requirement. Reliable triggering normally requires administrative control over md array assembly or the ability to present a crafted block device to the host.

Paranoid additional signal:

* +1 Constrained active metadata corruption concern. The overflow may propagate into badblocks state handling, so a limited kernel metadata corruption concern is defensible for triage, but no arbitrary write or controlled memory corruption primitive is established.

Important non-awarded signals:

* Real UAF not awarded. No stale lifetime use, double release, or refcount mutation is shown.
* CLASS_A not awarded. No controlled overwrite, arbitrary write, controlled reclaim, authorization bypass, COW violation, or demonstrated LPE is established.
* Strong LPE not awarded. The patch supports integer overflow in metadata parsing, not a concrete privilege-escalation primitive.
* Confidentiality and integrity high-impact bonuses not awarded. The supplied evidence does not establish information disclosure, protected-object modification, or attacker-directed kernel write.
* Remote reachability not awarded. This is storage metadata parsing, not a network-triggered path.
* Extra availability not awarded beyond the local storage DoS concern. No host-wide reliable kernel panic is demonstrated.
* Concrete repeated release or refcount signal not applicable.
* FLOOR 5 not applicable because there is no UAF, repeated lifetime mutation, controlled corruption, or meaningful below-host-admin exploitation surface.

3. Reachability analysis

The evaluated attacker must cause the kernel to load md v1 metadata containing a crafted `bblog_shift` value. In normal operation this requires host administrative control over md assembly, block-device provisioning, or an equivalent ability to make the host parse attacker-controlled md superblock metadata. This supports `HostAdminRequired=YES`.

No namespace-scoped capability path is established. There is no evidence that an ordinary unprivileged user can create the required md device and force `super_1_load()` to process crafted metadata entirely inside an attacker-controlled namespace.

Prior privileged setup is material if the attacker is not already host admin, because a block device or md member containing the crafted superblock must be presented to the host. No network trigger is established. No race or timing requirement is involved. Special hardware is not required beyond storage/block-device access.

4. Primitive and severity interpretation

The exact primitive is an unchecked integer shift count loaded from on-disk md superblock metadata. The value can exceed the width of `sector_t`, causing overflow or invalid arithmetic when calculating badblock sector and count values and when passing them into the badblocks API.

This is not a UAF and not a repeated lifetime/refcount mutation. It is best classified as CLASS_C because the demonstrated behavior is invalid arithmetic and potentially invalid badblocks state during storage metadata loading, without an independently useful memory corruption primitive.

The demonstrated consequence is local denial of service or failure during md array metadata loading. The speculative consequence is limited corruption of badblocks bookkeeping if the overflow reaches deeper badblocks handling. That concern justifies manual review but does not establish CLASS_A, Important severity, arbitrary kernel write, or privilege escalation.

ImportantCandidate=NO because the path appears host-admin gated and no strong exploitation primitive is shown. AutoDowngradeAllowed=YES because the issue is a constrained storage metadata validation bug without UAF, controlled corruption, boundary bypass, or demonstrated LPE.

Likely operational severity is Low to ordinary Moderate, with manual review recommended mainly because integer overflow reaches kernel storage metadata handling.

5. One-sentence report phrase

An unchecked `bblog_shift` value from md v1 on-disk metadata can overflow sector and count calculations during badblocks import, allowing a crafted md member device to cause local storage availability failures or limited badblocks state corruption under host-admin-controlled md assembly conditions.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is not an Important candidate from the supplied evidence, but it is an integer overflow in a kernel storage metadata path and the commit explicitly mentions possible overflow inside badblocks handling.

7. Call-site confidence

Call-site confidence: medium

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

Patch: md: do overflow check for sb->bblog_shift in super_1_load()
Commit: 75d15738fd33a782606d0dc80cfeff47edf2ddd8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=75d15738fd33a782606d0dc80cfeff47edf2ddd8

Commit description:

In super_1_load(), sb->bblog_shift is an __u8 type value loaded from on-
disk superblock. It is used for badblocks API badblocks_set() by the
following sequence,

 1930   rdev->badblocks.shift = sb->bblog_shift;
 1931   for (i = 0 ; i < (sectors << (9-3)) ; i++, bbp++) {
 1932           u64 bb = le64_to_cpu(*bbp);
 1933           int count = bb & (0x3ff);
 1934           u64 sector = bb >> 10;
 1935           sector <<= sb->bblog_shift;
 1936           count <<= sb->bblog_shift;
 1937           if (bb + 1 == 0)
 1938                   break;
 1939           if (!badblocks_set(&rdev->badblocks, sector, count, 1))
 1940                   return -EINVAL;
 1941   }

bb->bblog_shit is in range of 0-255, variable sector is 64bit width, for
an invalid bb->bblog_shit, it is possible to make sector be overflowed
by the following calculation,
 1935           sector <<= sb->bblog_shift;
Then in turn when call badblocks_set() at line 1939 with the invalid
rdev->badblocks.shift set at line 1930, may result an overflow inside
_badblocks_clear() in block/badblocks.c.

Although there are many places to call badblocks APIs, the non-zero
shift value is only used in super_1_load(), other places always use 0 as
the shift value. Therefore it is unnecessary to do a general shift value
overflow check inside badblock API, and just check here as the caller.

This may avoid unnecessary check, make the badblocks API code more simple
and elegant.

Fixes: 2699b67 ("md: load/store badblock list from v1.x metadata")
Fixes: 1726c77 ("badblocks: improve badblocks_set() for multiple ranges handling")
Cc: stable@vger.kernel.org
Cc: Ramesh Adhikari <adhikari.resume@gmail.com>
Signed-off-by: Coly Li <colyli@fygo.io>
Reviewed-by: Yu Kuai <yukuai@fygo.io>
Link: https://patch.msgid.link/20260720111400.2120834-1-colyli@fygo.io
Signed-off-by: Yu Kuai <yukuai@fygo.io>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  block/badblocks.c
  drivers/md/md.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=75d15738fd33a782606d0dc80cfeff47edf2ddd8

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

The original Linux kernel CVE announcement for CVE-2026-89557 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89557

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:H/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.6

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: 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: 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).