[CVE-2026-97926][MODERATE 7.0] ufs: validate cylinder group metadata before caching it

[CVE-2026-97926][MODERATE 7.0] ufs: validate cylinder group metadata before caching it

[AL-KERNEL]

CVE: CVE-2026-97926
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ufs: validate cylinder group metadata before caching it
Commit: 5902a95066883cf96fa15b2680694fc5dd0c7d11
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5902a95066883cf96fa15b2680694fc5dd0c7d11
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97926
Analysis date: Fri, 25 Sep 2026 21:24:10 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-97926 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97926

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.

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

CVE-2026-97926	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-97926 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-125;CWE-129;*CWE-20;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:H/A:H';BEST CVSS score: '7.8';DESCR 'A crafted UFS filesystem image can place unchecked values in cg_cgx and the cylinder group rotor fields. The c_cgx value is later used to index the cylinder summary array and can cause a 32 bit write outside the s_csp allocation. Invalid c_frotor or c_irotor values can also make an unsigned bitmap length wrap and cause ubh_scanc to walk beyond the cylinder group buffers. This provides concrete out of bounds memory access in the kernel and can cause a kernel crash, corrupt kernel data, or potentially support local privilege escalation. For the CVSS the PR:L value is used for the paranoid score because a local user, container root, or service account may have delegated access to mount or activate filesystem images without having unrestricted host administrator privileges. The issue is not directly network reachable because the crafted image must be mounted locally, although the image may be delivered over a network or removable media. Impact can include confidentiality, integrity, and availability loss because the vulnerable paths include both an out of bounds scan and a 32 bit out of bounds write.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)  SKIP CVE-2026-97926 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	OOB DISK LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* No floor or cap was applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Strong corruption primitive. Attacker-controlled `cg_cgx` becomes an unchecked index and causes a concrete 32-bit write outside the `s_csp` allocation. The destination is constrained to an index-derived location and the written value is not shown to be arbitrary, but this remains an attacker-influenced kernel OOB write.
* +1 Reliable kernel crash or strong DoS. Invalid rotor values can wrap the bitmap scan length and make `ubh_scanc()` traverse far beyond cylinder-group buffers. The OOB write can also corrupt kernel memory and plausibly crash the system.
* +1 Integrity impact. The OOB write modifies kernel memory outside its intended allocation. This scores the demonstrated consequence, not a second corruption primitive.
* +1 Important filesystem/storage path. The malformed values are consumed during ordinary UFS allocation and free operations after the cylinder-group metadata has been cached.
* +0 Confidentiality. The OOB scan is concrete, but no path returning scanned kernel bytes to userspace, storage, or the network is established.
* +0 Local unprivileged trigger. Supplying a crafted image is established, but the complete writable mount and allocation path below host-admin privilege is not.
* +0 Strong LPE leverage beyond the OOB write. Controlled target selection, arbitrary payload, credential overwrite, controlled reclaim, and a demonstrated privilege-escalation chain are not established.
* +0 Extra Availability. The same potential crash is not counted twice.
* +0 Remote reachability. Network delivery of an image is not direct network triggering of the vulnerable kernel operation.
* No negative privilege penalty was applied because host-admin-equivalent privilege is not conclusively established for every deployment.
* No rarity penalty was applied merely because UFS is less common than major Linux filesystems.

3. Reachability analysis

The attacker must construct a malformed UFS image, have it mounted, and cause an allocation or free operation that consumes the cached cylinder-group fields. Direct mounting of a writable block-backed UFS image commonly requires administrative mount authority and access to the backing device. However, the supplied evidence does not resolve whether the evaluated deployment uses host mounting, a delegated mounting service, removable-media automounting, or another below-host-admin path.

Accordingly, `HostAdminRequired` and `NamespaceScopedPrivilege` remain `UNKNOWN`. The complete operation is not shown to work inside an attacker-created user namespace. Prior setup requires UFS support and a writable mounted image, but no special hardware, race, or timing condition is involved.

4. Primitive and severity interpretation

The exact primitive is an attacker-influenced kernel OOB write through `fs_cs(ucpi->c_cgx)` and an OOB bitmap scan caused by wrapped length calculations derived from `c_frotor` or `c_irotor`. This is not a UAF, repeated release, stale-object operation, or cleanup-only fault.

The 32-bit write is concrete and occurs outside the `s_csp` allocation, so the issue is `CLASS_A`. Its value and final target are not demonstrated to provide arbitrary kernel write, but the attacker controls the malformed index and can reach the write through normal filesystem allocation or free processing.

The demonstrated consequences are kernel memory corruption and potential crash. Local privilege escalation is plausible enough to require review because a concrete OOB write exists, but it is not demonstrated. `ImportantCandidate=YES` and `AutoDowngradeAllowed=NO` preserve this unresolved corruption primitive without claiming proven LPE or arbitrary code execution.

5. One-sentence report phrase

A crafted UFS image can supply unchecked cylinder-group indices and rotor values that cause an index-directed 32-bit kernel OOB write or an unbounded bitmap scan during allocation and free operations, requiring manual exploitation review.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch establishes a concrete attacker-influenced kernel OOB write, while the degree of target control, practical mount privilege, and privilege-escalation potential remain unresolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: ufs: validate cylinder group metadata before caching it
Commit: 5902a95066883cf96fa15b2680694fc5dd0c7d11
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5902a95066883cf96fa15b2680694fc5dd0c7d11

Commit description:

ufs_read_cylinder() copies the cylinder group index and the rotor
positions straight from the on-disk group and caches them without any
check:

	ucpi->c_cgx    = fs32_to_cpu(sb, ucg->cg_cgx);
	ucpi->c_rotor  = fs32_to_cpu(sb, ucg->cg_rotor);
	ucpi->c_frotor = fs32_to_cpu(sb, ucg->cg_frotor);
	ucpi->c_irotor = fs32_to_cpu(sb, ucg->cg_irotor);

They are then used as indices during allocation and free:

  - c_cgx indexes the cylinder summary array as
    UFS_SB(sb)->fs_cs(ucpi->c_cgx), so a value past s_ncg writes a 32
    bit count outside the s_csp allocation.

  - c_frotor becomes a bitmap scan start, start = c_frotor >> 3, and
    then length = ((s_fpg + 7) >> 3) - start. A start beyond the block
    bitmap wraps the unsigned length to a huge value, so ubh_scanc()
    walks far past the cylinder group buffers. c_irotor drives the
    inode bitmap the same way.

A crafted image can set any of these freely, turning an ordinary
allocation into an out of bounds access.

Reject a cylinder group whose recorded index does not match the group
being read, or whose rotors fall outside the group, before the metadata
is cached. Valid filesystems keep cg_cgx equal to the group number and
the rotors within the group, so only malformed images are rejected.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Ali Ahmet Memis <ali@iusegentoo.com>
Link: https://patch.msgid.link/20260801071306.59484-3-ali@iusegentoo.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/ufs/cylinder.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=5902a95066883cf96fa15b2680694fc5dd0c7d11

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

The original Linux kernel CVE announcement for CVE-2026-97926 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97926

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:L/I:H/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: unknown
  Paranoid ActionableScore: 5

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).