[CVE-2026-43068][MODERATE REGULAR] ext4: avoid allocate block from corrupted group in ext4_mb_find_by_goal()

[CVE-2026-43068][MODERATE REGULAR] ext4: avoid allocate block from corrupted group in ext4_mb_find_by_goal()

[AL-KERNEL]

CVE: CVE-2026-43068
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ext4: avoid allocate block from corrupted group in ext4_mb_find_by_goal()
Commit: fea6b2e250ff48f10d166011b57a8516ae5438c9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fea6b2e250ff48f10d166011b57a8516ae5438c9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43068
Analysis date: Tue, 09 Jun 2026 17:42:18 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline Moderate / not Important
Manual review required: NO

Summary:
An ext4 allocation error-handling bug can repeatedly select a block group with a corrupted bitmap, causing delayed allocation failures and possible local data loss, but the patch does not indicate memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-43068 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43068

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.

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

CVE-2026-43068	MODERATE	CHECK	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-703;CWE-754;CWE-354;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.3';DESCR 'ext4 block allocation can repeatedly fail when ext4_mb_find_by_goal() selects a goal block group whose block bitmap is already marked corrupted. This can make delayed allocation fail and can lead to data loss for writes on the affected filesystem. For the CVSS the PR:L is used because a local user or process generally needs write access to a mounted ext4 filesystem that is already corrupted or attacker influenced. The issue is not network reachable and does not show a memory corruption primitive such as UAF or OOB write. Impact is mainly local denial of service and limited filesystem integrity impact due to failed allocation and lost write data.';NO MANUAL CHECK; ,and ActionableScore result is Borderline Moderate / not Important (with actual score 4)	YES	DISK IMPROVEONLY  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline Moderate / not Important**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local process with write access to the affected mounted ext4 filesystem can enter the delayed allocation path.

* Integrity impact plausible: +1
  The commit log explicitly reports repeated delayed allocation failures with "Data will be lost", so limited filesystem data integrity impact is plausible.

* Important filesystem/storage path: +1
  This is ext4 block allocation and delayed allocation handling, which is an important filesystem write path.

* Broad/default/common subsystem: +1
  ext4 is widely deployed and commonly used.

Negative signal:

* Special corrupted filesystem metadata precondition: -1
  The issue requires a block group bitmap already marked corrupted or an attacker-influenced corrupted filesystem state. The patch does not show that an ordinary user can create the corruption from a clean filesystem.

Paranoid additional signal:

* Availability impact realistic: +1
  Repeated allocation failure on an affected filesystem can cause persistent write failures or service-level disruption, but not a demonstrated kernel crash or privilege escalation.

Not awarded:

* No generic memory corruption.
  The patch does not show UAF, OOB write, double free, type confusion, stale pointer reuse, or arbitrary write.

* No privilege escalation signal.
  The failure mode is allocation failure and possible data loss, not a demonstrated LPE primitive.

* No network reachability.
  This is a local filesystem write/allocation path.

## 3. Reachability analysis

A local user or process with write access to the mounted ext4 filesystem can trigger the affected allocation path if allocation keeps selecting a corrupted goal block group. Mounting a crafted ext4 image would normally require privileges, but writing to an already mounted affected filesystem may not. Containers only matter if they can write to the affected ext4 mount or influence the underlying filesystem image. The path is common because ext4 and delayed allocation are widely deployed, but realistic exploitation depends on the filesystem already having corrupted block group bitmap state.

## 4. Severity interpretation

This behaves like an ordinary to borderline Moderate filesystem reliability issue, not an Important-class kernel vulnerability. The realistic impact is local data loss or write failure on an affected ext4 filesystem. Theoretical security impact is limited because the patch does not indicate memory corruption, page-cache corruption, permission bypass, credential exposure, or privilege escalation. The paranoid score is slightly higher only because persistent write failures in an important filesystem path can have operational impact.

## 5. One-sentence report phrase

An ext4 allocation error-handling bug can repeatedly select a block group with a corrupted bitmap, causing delayed allocation failures and possible local data loss, but the patch does not indicate memory corruption or privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

This is not an Important candidate, but the score is borderline because ext4 is broadly deployed and the impact includes possible data loss in a core filesystem write path.

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

Patch: ext4: avoid allocate block from corrupted group in ext4_mb_find_by_goal()
Commit: fea6b2e250ff48f10d166011b57a8516ae5438c9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fea6b2e250ff48f10d166011b57a8516ae5438c9

Commit description:

There's issue as follows:
...
EXT4-fs (mmcblk0p1): Delayed block allocation failed for inode 206 at logical offset 0 with max blocks 1 with error 117
EXT4-fs (mmcblk0p1): This should not happen!! Data will be lost

EXT4-fs (mmcblk0p1): Delayed block allocation failed for inode 206 at logical offset 0 with max blocks 1 with error 117
EXT4-fs (mmcblk0p1): This should not happen!! Data will be lost

EXT4-fs (mmcblk0p1): Delayed block allocation failed for inode 206 at logical offset 0 with max blocks 1 with error 117
EXT4-fs (mmcblk0p1): This should not happen!! Data will be lost

EXT4-fs (mmcblk0p1): Delayed block allocation failed for inode 206 at logical offset 0 with max blocks 1 with error 117
EXT4-fs (mmcblk0p1): This should not happen!! Data will be lost

EXT4-fs (mmcblk0p1): Delayed block allocation failed for inode 2243 at logical offset 0 with max blocks 1 with error 117
EXT4-fs (mmcblk0p1): This should not happen!! Data will be lost

EXT4-fs (mmcblk0p1): Delayed block allocation failed for inode 2239 at logical offset 0 with max blocks 1 with error 117
EXT4-fs (mmcblk0p1): This should not happen!! Data will be lost

EXT4-fs (mmcblk0p1): error count since last fsck: 1
EXT4-fs (mmcblk0p1): initial error at time 1765597433: ext4_mb_generate_buddy:760
EXT4-fs (mmcblk0p1): last error at time 1765597433: ext4_mb_generate_buddy:760
...

According to the log analysis, blocks are always requested from the
corrupted block group. This may happen as follows:
ext4_mb_find_by_goal
  ext4_mb_load_buddy
   ext4_mb_load_buddy_gfp
     ext4_mb_init_cache
      ext4_read_block_bitmap_nowait
      ext4_wait_block_bitmap
       ext4_validate_block_bitmap
        if (!grp || EXT4_MB_GRP_BBITMAP_CORRUPT(grp))
         return -EFSCORRUPTED; // There's no logs.
 if (err)
  return err;  // Will return error
ext4_lock_group(ac->ac_sb, group);
  if (unlikely(EXT4_MB_GRP_BBITMAP_CORRUPT(e4b->bd_info))) // Unreachable
   goto out;

After commit 9008a58 ("ext4: make the bitmap read routines return
real error codes") merged, Commit 163a203 ("ext4: mark block group
as corrupt on block bitmap error") is no real solution for allocating
blocks from corrupted block groups. This is because if
'EXT4_MB_GRP_BBITMAP_CORRUPT(e4b->bd_info)' is true, then
'ext4_mb_load_buddy()' may return an error. This means that the block
allocation will fail.
Therefore, check block group if corrupted when ext4_mb_load_buddy()
returns error.

Fixes: 163a203 ("ext4: mark block group as corrupt on block bitmap error")
Fixes: 9008a58 ("ext4: make the bitmap read routines return real error codes")
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260302134619.3145520-1-yebin@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/ext4/mballoc.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=fea6b2e250ff48f10d166011b57a8516ae5438c9

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

The original Linux kernel CVE announcement for CVE-2026-43068 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43068

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: 3
  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).