[CVE-2026-68145][IMPORTANT] iomap: fix out-of-bounds bitmap_set() with zero-length range

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

CVE: CVE-2026-68145
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: iomap: fix out-of-bounds bitmap_set() with zero-length range
Commit: fb4fad9105c88b1d82f1b3c39e3b6abea8249af6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fb4fad9105c88b1d82f1b3c39e3b6abea8249af6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68145
Analysis date: Mon, 10 Aug 2026 18:45:34 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local out-of-bounds bitmap_set() write in the iomap buffered write path can be triggered by a zero-length dirty range after a failed user-buffer copy, causing kernel memory corruption and at least a local DoS, with plausible but unproven privilege-escalation impact.

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

The original Linux kernel CVE announcement for CVE-2026-68145 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68145

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: IMPORTANT
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-2026-68145	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-191;CWE-682;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'An out of bounds write in the iomap buffered write path can occur because ifs_set_range_dirty computes the last block from off plus len minus 1 without first rejecting a zero length range. When len is 0, the unsigned calculation can underflow and produce a very large bitmap range, causing bitmap_set to write beyond the allocated iomap folio state bitmap. A local user process may reach this path when a buffered write copies zero bytes from user memory while the target folio is already uptodate. For the CVSS the PR:L is used because reliable triggering requires local code execution as a normal user, not administrator privileges. The issue is not directly network reachable and is triggered through local filesystem write operations. Impact is at least local denial of service via kernel memory corruption and kernel crash. In the worst case, because this is an out of bounds kernel write rather than a pure crash or leak, it may allow confidentiality or integrity impact and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	WRITE OOB DANGER DISK SIMPLEFIX KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A normal local process can plausibly reach the path through buffered filesystem writes and a user-buffer fault causing copied == 0.
* Memory corruption, strong corruption primitive: +2. The patch fixes an out-of-bounds bitmap_set() write beyond ifs->state due to unsigned underflow.
* Corruption primitive constrained or fixed-pattern: -1. The overwrite is a bitmap_set() pattern, not an arbitrary attacker-chosen payload or chosen address.
* Weak LPE concern: +1. Kernel heap memory corruption is present, but there is no demonstrated reclaim, target selection, callback control, or arbitrary write.
* Reliable kernel crash / strong DoS: +1. Writing far beyond the allocated bitmap can realistically corrupt kernel memory and crash the system.
* Important filesystem/storage path: +1. The bug is in iomap buffered write handling, a core filesystem write path used by major filesystems.

Paranoid additions:

* Confidentiality impact plausible: +1. If adjacent kernel objects are corrupted in an exploitable layout, limited or broader disclosure cannot be ruled out.
* Integrity impact plausible: +1. Kernel memory corruption in a filesystem write path can plausibly affect privileged kernel state, even though the primitive is constrained.

Not counted:

* Remote reachable: +0. No realistic network trigger is shown.
* Admin privilege requirement: +0. The described trigger does not require CAP_SYS_ADMIN.
* Dirty-Pipe-like ownership bypass: +0. This is kernel bitmap memory corruption, not demonstrated page-cache ownership or COW bypass.

## 3. Reachability analysis

The likely trigger is a local userspace write path where copy_folio_from_iter_atomic() returns 0 while the folio is already uptodate, causing __iomap_write_end() to call iomap_set_range_dirty() with copied == 0. This makes the bug reachable by a local process writing to a file on an affected iomap-based filesystem.

Namespaces and containers may matter because unprivileged container workloads can often write to filesystems backed by host kernel iomap code, although the exact exposure depends on mounted filesystems and write permissions. The path is not network reachable by itself, but it is in a common local filesystem write path rather than a rare debug-only interface.

Call-site confidence: high. The commit message explicitly identifies __iomap_write_end(), copy_folio_from_iter_atomic(), iomap_set_range_dirty(), and the zero-length copied path.

## 4. Severity interpretation

This is more than an ordinary Moderate issue because the patch fixes a concrete out-of-bounds kernel write, not just a warning, NULL dereference, leak, or validation cleanup. Realistic impact is at least local DoS through kernel memory corruption.

Theoretical privilege escalation is plausible because this is a kernel OOB write in a core filesystem path, but the exploitation evidence is not strong enough to assume reliable LPE. The overwrite is constrained to bitmap_set() behavior and lacks demonstrated attacker-controlled target selection or arbitrary payload control. Therefore the conservative score is Actionable Moderate, while the paranoid score reaches Strong Important candidate.

## 5. One-sentence report phrase

A local out-of-bounds bitmap_set() write in the iomap buffered write path can be triggered by a zero-length dirty range after a failed user-buffer copy, causing kernel memory corruption and at least a local DoS, with plausible but unproven privilege-escalation impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because the bug is a local unprivileged reachable out-of-bounds write in kernel memory on an important filesystem write path, even though the overwrite pattern appears constrained.

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

Patch: iomap: fix out-of-bounds bitmap_set() with zero-length range
Commit: fb4fad9105c88b1d82f1b3c39e3b6abea8249af6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fb4fad9105c88b1d82f1b3c39e3b6abea8249af6

Commit description:

ifs_set_range_dirty() and ifs_set_range_uptodate() compute last_blk
as (off + len - 1) >> i_blkbits.  When off is 0 and len is 0, the
unsigned subtraction underflows to SIZE_MAX, producing a huge
last_blk and nr_blks value that causes bitmap_set() to write far
beyond the ifs->state allocation.

Regarding ifs_set_range_uptodate(), it is temporarily safe because len
cannot be passed in as 0. However, for ifs_set_range_dirty() this is
reachable from __iomap_write_end(): when copy_folio_from_iter_atomic()
returns 0 (e.g. user buffer fault) and the folio is already uptodate,
the guard at the top of __iomap_write_end() does not trigger because
!folio_test_uptodate() is false, and iomap_set_range_dirty() is called
with copied == 0.

Add a !len guard to both functions before the computation, so that a
zero-length range is a no-op.

Fixes: 4ce02c6 ("iomap: Add per-block dirty state tracking to improve performance")
Cc: stable@vger.kernel.org # v6.6
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20260714082325.325163-5-yi.zhang@huaweicloud.com
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/iomap/buffered-io.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=fb4fad9105c88b1d82f1b3c39e3b6abea8249af6

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

The original Linux kernel CVE announcement for CVE-2026-68145 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68145

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 6
  Paranoid ActionableScore: 8

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).