[CVE-2026-68187][MODERATE REGULAR] exec: fix unsigned loop counter wrap in transfer_args_to_stack()

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

CVE: CVE-2026-68187
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: exec: fix unsigned loop counter wrap in transfer_args_to_stack()
Commit: c62bb00caba66e01fb578d5f0302f247dc64930a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c62bb00caba66e01fb578d5f0302f247dc64930a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68187
Analysis date: Mon, 10 Aug 2026 11:30:38 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended / not Strong Important by default
Manual review required: YES

Summary:
An unsigned loop counter underflow in transfer_args_to_stack on CONFIG_MMU=n systems can let a local user trigger an out of bounds bprm page pointer read during execve, causing a kernel panic and possibly copying unintended page contents into the new process stack.

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

The original Linux kernel CVE announcement for CVE-2026-68187 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68187

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
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CVE-2026-68187	MODERATE	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:L/A:H';CWE-191;CWE-125;CWE-754;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.3';DESCR 'An unsigned loop counter in transfer_args_to_stack can wrap when bprm p falls below one page on non MMU builds. After the index 0 iteration the next index becomes ULONG_MAX, so the kernel reads before the bprm page array, treats the value as a page pointer, maps it with kmap_local_page, and copies a page into the stack of the process being created. A local user can potentially trigger this through execve with a very large argument or environment string on affected binfmt_flat or binfmt_elf_fdpic systems. For the CVSS the PR:L is used because local code execution by an unprivileged user is sufficient in the affected configuration. The issue is not network reachable. The base impact is local denial of service via kernel panic. The paranoid impact also accounts for a possible confidentiality leak and limited integrity concern because an out of bounds page pointer may cause data from an unintended page to be copied into user visible process memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / not Strong Important by default (with actual score 4)	YES	DANGER DISK HARDWARE LINUS MEMORY  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended / not Strong Important by default**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local user can potentially trigger the path through execve with a large argv or env string on affected noMMU systems.
* Memory corruption, weak indirect corruption candidate: +1. The bug causes an unsigned underflow and an out of bounds read from bprm page array, producing a bogus page pointer passed to kmap_local_page. This is not a strong write or reclaim primitive.
* Reliable kernel crash / strong DoS: +1. The commit includes a concrete Oops and kernel panic in transfer_args_to_stack.
* Availability impact realistic: +1. The demonstrated impact is host wide kernel panic, not just a warning.
* Rare AND difficult to reach subsystem/configuration: -1. Only CONFIG_MMU=n builds are affected, through binfmt_flat and binfmt_elf_fdpic on nommu.

Paranoid additional signal:

* Confidentiality impact plausible: +1. A bogus page pointer may cause PAGE_SIZE bytes from an unintended mapped page to be copied into the stack of the newly created process, making limited data exposure technically plausible.

Not counted:

* Remote reachable: +0. No network path.
* Strong LPE plausibility: +0. No attacker controlled reclaim, write primitive, type confusion, callback control, or refcount takeover is shown.
* Generic strong memory corruption: +0. The concrete primitive is OOB pointer read plus invalid mapping/copy, not an OOB write or UAF.
* Integrity impact: +0. The copied data goes into the new process stack and does not show privileged object modification.

## 3. Reachability analysis

The trigger is local via execve with carefully sized argument or environment data. PR is effectively local unprivileged in the affected configuration because ordinary users can call execve and control argv and envp. Namespaces do not materially change the trigger because this is a core exec path once the affected binfmt and noMMU build are present.

The main limiting factor is exposure. CONFIG_MMU=n is uncommon for general purpose Linux systems and is mostly relevant to embedded or special architecture deployments. The affected transfer_args_to_stack path is used by binfmt_flat and binfmt_elf_fdpic on nommu only. Realistic exploitation is therefore narrow, but the crash path is concrete and the possible unintended page copy makes this worth manual review.

Call-site confidence: high. The patch and commit message include the affected function, the loop condition, the bad index behavior, and the crash trace.

## 4. Severity interpretation

This behaves primarily like a local DoS in a rare noMMU configuration. The realistic demonstrated impact is kernel panic.

The theoretical concern is stronger than an ordinary NULL dereference because the bug reads outside the bprm page array, treats the result as a page pointer, and copies a full page into user visible process stack memory. That supports a paranoid confidentiality concern, but not an Important-class privilege escalation assessment without additional evidence of controllable target selection, kernel write, object reuse, or sensitive data disclosure reliability.

Overall this is a borderline manual-review issue rather than a clear Strong Important candidate.

## 5. One-sentence report phrase

An unsigned loop counter underflow in transfer_args_to_stack on CONFIG_MMU=n systems can let a local user trigger an out of bounds bprm page pointer read during execve, causing a kernel panic and possibly copying unintended page contents into the new process stack.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the common outcome is DoS, but the bug includes an out of bounds page pointer read followed by copying PAGE_SIZE bytes into user visible memory, so limited confidentiality impact should be reviewed manually despite the rare noMMU exposure.

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

Patch: exec: fix unsigned loop counter wrap in transfer_args_to_stack()
Commit: c62bb00caba66e01fb578d5f0302f247dc64930a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c62bb00caba66e01fb578d5f0302f247dc64930a

Commit description:

The stop value is derived from bprm->p >> PAGE_SHIFT. The index variable
is an unsigned long. If bprm->p drops below PAGE_SIZE and stop becomes
zero the loop condition index >= stop is always true.

After the index == 0 iteration the decrement wraps to ULONG_MAX and
bprm->page[ULONG_MAX] reads sizeof(void *) bytes in front of the array.
The pointer has wrapped to -1. That garbage pointer is then passed to
kmap_local_page() and PAGE_SIZE bytes are copied from wherever that
lands into the stack of the process being created. And the loop doesn't
terminate either...

Getting there only requires bprm->p < PAGE_SIZE. On !MMU
bprm_set_stack_limit() and bprm_hit_stack_limit() are empty. So the only
constraint on how far bprm->p is pushed down is valid_arg_len(), i.e.
that each individual string still fits in what is left.

bprm->p starts at PAGE_SIZE * MAX_ARG_PAGES - sizeof(void *) so a
single argument or environment string of a little over 31 pages leaves
it in the first page:

  Oops - load access fault [#1]
  CPU: 0 UID: 0 PID: 1 Comm: victim Not tainted 7.2.0-rc4 #1
  epc : __memcpy+0xd4/0xf8
   ra : transfer_args_to_stack+0xaa/0xae
   s4 : ffffffffffffffff   s2 : 0000000000000000
   a1 : ffffffdc98000000   a2 : 0000000000001000
  status: 0000000a00001880 badaddr: ffffffdc98000000 cause: 0000000000000005
  [<801a5324>] __memcpy+0xd4/0xf8
  [<800d5f6a>] load_flat_binary+0x43a/0x65e
  [<800a2de4>] bprm_execve+0x1d4/0x316
  [<800a351a>] do_execveat_common+0x12e/0x138
  [<800a3d44>] __riscv_sys_execve+0x38/0x4e
  Kernel panic - not syncing: Fatal exception in interrupt

This is an arcane bug but we should still fix it.

Count down from MAX_ARG_PAGES so the loop ends when index reaches stop,
stop == 0 included. The iterations performed are unchanged for every
other value of stop.

Only CONFIG_MMU=n builds are affected, transfer_args_to_stack() is used
by binfmt_flat and binfmt_elf_fdpic on nommu only.

The loop predates git history. commit 7e7ec6a
("elf_fdpic_transfer_args_to_stack(): make it generic") only moved it
from binfmt_elf_fdpic.c into fs/exec.c and narrowed the copy to the used
part of the first page. The condition and the decrement are unchanged
from 2.6.12-rc2.

Link: https://patch.msgid.link/20260721-hochachtung-staumauer-pigmente-15d71f7d7d04@brauner
Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/exec.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=c62bb00caba66e01fb578d5f0302f247dc64930a

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

The original Linux kernel CVE announcement for CVE-2026-68187 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68187

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:H
  The Best / paranoid CVSS score: 7.3

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).