[CVE-2026-64191][IMPORTANT] i2c: stub: Reject I2C block transfers with invalid length

[CVE-2026-64191][IMPORTANT] i2c: stub: Reject I2C block transfers with invalid length

[AL-KERNEL]

CVE: CVE-2026-64191
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: i2c: stub: Reject I2C block transfers with invalid length
Commit: 7e9072dbd5f2f17934751873450d2c22080ead80
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7e9072dbd5f2f17934751873450d2c22080ead80
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64191
Analysis date: Mon, 20 Jul 2026 13:07:57 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local user with access to an i2c-stub `/dev/i2c-*` device can submit an oversized `I2C_SMBUS_I2C_BLOCK_DATA` transfer and trigger kernel stack out-of-bounds read or write in `stub_xfer()`, causing DoS and requiring manual review for possible local privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-64191 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64191

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-2026-64191	IMPORTANT	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:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-125;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'An out-of-bounds access in the i2c-stub driver occurs because stub_xfer uses data block 0 as the I2C block transfer length without enforcing the I2C_SMBUS_BLOCK_MAX limit of 32. A local process with access to /dev/i2c-* can issue an I2C_SMBUS ioctl with I2C_SMBUS_I2C_BLOCK_DATA and an oversized length, causing reads or writes past the end of the union i2c_smbus_data block buffer on the kernel stack. For the CVSS the PR:L is used because reliable triggering needs local code execution with access to the i2c device node, while loading and configuring i2c-stub is usually an administrator action on the target system. The issue is not network reachable and i2c-stub is a development or test driver that is not built by default. Impact is at least local denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because the bug provides a kernel stack out-of-bounds read or write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	READ KASAN OOB USB IMPROVEONLY LINUS  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1. A local process with access to `/dev/i2c-*` can issue the relevant `I2C_SMBUS` ioctl.
* Memory corruption, strong corruption primitive: +2. The bug can read or write past `union i2c_smbus_data.block` on the kernel stack.
* Weak to moderate LPE concern: +1 conservative, +2 paranoid. Stack OOB write in kernel context is a plausible LPE review item, but the patch does not demonstrate target selection, reclaim, or a reliable exploit primitive.
* Reliable kernel crash / strong DoS: +1. KASAN shows stack-out-of-bounds in `stub_xfer()`.
* Confidentiality impact plausible: +1. The OOB read side may expose adjacent kernel stack bytes through the emulated I2C data path.
* Integrity impact plausible: +1. The OOB write side can corrupt adjacent kernel stack state.
* Rare AND difficult-to-reach subsystem/configuration: -1. `i2c-stub` is a development/test driver, not built by default, and must be loaded/configured with `chip_addr`.
* Constrained primitive uncertainty: -1 in conservative score. The patch does not show attacker-controlled overwrite target selection or a demonstrated LPE chain.

Call-site confidence: high. The commit provides the ioctl call path through `i2cdev_ioctl_smbus()` to `stub_xfer()` and a KASAN trace.

## 3. Reachability analysis

The bug is locally reachable through `/dev/i2c-*` when `i2c-stub` is loaded and configured. It is not network reachable. Triggering does not require full root after the device node is exposed, but typical deployment requires an administrator to load and configure the test driver. Namespaces do not materially improve reachability unless the host exposes the relevant I2C device node into a container.

## 4. Severity interpretation

This is not an ordinary validation-only bug because the missing length check leads to concrete kernel stack out-of-bounds access. Realistic exposure is limited by the non-default test driver and device-node access requirement, so the conservative score is Actionable Moderate. The paranoid score reaches Strong Important candidate because kernel stack OOB write/read is a memory corruption primitive that could plausibly affect confidentiality, integrity, and availability.

## 5. One-sentence report phrase

A local user with access to an i2c-stub `/dev/i2c-*` device can submit an oversized `I2C_SMBUS_I2C_BLOCK_DATA` transfer and trigger kernel stack out-of-bounds read or write in `stub_xfer()`, causing DoS and requiring manual review for possible local privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.
YES REQUIRES MANUAL CHECK because this is concrete kernel stack memory corruption, even though practical exposure is limited by the non-default `i2c-stub` configuration.

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

Patch: i2c: stub: Reject I2C block transfers with invalid length
Commit: 7e9072dbd5f2f17934751873450d2c22080ead80
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7e9072dbd5f2f17934751873450d2c22080ead80

Commit description:

The I2C_SMBUS_I2C_BLOCK_DATA case in stub_xfer() uses data->block[0]
as the transfer length. The existing check only clamps it to avoid
overrunning the chip->words[256] register array, but does not validate
it against I2C_SMBUS_BLOCK_MAX (32), which is the limit of the union
i2c_smbus_data.block buffer (34 bytes total). The driver is a
development/test tool (CONFIG_I2C_STUB=m, not built by default)
that must be loaded with a chip_addr= parameter.

A local user with access to /dev/i2c-* can issue an I2C_SMBUS ioctl
with I2C_SMBUS_I2C_BLOCK_DATA and data->block[0] > 32, causing
stub_xfer() to read or write past the end of the union
i2c_smbus_data.block buffer:

 BUG: KASAN: stack-out-of-bounds in stub_xfer (drivers/i2c/i2c-stub.c:223)
 Read of size 1 at addr ffff88800abcfd92 by task exploit/81
 Call Trace:
  <TASK>
  stub_xfer (drivers/i2c/i2c-stub.c:223)
  __i2c_smbus_xfer (drivers/i2c/i2c-core-smbus.c:593)
  i2c_smbus_xfer (drivers/i2c/i2c-core-smbus.c:536)
  i2cdev_ioctl_smbus (drivers/i2c/i2c-dev.c:391)
  i2cdev_ioctl (drivers/i2c/i2c-dev.c:478)
  __x64_sys_ioctl (fs/ioctl.c:583)
  do_syscall_64 (arch/x86/entry/syscall_64.c:94)
  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
  </TASK>

The bug exists because i2c-stub implements .smbus_xfer directly,
bypassing the I2C_SMBUS_BLOCK_MAX validation in
i2c_smbus_xfer_emulated(). The I2C_SMBUS_BLOCK_DATA case in the same
function correctly validates against I2C_SMBUS_BLOCK_MAX, but the
I2C_SMBUS_I2C_BLOCK_DATA case does not.

Fix by rejecting transfers with data->block[0] == 0 or
data->block[0] > I2C_SMBUS_BLOCK_MAX with -EINVAL, consistent with
both the I2C_SMBUS_BLOCK_DATA case in the same function and the
I2C_SMBUS_I2C_BLOCK_DATA validation in i2c_smbus_xfer_emulated().

Fixes: 4710317 ("i2c-stub: Implement I2C block support")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/i2c/i2c-stub.c
  drivers/i2c/i2c-core-smbus.c
  drivers/i2c/i2c-dev.c
  fs/ioctl.c
  arch/x86/entry/syscall_64.c
  arch/x86/entry/entry_64.S

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=7e9072dbd5f2f17934751873450d2c22080ead80

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

The original Linux kernel CVE announcement for CVE-2026-64191 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64191

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: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: 5
  Paranoid ActionableScore: 7

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).