[CVE-2026-74673][MODERATE REGULAR] Input: evdev - fix information leak in evdev_pass_values()

[CVE-2026-74673][MODERATE REGULAR] Input: evdev - fix information leak in evdev_pass_values()

[AL-KERNEL]

CVE: CVE-2026-74673
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: Input: evdev - fix information leak in evdev_pass_values()
Commit: d2e3839419ac4047835762c4d7712bda1101b57e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d2e3839419ac4047835762c4d7712bda1101b57e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74673
Analysis date: Sat, 22 Aug 2026 18:00:20 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline Moderate / manual review recommended
Manual review required: YES

Summary:
evdev may leak uninitialized padding bytes from kernel stack allocated `struct input_event` objects to local userspace readers of accessible input event devices, causing a limited kernel information disclosure without integrity or availability impact.

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

The original Linux kernel CVE announcement for CVE-2026-74673 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74673

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.

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

CVE-2026-74673	MODERATE	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:N/A:N';*CWE-200;CWE-908;*CWE-457;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.5';DESCR 'evdev could copy uninitialized padding bytes from struct input_event to userspace because the event object was allocated on the kernel stack and only populated field by field. On architectures where struct input_event contains padding, repeated reads from an accessible /dev/input/eventX device may disclose small fragments of kernel stack memory. For the CVSS the PR:L is used because an attacker needs local access to an evdev device node or an equivalent input permission, but full administrative privileges are not normally required. The issue is not network reachable and does not provide a direct write primitive. Impact is confidentiality only, with no integrity or availability impact expected from this bug itself.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Moderate / manual review recommended (with actual score 3)	YES	SIMPLEFIX LEAK HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline Moderate / manual review recommended**::

## 2. Signal breakdown

* Local unprivileged trigger: +1. A local process with read access to `/dev/input/eventX` can receive evdev events copied from the kernel buffer.
* Confidentiality impact plausible: +1. Uninitialized padding bytes in `struct input_event` can disclose small fragments of kernel stack memory to userspace.
* Broad/default/common subsystem: +1 in paranoid scoring. evdev is a common Linux input subsystem, although practical exposure depends on device-node permissions.
* Rare or configuration dependent condition: -1 in conservative scoring. The leak depends on architectures where `struct input_event` has relevant padding, with SPARC64 explicitly mentioned.

No memory corruption, write primitive, UAF, privilege escalation primitive, or DoS signal is supported by the patch.

## 3. Reachability analysis

The bug is locally reachable by userspace reading evdev events from an accessible input device node. In typical desktop or seat-managed systems, access may be granted to the active logged-in user or members of an input-related group, but it is not generally reachable by arbitrary remote attackers. Containers usually do not receive access to host evdev nodes unless explicitly passed through. The path is common, but the actual information leak depends on architecture-specific structure padding and local device permission policy.

## 4. Severity interpretation

This behaves like an information disclosure Moderate, not an Important-class kernel memory corruption issue. The realistic primitive is leakage of limited uninitialized stack padding bytes. Theoretical usefulness exists because repeated small kernel stack disclosures can assist other attacks, but the patch does not show corruption, control-flow influence, arbitrary read, arbitrary write, or direct privilege escalation.

## 5. One-sentence report phrase

evdev may leak uninitialized padding bytes from kernel stack allocated `struct input_event` objects to local userspace readers of accessible input event devices, causing a limited kernel information disclosure without integrity or availability impact.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED.

This should not be auto-closed solely as Low because it is a kernel stack information leak, but it also does not justify Important handling without evidence of a stronger disclosure primitive or a follow-on exploit chain.

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

Patch: Input: evdev - fix information leak in evdev_pass_values()
Commit: d2e3839419ac4047835762c4d7712bda1101b57e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d2e3839419ac4047835762c4d7712bda1101b57e

Commit description:

In evdev_pass_values(), the input_event structure is allocated on the
kernel stack and populated field-by-field. However, it is never fully
initialized. On architectures where struct input_event contains explicit
or implicit padding (such as the 32-bit __pad field on SPARC64), these
padding bytes are left uninitialized.

When this event structure is subsequently passed to the client buffer
and later copied to userspace, the uninitialized padding bytes leak
kernel stack memory, potentially exposing sensitive information.

Similar issues exist in __evdev_queue_syn_dropped and __pass_event.

Fix this by explicitly zeroing the entire event structure with memset()
before populating its fields. This ensures all padding bytes are cleared
before the data crosses the security boundary.

Reported-by: sashiko-bot@kernel.org
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/ampGGKo4UMKru6f5@google.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/input/evdev.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=d2e3839419ac4047835762c4d7712bda1101b57e

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

The original Linux kernel CVE announcement for CVE-2026-74673 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74673

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:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS score: 5.5

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: 2
  Paranoid ActionableScore: 3

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).