[CVE-2026-98201][MODERATE 7.0] Input: zero ff_effect before compat copy in input_ff_effect_from_user

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

CVE: CVE-2026-98201
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: Input: zero ff_effect before compat copy in input_ff_effect_from_user
Commit: ed0905fda39682d3da7937184397d3276abbffa8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ed0905fda39682d3da7937184397d3276abbffa8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98201
Analysis date: Fri, 09 Oct 2026 03:43:19 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98201 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98201

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 7.0
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-2026-98201	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98201 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE_KPANIC RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both MODERATE_KPANIC)	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-457;*CWE-200;CWE-665;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.5';DESCR 'A kernel stack information disclosure occurs in input_ff_effect_from_user() because the 32-bit compat path copies only struct ff_effect_compat into a larger native struct ff_effect. The remaining bytes retain previous kernel stack contents and input_ff_upload() stores the complete native structure in the force feedback effect table. A uinput based force feedback daemon can later receive these bytes through UI_BEGIN_FF_UPLOAD. For the CVSS the PR:L value is used because a local user or process must access the relevant input device and trigger the compat force feedback upload path. The base score uses AC:H and C:L because exploitation depends on a 32-bit compat caller, a suitable force feedback device, and a cooperating uinput based daemon, while the directly exposed data is limited to the uninitialized structure tail. The paranoid score uses AC:L and C:H when this environment is already available because repeated kernel stack disclosures may expose addresses or other sensitive values and can weaken protections such as KASLR. The issue is not network reachable. It does not provide a direct write primitive or a demonstrated standalone privilege escalation path, but the disclosed kernel data may assist exploitation of another vulnerability.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	DANGER SIMPLEFIX LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 5
* Raw paranoid score: 3
* Paranoid score: 5
* Applied conservative floor: 3 -> 5 because a direct, repeatable kernel-stack disclosure to userspace establishes an Important-preserving Class A primitive requiring manual review.
* Applied paranoid floor: 3 -> 5 for the same established disclosure primitive. No speculative integrity, availability, or privilege-escalation impact was added.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive signals:

* +2 Strong disclosure primitive. Uninitialized kernel-stack bytes are copied into persistent force-feedback state and exposed to userspace through `UI_BEGIN_FF_UPLOAD`.
* +1 Confidentiality plausible. The disclosure path is explicit and user-visible rather than an internal-only invalid read.

No double-counting:

* No separate practical-exploit-chain point. The compat copy, full native-structure storage, and userspace readback are the same mechanism already counted as the disclosure primitive.
* No local-unprivileged point. Local interaction is established, but ordinary-user access to all required input and uinput device nodes is not.
* No integrity or availability point. The patch establishes disclosure, not overwrite, crash, or persistent service loss.
* No LPE point. Leaked stack data may assist another exploit, but standalone privilege escalation is not demonstrated.
* No UAF, controlled reclaim, arbitrary write, callback influence, or ownership violation is established.
* B5-R does not apply. There is no heap OOB write, retry-accumulated offset, or later consumption of corrupted descriptors.

3. Reachability analysis

The trigger requires a 32-bit compat caller to submit `EVIOCSFF` through the affected force-feedback path. A uinput-based force-feedback daemon can subsequently retrieve the full native effect through `UI_BEGIN_FF_UPLOAD`.

The complete privilege boundary is unresolved. Access depends on permissions for the relevant input and uinput device nodes and on the presence of the uinput force-feedback arrangement. The evidence does not prove host-administrator privilege, but it also does not establish ordinary unprivileged access. Therefore `HostAdminRequired=UNKNOWN`.

This is not a namespace-capability path. Device-node delegation may permit a service account or selected local user to reach it, but that is discretionary host setup rather than an attacker-obtainable namespace capability. No network trigger, special physical hardware, race, or timing window is required. Repeated uploads appear capable of producing additional stack samples once the complete device path is available.

4. Primitive and severity interpretation

The exact primitive is a bounded but potentially repeatable kernel-stack information disclosure. The smaller compat structure overwrites only a prefix of an uninitialized native `ff_effect`. The untouched native tail is then copied into `ff->effects[id]` and exposed to userspace.

This is not an OOB read, OOB write, UAF, or corrupted-state execution. It is a direct initialization failure with a concrete disclosure sink. The demonstrated consequence is disclosure of stale stack bytes. Potential KASLR weakening or exposure of other sensitive stack values is technically plausible, but the available evidence does not establish the contents of each sample or a completed privilege-escalation chain.

`PrimitiveClass=CLASS_A` is used because kernel memory is directly transferred across the kernel-to-userspace boundary. `ImportantCandidate=YES` and `AutoDowngradeAllowed=NO` preserve the issue for review despite uncertain device permissions and limited bytes per operation. Operationally, this is at least Actionable Moderate and may warrant Important treatment if testing confirms useful or repeatable address disclosure to a below-host-admin actor.

5. One-sentence report phrase

A 32-bit force-feedback compat upload can expose uninitialized kernel-stack bytes through UI_BEGIN_FF_UPLOAD, creating a local information-disclosure primitive whose practical reachability and leaked contents require manual validation.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Confirm the native-versus-compat size difference, leaked byte count and contents, repeatability, device-node permissions, and whether a below-host-admin actor can control both the upload and readback path.

7. Call-site confidence

Call-site confidence: high

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

Patch: Input: zero ff_effect before compat copy in input_ff_effect_from_user
Commit: ed0905fda39682d3da7937184397d3276abbffa8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ed0905fda39682d3da7937184397d3276abbffa8

Commit description:

In the compat path input_ff_effect_from_user() aliases the caller's
native struct ff_effect with the smaller struct ff_effect_compat and
copies only the compat sized prefix:

	compat_effect = (struct ff_effect_compat *)effect;

	if (copy_from_user(compat_effect, buffer,
			   sizeof(struct ff_effect_compat)))

The tail of the native structure is never written. Callers pass an
uninitialized on-stack object, for example evdev_do_ioctl() for
EVIOCSFF, so those bytes keep their previous stack contents.
input_ff_upload() then stores the full native structure in
ff->effects[id], from where a uinput based force feedback daemon can
read it back via UI_BEGIN_FF_UPLOAD, disclosing kernel stack memory to
userspace.

Zero the effect before the compat copy.

Fixes: 2d56f3a ("Input: refactor evdev 32bit compat to be shareable with uinput")
Cc: stable@vger.kernel.org
Signed-off-by: Iván Ezequiel Rodriguez <ivanrwcm25@gmail.com>
Link: https://patch.msgid.link/20260901130629.24078-3-ivanrwcm25@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/input/input-compat.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=ed0905fda39682d3da7937184397d3276abbffa8

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

The original Linux kernel CVE announcement for CVE-2026-98201 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98201

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: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: unknown
  Paranoid ActionableScore: 5

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).