[CVE-2026-52937][MODERATE REGULAR] tap: fix stack info leak in tap_ioctl() SIOCGIFHWADDR [ Upstream

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

CVE: CVE-2026-52937
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: tap: fix stack info leak in tap_ioctl() SIOCGIFHWADDR [ Upstream
Commit: 719007c3492f0f1f9e9cdbed8ac45ba45bb13eeb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=719007c3492f0f1f9e9cdbed8ac45ba45bb13eeb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52937
Analysis date: Wed, 24 Jun 2026 04:56:04 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local stack information leak in `tap_ioctl()` for `SIOCGIFHWADDR` can expose kernel stack bytes including kernel pointers through a tap or macvtap character device, potentially defeating KASLR and assisting later kernel exploitation.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-52937 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52937

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-2026-52937	MODERATE	CHECK	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-457;CWE-908;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.5';DESCR 'A stack information leak in tap_ioctl() SIOCGIFHWADDR occurs because sockaddr_storage ss is not initialized before data is copied back through ifr_hwaddr. netif_get_mac_address() only fills sa_family and the device address bytes, so trailing stack bytes can be exposed to userspace. The leaked bytes may include kernel .text and direct-map pointers, which can defeat KASLR and assist later kernel exploitation. For the CVSS the PR:L is selected because an attacker needs local user or process access to an opened tap or macvtap character device, but does not need full administrative privileges once that device access is available. The issue is not network reachable as a packet processing bug and is triggered through a local ioctl path. Impact is information disclosure only by itself, with no direct write primitive or standalone privilege escalation shown by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Actionable Moderate at minimum (with actual score 4)	YES	KASAN SIMPLEFIX RACE LEAK NETWORK  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1
  Trigger is a local ioctl path on a tap or macvtap character device. Creating such a device may require privileges, but using an already accessible fd or delegated device access can be lower than full host root.

* Confidentiality impact plausible: +1
  The bug leaks uninitialized kernel stack bytes to userspace.

* Broad or common deployment exposure: +1
  tap and macvtap are common in virtualization, container, and networking setups, although they are not universally exposed to untrusted users by default.

* Paranoid extra signal for kernel pointer disclosure: +1
  The commit explicitly says the leak can expose kernel .text and direct-map pointers, defeating KASLR. This does not prove standalone LPE, but it is exploitation-assisting information and should not be auto-closed without review.

No points for memory corruption, integrity impact, availability impact, remote reachability, or direct privilege escalation.

## 3. Reachability analysis

The bug is reachable through `tap_ioctl()` with `SIOCGIFHWADDR` on a tap or macvtap character device. It is not a network packet-processing vulnerability and is not remotely triggerable by sending traffic. Typical creation or management of tap or macvtap devices may require administrative privileges or `CAP_NET_ADMIN`, but practical access can be delegated to VMs, containers, service accounts, or processes that already hold an open device fd. Namespaces can therefore reduce the practical privilege requirement in some deployments.

## 4. Severity interpretation

This behaves as an information disclosure issue, not memory corruption. The immediate primitive is an 8 byte kernel stack leak through `ifr_hwaddr`. Realistic standalone impact is KASLR bypass or kernel pointer disclosure, not direct privilege escalation. However, because the disclosed data may include kernel text and direct-map pointers, it can materially assist exploitation of a separate kernel memory corruption bug. This is therefore stronger than an ordinary low-risk info leak, but not an Important-class issue by itself.

## 5. One-sentence report phrase

A local stack information leak in `tap_ioctl()` for `SIOCGIFHWADDR` can expose kernel stack bytes including kernel pointers through a tap or macvtap character device, potentially defeating KASLR and assisting later kernel exploitation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is not standalone LPE, but it is a kernel pointer information leak explicitly described as defeating KASLR, which is commonly useful in exploit chains and should not be automatically closed.

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

Patch: tap: fix stack info leak in tap_ioctl() SIOCGIFHWADDR [ Upstream
Commit: 719007c3492f0f1f9e9cdbed8ac45ba45bb13eeb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=719007c3492f0f1f9e9cdbed8ac45ba45bb13eeb

Changed files:
  drivers/net/tap.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=719007c3492f0f1f9e9cdbed8ac45ba45bb13eeb

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

The original Linux kernel CVE announcement for CVE-2026-52937 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52937

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: 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).