[CVE-2026-53400][MODERATE 7.0] i2c: core: fix adapter registration race

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

CVE: CVE-2026-53400
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: i2c: core: fix adapter registration race
Commit: 78793c75dc6d0ff2e4d50ad617349b328a99054e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=78793c75dc6d0ff2e4d50ad617349b328a99054e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53400
Analysis date: Sun, 19 Jul 2026 10:55:10 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An I2C adapter registration race can expose an incompletely initialized adapter through the IDR, allowing a concurrent `i2c_get_adapter()` lookup to reference invalid device state and potentially trigger a NULL pointer dereference or use-after-free, with typical exploitation requiring local device-management control.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-53400 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53400

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-53400	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-362;CWE-665;*CWE-416;**CWE-476;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'An i2c adapter registration race can expose an adapter through the IDR before the embedded struct device is fully initialized. A concurrent i2c_get_adapter() lookup can then take a reference to an incompletely initialized object, which can lead to NULL pointer dereference or use after free in kernel object lifetime handling. For the CVSS the PR:L is used for the paranoid score because some systems may delegate device management, driver bind operations, or hotplug control to a local service account or reduced capability root rather than full host root. The base scenario is still more restrictive because normal adapter registration is usually administrative. The issue is not network reachable and requires local ability to influence adapter registration or race with registration activity. Impact is at least local denial of service via kernel crash and in worst case may allow broader confidentiality and integrity impact due to use after free risk, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	LOCK USB RACE UAF HARDWARE DEBUGFS LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Memory corruption, weak/indirect corruption candidate: +1
  The commit explicitly mentions possible NULL pointer dereference or use-after-free, but the patch does not show attacker-controlled reclaim, overwrite, callback control, or type confusion.

* Real lifetime corruption: +1
  The race exposes an `i2c_adapter` through the IDR before the embedded `struct device` is fully initialized, and `i2c_get_adapter()` can take a reference to that object.

* Reliable kernel crash / strong DoS: +1
  The described outcomes include NULL pointer dereference and UAF-style lifetime failure, both plausibly leading to kernel crash.

* Privileged kernel/device-management memory corruption path: +1
  The bug is in adapter registration and device-management lifetime handling in the I2C core.

* Hard or unreliable race / special timing required: -1
  Exploitation depends on a narrow registration lookup race.

* Requires admin/root/CAP_* in typical deployments: -2
  Reliable triggering normally requires adapter registration, driver binding, module loading, hotplug control, or similar privileged device-management activity.

Conservative total: 3

Paranoid adjustments:

* Local unprivileged or reduced-privilege trigger concern: +1
  Some systems delegate device management, driver bind/unbind, or hotplug operations to service accounts, container root, or reduced-capability root contexts rather than full host root.

* Broad/common subsystem exposure: +1
  I2C core is widely used across embedded, laptop, server-management, and hardware-control environments, even though this specific registration path is not a normal packet or syscall hot path.

Paranoid total: 5

Race-UAF cap applied: yes. This is a race-dependent lifetime bug without demonstrated attacker-controlled reclaim, stale writable object reuse, callback dispatch, type confusion, or refcount takeover. Conservative scoring stays below Important. Paranoid scoring is capped at Actionable Moderate.

## 3. Reachability analysis

The bug is local and not network reachable. A realistic trigger requires influencing I2C adapter registration or racing a lookup such as `i2c_get_adapter()` while registration is in progress. In normal deployments this is usually restricted to root or privileged device-management paths such as driver load, bind, unbind, firmware or hotplug handling.

Namespaces and containers may matter if a product delegates device control to container root, a hardware-management daemon, or a reduced-capability service account. In that case PR should not automatically be treated as full host-root, and the paranoid interpretation becomes more relevant.

The path is not generally exposed to ordinary unprivileged users by default. Exploitability depends on whether the attacker can repeatedly trigger adapter registration or otherwise cause concurrent lookup during registration.

Call-site confidence: medium. The commit explicitly identifies `i2c_get_adapter()` as the racing lookup and explains the embedded-device reference issue, but the full caller graph and practical trigger sources are not included in the provided patch.

## 4. Severity interpretation

This behaves above an ordinary Moderate cleanup because it is a real publication-before-initialization race in kernel object lifetime handling, and the commit explicitly names UAF as a possible consequence. However, it is not a strong Important by default because the patch does not show a controlled reclaim primitive, arbitrary write, type confusion, callback hijack, or reliable privilege-escalation chain.

Realistic impact is local DoS through kernel crash. Theoretical impact includes broader memory-corruption consequences from UAF, but the available patch evidence supports only weak LPE concern, not strong LPE plausibility.

## 5. One-sentence report phrase

An I2C adapter registration race can expose an incompletely initialized adapter through the IDR, allowing a concurrent `i2c_get_adapter()` lookup to reference invalid device state and potentially trigger a NULL pointer dereference or use-after-free, with typical exploitation requiring local device-management control.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is a kernel object lifetime race with an explicitly mentioned UAF possibility, and delegated device-management environments may lower the practical privilege requirement even though normal triggering is privileged.

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

Patch: i2c: core: fix adapter registration race
Commit: 78793c75dc6d0ff2e4d50ad617349b328a99054e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=78793c75dc6d0ff2e4d50ad617349b328a99054e

Commit description:

Adapters can be looked up based on their id using i2c_get_adapter()
which takes a reference to the embedded struct device.

Make sure that the adapter (including its struct device) has been
initialised before adding it to the IDR to avoid accessing uninitialised
data which could, for example, lead to NULL-pointer dereferences or
use-after-free.

Note that the i2c-dev chardev, which is registered from a bus notifier,
currently uses i2c_get_adapter() so the adapter needs to be added to the
IDR before registration.

Fixes: 6e13e64 ("i2c: Add i2c_add_numbered_adapter()")
Cc: stable@vger.kernel.org	# 2.6.22
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/i2c/i2c-core-base.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=78793c75dc6d0ff2e4d50ad617349b328a99054e

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

The original Linux kernel CVE announcement for CVE-2026-53400 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53400

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:H/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 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: NO
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).