[CVE-2026-53157][MODERATE 7.0] net: phonet: free phonet_device after RCU grace period

[CVE-2026-53157][MODERATE 7.0] net: phonet: free phonet_device after RCU grace period

[AL-KERNEL]

CVE: CVE-2026-53157
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: phonet: free phonet_device after RCU grace period
Commit: 52b8f5ef82c886f7cd24617915e4b1579ddfd001
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=52b8f5ef82c886f7cd24617915e4b1579ddfd001
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53157
Analysis date: Thu, 25 Jun 2026 21:18:57 -0400
ActionableScore: 7
ActionableScore lower bound: 2
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A Phonet RCU lifetime bug frees phonet_device immediately after list_del_rcu(), allowing concurrent RCU readers to access freed slab memory and potentially causing a kernel crash or memory corruption.

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

The original Linux kernel CVE announcement for CVE-2026-53157 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53157

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
======================================================================

CVE-2026-53157	MODERATE	CHECK	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-416;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in the Phonet device lifetime handling can lead to slab use after free because phonet_device_destroy removes an object from an RCU protected per net device list with list_del_rcu and then frees it immediately. RCU readers walking the same list may still hold a pointer to the removed phonet_device after it has been freed, which can cause a kernel crash and in the worst case may expose a memory corruption primitive. For the CVSS the PR:L is used in the paranoid score because triggering may be possible for a local user with delegated network administration in a namespace or through a constrained service account, although typical host level network device removal is more privileged. The issue is not directly network reachable because the dangerous free path is tied to local network device teardown rather than packet traffic. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to UAF memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-53157  SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	UAF HARDWARE LINUS KPANIC 	-	-	checked

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

ActionableScore=7
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Paranoid score signals:

* Local unprivileged trigger: +1
  Possible in environments where unprivileged users can obtain CAP_NET_ADMIN inside user or network namespaces and trigger Phonet device teardown paths.

* Generic memory corruption, strong corruption primitive: +2
  The patch fixes a real slab-use-after-free caused by freeing an RCU-protected object immediately after list_del_rcu().

* Real lifetime corruption: +1
  This is an RCU lifetime bug. Readers may still hold phonet_device after removal from the list.

* Reliable kernel crash / strong DoS: +1
  A stale RCU reader dereferencing freed slab memory can realistically crash the kernel.

* Weak LPE concern: +1
  UAF exists, but the patch does not show a controlled reclaim primitive or attacker-controlled overwrite target. This supports manual review but not strong LPE by itself.

* Hard or unreliable race / special timing required: -1
  Exploitation depends on overlap between RCU readers and device destruction.

* Confidentiality impact plausible: +1
  In the paranoid interpretation, UAF memory corruption may expose stale or reused object contents.

* Integrity impact plausible: +1
  In the paranoid interpretation, reuse of freed slab memory could affect kernel object integrity, although no concrete write primitive is demonstrated.

Conservative score signals:

* Generic memory corruption: +2
* Real lifetime corruption: +1
* Reliable kernel crash / strong DoS: +1
* Hard RCU timing window: -1
* Requires CAP_NET_ADMIN or equivalent in typical host deployments: -2
* Rare subsystem / Phonet-specific exposure: -1

Conservative total: 2

## 3. Reachability analysis

The vulnerable free path is tied to Phonet network device destruction, not to direct packet processing. In a normal host deployment, triggering device teardown or address management usually requires CAP_NET_ADMIN or equivalent administrative control, so the conservative interpretation applies a privilege penalty.

Namespaces can change the practical risk. If unprivileged user namespaces are enabled and a local user can create or control a network namespace with CAP_NET_ADMIN inside it, the effective trigger requirement may be closer to PR:L rather than host-root-only. Phonet is not a broad default Internet-facing protocol path, and the bug is not directly remote reachable.

Realistic exploitation requires racing an RCU reader walking the per-net Phonet device list against phonet_device_destroy(). The most likely demonstrated impact is kernel crash. The technically concerning part is that the object is freed before the RCU grace period, so allocator reuse before reader completion creates a real UAF candidate.

## 4. Severity interpretation

This is not an ordinary resource leak or NULL dereference. It is a genuine RCU use-after-free in kernel networking code.

Conservatively, it behaves like a privileged local DoS in a relatively niche subsystem. Under the paranoid but defensible interpretation, it is a kernel lifetime corruption issue reachable from delegated network administration contexts, with possible but unproven confidentiality or integrity impact. There is not enough evidence to claim reliable privilege escalation, but the UAF class makes it unsuitable for automatic closure.

## 5. One-sentence report phrase

A Phonet RCU lifetime bug frees phonet_device immediately after list_del_rcu(), allowing concurrent RCU readers to access freed slab memory and potentially causing a kernel crash or memory corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a real RCU use-after-free in kernel networking code. Even though the common trigger likely requires CAP_NET_ADMIN and the subsystem is niche, namespace delegation and UAF lifetime semantics make it a strong manual-review candidate.

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

Patch: net: phonet: free phonet_device after RCU grace period
Commit: 52b8f5ef82c886f7cd24617915e4b1579ddfd001
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=52b8f5ef82c886f7cd24617915e4b1579ddfd001

Commit description:

phonet_device_destroy() removes a phonet_device from the per-net device
list with list_del_rcu(), but frees it immediately. RCU readers walking
the same list can still hold a pointer to the object after it has been
removed, leading to a slab-use-after-free.

Use kfree_rcu(), matching the lifetime rule already used by
phonet_address_del() for the same object type.

Fixes: eeb74a9 ("Phonet: convert devices list to RCU")
Cc: stable@vger.kernel.org
Signed-off-by: Santosh Kalluri <santosh.kalluri129@gmail.com>
Acked-by: Rémi Denis-Courmont <remi@remlab.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/phonet/pn_dev.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=52b8f5ef82c886f7cd24617915e4b1579ddfd001

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

The original Linux kernel CVE announcement for CVE-2026-53157 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53157

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:N/I:N/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: 2
  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: 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).