[CVE-2026-53111][LOW] bpf: test_run: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap [ Upstream

[CVE-2026-53111][LOW] bpf: test_run: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53111
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: bpf: test_run: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap [ Upstream
Commit: 5c8d1f91fc4898d79f29d79c1a6f7c2b3ee66fb0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5c8d1f91fc4898d79f29d79c1a6f7c2b3ee66fb0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53111
Analysis date: Fri, 26 Jun 2026 14:19:54 -0400
ActionableScore: 3
ActionableScore lower bound: 0
Actionable bucket: Borderline manual review recommended
Manual review required: YES

Summary:
A local BPF test_run path can create an skb without dst state and trigger a NULL pointer dereference in bpf_lwt_xmit_push_encap, causing a kernel crash when a privileged or delegated-capability user runs an LWT_XMIT BPF program.

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

The original Linux kernel CVE announcement for CVE-2026-53111 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53111

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: LOW
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-2026-53111	LOW	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:N/I:N/A:H';**CWE-476;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference can occur in the BPF test run path because bpf_lwt_xmit_push_encap may access skb_dst(skb) when the test skb has no dst attached. A local process can trigger the crash by running a BPF_PROG_TYPE_LWT_XMIT program through BPF_PROG_TEST_RUN where the program reaches bpf_lwt_push_ip_encap and needs skb_dst(skb) dev for headroom calculation. For the CVSS the PR:L is used in the paranoid score because a local process with delegated BPF or network administration capability may trigger the issue without being full root in practical deployments. The issue is not network reachable because it is triggered through the local BPF test_run interface rather than by packet traffic. Impact is denial of service via kernel crash. The observed primitive is a NULL pointer dereference and the patch context does not support privilege escalation or memory corruption impact.';NO MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended (with actual score 2) 	MAYBE	NULLPTR BPF SKB HARDWARE DEBUGFS LINUS TEST IPV6 SYZBOT  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=3
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 3
* Final recommended bucket: **Borderline manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The patch and trace show a reproducible NULL pointer dereference in `bpf_lwt_push_ip_encap()`.
* Requires admin/root/CAP_* in typical deployments: -2. Loading or using `BPF_PROG_TYPE_LWT_XMIT` normally requires privileged BPF or network administration capabilities.
* No memory corruption score. The demonstrated primitive is a NULL pointer dereference, not UAF, OOB write, double free, or controlled overwrite.
* No remote/network score. The trigger is `BPF_PROG_TEST_RUN` through the local `bpf()` syscall, not packet traffic.
* Conservative score floors to 0.

Paranoid signals:

* Reliable kernel crash / strong DoS: +1.
* Local low-privileged or delegated-capability trigger concern: +1. In containerized or delegated environments, a service account or reduced-capability root may have enough BPF or network capability without being equivalent to full host root.
* Local high-control kernel data-plane API: +1. BPF test_run gives detailed local control over a synthetic skb and program execution path.
* No LPE bonus. The patch context supports DoS only.
* No generic memory corruption bonus. NULL dereference is weak corruption class with no reclaim or overwrite primitive.

## 3. Reachability analysis

The bug is locally triggerable through `bpf_prog_test_run_skb()` using a `BPF_PROG_TYPE_LWT_XMIT` program that reaches `bpf_lwt_xmit_push_encap()`. Typical production systems require privileged BPF access, CAP_NET_ADMIN, CAP_BPF, or equivalent permissions to load and test such programs, so the conservative privilege model treats this as admin-gated. Namespaces or delegated capability models may reduce the practical privilege requirement, especially where container root or service accounts are allowed to use BPF-related interfaces. The path is not remotely reachable through normal packet processing because the crash occurs in the synthetic BPF test_run path.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like local DoS under the conservative model. The paranoid score reaches borderline manual-review territory only because BPF is a high-control local kernel API and delegated capability deployments may make the trigger more accessible than full host root. There is no supported evidence of privilege escalation, UAF reclaim, object confusion, information disclosure, or attacker-controlled memory corruption.

## 5. One-sentence report phrase

A local BPF test_run path can create an skb without dst state and trigger a NULL pointer dereference in bpf_lwt_xmit_push_encap, causing a kernel crash when a privileged or delegated-capability user runs an LWT_XMIT BPF program.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

This is not an Important-class candidate based on the patch alone, but a quick manual check is reasonable because the affected interface is BPF and some deployments delegate BPF or network capabilities to reduced-privilege users.

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

Patch: bpf: test_run: Fix the null pointer dereference issue in bpf_lwt_xmit_push_encap [ Upstream
Commit: 5c8d1f91fc4898d79f29d79c1a6f7c2b3ee66fb0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5c8d1f91fc4898d79f29d79c1a6f7c2b3ee66fb0

Changed files:
  net/bpf/test_run.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=5c8d1f91fc4898d79f29d79c1a6f7c2b3ee66fb0

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

The original Linux kernel CVE announcement for CVE-2026-53111 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53111

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  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: 0
  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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).