[CVE-2026-53095][IMPORTANT] bpf: Fix abuse of kprobe_write_ctx via freplace [ Upstream

[CVE-2026-53095][IMPORTANT] bpf: Fix abuse of kprobe_write_ctx via freplace [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53095
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: bpf: Fix abuse of kprobe_write_ctx via freplace [ Upstream
Commit: b312cf41b9e43f442613053f6cad39898e1baf96
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b312cf41b9e43f442613053f6cad39898e1baf96
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53095
Analysis date: Wed, 24 Jun 2026 15:44:18 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A BPF freplace program could bypass the intended kprobe_write_ctx restriction and modify pt_regs for kprobe target kernel functions, allowing local privileged or delegated BPF users to alter kernel function arguments with plausible DoS or privilege escalation impact.

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

The original Linux kernel CVE announcement for CVE-2026-53095 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53095

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: IMPORTANT
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-2026-53095	IMPORTANT	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:H/A:H';CWE-284;CWE-269;CWE-693;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '7.8';DESCR 'A BPF freplace program could abuse a kprobe_write_ctx mismatch and attach to a kprobe target that was not allowed to write struct pt_regs. When the target kernel function runs, the freplace program can modify pt_regs and change register arguments of that kernel function, giving the BPF program unintended control over kernel execution state. For the CVSS the PR:L value is used in the paranoid score because exploitation requires local ability to load and attach tracing or extension BPF programs, often via delegated CAP_BPF CAP_PERFMON or similar service privileges rather than full root in practical deployments. The issue is not network reachable. Impact may include kernel logic corruption, denial of service, and plausible local privilege escalation because attacker controlled register changes can affect privileged kernel code paths.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8) 	MAYBE	INIT BPF LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: 7
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Privilege escalation plausible: +2
  The bug allows a BPF freplace program to modify `struct pt_regs` for a kprobe target kernel function, effectively changing kernel function arguments at runtime. This is a plausible LPE class even without a classic UAF or OOB write.

* Security-boundary bypass with practical sensitive-object access: +2
  The patch enforces an intended BPF attach-time boundary around `kprobe_write_ctx`. Without it, a program with writeable uprobe-style context semantics can be attached in a way that affects kprobe execution on kernel functions.

* Local high-control kernel data-plane API: +1
  BPF tracing and freplace are high-control local kernel instrumentation APIs.

* Reliable kernel crash / strong DoS: +1
  Modifying register arguments of arbitrary probed kernel functions can plausibly crash the kernel or corrupt kernel control flow assumptions.

* Integrity impact plausible: +1
  Attacker-controlled modification of kernel function arguments can change privileged kernel behavior.

* Confidentiality impact plausible: +1
  If register modification affects security checks or object selection, sensitive data exposure is plausible, although not directly demonstrated.

* Broad/common subsystem exposure: +1
  BPF tracing is a common kernel facility on many production systems, even though access is usually restricted.

* Requires CAP_BPF/CAP_PERFMON/CAP_SYS_ADMIN-like privileges in typical deployments: -1 conservative / 0 paranoid
  Conservative scoring applies a partial privilege penalty because BPF tracing access is normally restricted. The full -2 is not applied because this is a capability-boundary bypass and reduced-capability root, service accounts, or delegated BPF permissions are realistic deployment models.

## 3. Reachability analysis

The bug is locally reachable by a user or service able to load and attach BPF tracing or freplace programs. It is not network reachable. In typical hardened deployments this requires `CAP_BPF`, `CAP_PERFMON`, or `CAP_SYS_ADMIN`, and unprivileged BPF is commonly disabled. However, container root, service accounts, observability agents, or delegated BPF permissions may reduce the practical privilege barrier below full host root. The vulnerable path is not a rare driver path. It is part of the BPF tracing attach logic.

## 4. Severity interpretation

This behaves more like an Important candidate than an ordinary Moderate. The issue is not generic heap memory corruption, UAF, or OOB write, but it gives unintended control over `pt_regs` for kernel function execution. That is a strong kernel execution-state manipulation primitive and can plausibly lead to DoS, security-check bypass, or local privilege escalation depending on the chosen kprobe target. Realistic exploitation still depends on BPF privileges and target selection, so the conservative score is slightly lower than the paranoid score.

## 5. One-sentence report phrase

A BPF freplace program could bypass the intended kprobe_write_ctx restriction and modify pt_regs for kprobe target kernel functions, allowing local privileged or delegated BPF users to alter kernel function arguments with plausible DoS or privilege escalation impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because the primitive is not merely a warning or crash. It is unauthorized kernel register/context modification through BPF attach logic, with plausible integrity impact and local privilege escalation potential.

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

Patch: bpf: Fix abuse of kprobe_write_ctx via freplace [ Upstream
Commit: b312cf41b9e43f442613053f6cad39898e1baf96
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b312cf41b9e43f442613053f6cad39898e1baf96

Changed files:
  kernel/bpf/syscall.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=b312cf41b9e43f442613053f6cad39898e1baf96

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

The original Linux kernel CVE announcement for CVE-2026-53095 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53095

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:H/I:H/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 7
  Paranoid ActionableScore: 8

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).