[CVE-2026-53036][MODERATE 7.0] bpf, arm64: Fix off-by-one in check_imm signed range check [ Upstream

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

CVE: CVE-2026-53036
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: bpf, arm64: Fix off-by-one in check_imm signed range check [ Upstream
Commit: a5dfeb3b61065039488342d43ae06d4729d955d4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a5dfeb3b61065039488342d43ae06d4729d955d4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53036
Analysis date: Fri, 26 Jun 2026 15:03:48 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An off-by-one signed immediate range check in the arm64 BPF JIT can misencode out-of-range branches and make native JIT control flow diverge from verifier-approved BPF control flow, causing at least local DoS and requiring manual review for possible privilege-escalation impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-53036 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53036

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-2026-53036	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-193;CWE-682;CWE-670;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'An off-by-one signed range check in the arm64 BPF JIT can allow branch displacements that do not fit into the encoded immediate field. When such an out of range forward branch is passed to the arm64 instruction encoder, the immediate can be masked into the signed field and interpreted as a backward branch, causing native JIT code to execute a control flow path that does not match the verified BPF program. For the CVSS the PR:L value is used because an attacker needs local ability to load BPF programs, for example through unprivileged BPF if enabled or delegated BPF capability, not full administrator access. The issue is not network reachable by itself and requires local code execution on an affected arm64 system with BPF JIT enabled. Impact is at least local denial of service via wrong control flow, crash, or hang. In the paranoid interpretation, the verifier and JIT semantic mismatch may create a path to confidentiality or integrity impact because verified BPF safety assumptions may no longer match generated kernel code.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5) 	MAYBE	OOB BPF SIMPLEFIX HARDWARE TEST  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: **4**
* Paranoid score: **5**
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* **Local unprivileged or low-privilege trigger: +1**
  Triggering requires ability to load BPF programs. This is local and may be available to unprivileged users on systems with unprivileged BPF enabled, or to reduced-privilege users with delegated BPF capability.

* **Local unprivileged high-control kernel data-plane API: +1**
  BPF is a high-control kernel interface where users can shape program structure and control-flow layout.

* **Reliable kernel crash / strong DoS: +1**
  A misencoded branch can redirect JIT control flow and plausibly cause hangs, invalid execution, or kernel crash.

* **Broad/default/common subsystem exposure: +1**
  eBPF and BPF JIT are common, though this specific bug is arm64-specific.

* **Hard or special trigger conditions: -1**
  Exploitation requires crafting a BPF program whose JIT branch displacement falls into the incorrect signed immediate range. This is not a trivial always-on trigger.

Paranoid additional signal:

* **Weak LPE concern: +1**
  The key concern is verifier/JIT semantic mismatch. The verifier validates one BPF control-flow model, but the arm64 JIT may emit native code with a different branch direction. This does not prove arbitrary write or code execution, but it is a plausible privilege-escalation class that requires manual review.

## 3. Reachability analysis

The bug is locally reachable through BPF program loading on affected arm64 systems with BPF JIT enabled. It is not directly network reachable. In typical hardened deployments, unprivileged BPF may be disabled, which raises the practical privilege requirement to a process with CAP_BPF, CAP_SYS_ADMIN, or equivalent delegation. However, this should not be treated as full host-root only, because containerized or service-scoped environments may delegate BPF-related capabilities without granting complete host control.

The affected path is architecture-specific to arm64 JIT code generation. The realistic trigger requires a sufficiently large or carefully shaped BPF program that causes an out-of-range branch displacement to pass the broken check and then be truncated into the signed immediate field.

## 4. Severity interpretation

This is more than an ordinary DoS-only Moderate because it affects BPF JIT control-flow correctness. The directly supported impact is local DoS through miscompiled native code, crash, or hang. The stronger concern is not demonstrated memory corruption, but a verifier/JIT semantic mismatch, which historically deserves elevated triage because generated kernel code may violate assumptions used during BPF verification.

Conservative handling fits **borderline manual review**. Paranoid handling reaches **Actionable Moderate at minimum** because local BPF JIT miscompilation can plausibly become a privilege-escalation path even without a demonstrated primitive in the patch.

## 5. One-sentence report phrase

An off-by-one signed immediate range check in the arm64 BPF JIT can misencode out-of-range branches and make native JIT control flow diverge from verifier-approved BPF control flow, causing at least local DoS and requiring manual review for possible privilege-escalation impact.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is a BPF JIT control-flow miscompilation bug with possible verifier/JIT semantic mismatch. Even though the patch does not demonstrate an arbitrary write or concrete LPE primitive, this class is high-risk enough that it should not be auto-closed.

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UPSTREAM PATCH SUMMARY
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Patch: bpf, arm64: Fix off-by-one in check_imm signed range check [ Upstream
Commit: a5dfeb3b61065039488342d43ae06d4729d955d4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a5dfeb3b61065039488342d43ae06d4729d955d4

Changed files:
  arch/arm64/net/bpf_jit_comp.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=a5dfeb3b61065039488342d43ae06d4729d955d4

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

The original Linux kernel CVE announcement for CVE-2026-53036 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53036

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:L/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: 4
  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: 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).