[CVE-2026-63864][MODERATE 7.0] bpf: Propagate error from visit_tailcall_insn [ Upstream

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

CVE: CVE-2026-63864
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: bpf: Propagate error from visit_tailcall_insn [ Upstream
Commit: 945816e63c8677cf4bfde963a0774432ce8afc85
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=945816e63c8677cf4bfde963a0774432ce8afc85
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63864
Analysis date: Sun, 19 Jul 2026 11:22:24 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A missing return-value check in the BPF verifier allowed `visit_tailcall_insn()` errors to be ignored during tail-call stack-liveness analysis, potentially leaving verifier state inconsistent and requiring manual review for possible verifier-bypass and LPE impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63864 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63864

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-2026-63864	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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-252;CWE-703;CWE-693;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '7.8';DESCR 'A missing error propagation in the BPF verifier can cause visit_insn to ignore failures from visit_tailcall_insn during tail call stack liveness analysis. Because the verifier is the trust boundary for accepting untrusted or semi trusted BPF bytecode, an ignored error in tail call analysis may leave verifier state inconsistent and can potentially allow acceptance of a program that should have been rejected. For the CVSS the PR:L is used because a local user or process must be able to load BPF programs through bpf(), CAP_BPF, CAP_SYS_ADMIN, or a delegated BPF service. The issue is not network reachable by itself and requires local access to a BPF loading path. Impact is treated as possible privilege escalation in the paranoid score because verifier soundness bugs can lead to kernel memory corruption or arbitrary kernel memory access, while the base score keeps AC:H because the trigger depends on a verifier error path in tail call analysis.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)  SKIP CVE-2026-63864 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: e40f5a6bf88a781d5f81bc6b8aab9ac31d8c98dd YES  NO NO unknown 	MAYBE	BPF SIMPLEFIX IMPROVEONLY  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	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:

* Security-boundary bypass concern: +2
  The affected code is in the BPF verifier, which is a kernel security boundary for accepting or rejecting BPF programs.
* Weak LPE concern: +1
  Ignoring an error from `visit_tailcall_insn()` may allow verifier analysis to continue with incomplete tail-call stack-liveness state. This is plausibly security-sensitive, but the patch does not show a concrete arbitrary read, arbitrary write, UAF reclaim, or controlled object replacement primitive.
* Broad/common subsystem: +1
  BPF verifier bugs are high-sensitivity kernel attack-surface issues, although actual reachability depends on BPF privilege policy.
* Requires CAP_BPF/CAP_SYS_ADMIN/root in typical deployments: -2
  In the evaluated configuration, unprivileged BPF is assumed disabled, so this is not treated as a plain unprivileged local trigger.

Conservative total: 3

Paranoid additional signals:

* Confidentiality impact plausible: +1
  If the verifier incorrectly accepts an unsafe program, kernel memory disclosure can become possible in worst-case verifier-bypass scenarios.
* Integrity impact plausible: +1
  If the verifier state inconsistency permits unsafe memory access, kernel memory modification or privilege escalation may be possible in worst-case scenarios.

Paranoid total: 5

No strong memory-corruption primitive is awarded. The patch shows missing error propagation in verifier analysis, not a demonstrated UAF, OOB write, arbitrary write, type confusion, or attacker-controlled reclaim primitive.

## 3. Reachability analysis

The bug is triggered through BPF program verification involving `BPF_FUNC_tail_call` and `visit_tailcall_insn()`. A local process must be able to load BPF programs. In typical hardened enterprise/product configurations, this requires `CAP_BPF`, `CAP_SYS_ADMIN`, root, or a privileged BPF-loading service, so unprivileged local reachability is not assumed.

Namespaces and containers may matter if a container or service is delegated BPF-loading capability, but this should not be treated the same as fully unprivileged access. The path is not network reachable by itself. Realistic exploitation requires access to a BPF loading path and a verifier state pattern where the ignored error changes the accept/reject outcome.

Call-site confidence: high, because the changed call site is visible and directly shows the ignored return value.

## 4. Severity interpretation

This is not an ordinary low-risk cleanup because it affects BPF verifier correctness. It is also not a proven Important-class vulnerability from the patch alone, because no concrete exploit primitive is shown.

Realistically, it should be handled as Actionable Moderate or manual-review-worthy verifier logic. The paranoid interpretation reaches Actionable Moderate because BPF verifier soundness bugs can become LPE issues if they permit unsafe programs to be accepted.

## 5. One-sentence report phrase

A missing return-value check in the BPF verifier allowed `visit_tailcall_insn()` errors to be ignored during tail-call stack-liveness analysis, potentially leaving verifier state inconsistent and requiring manual review for possible verifier-bypass and LPE impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is BPF verifier control-flow and liveness logic. Even though the patch does not demonstrate a concrete memory-corruption primitive, verifier soundness bugs are historically security-sensitive and should not be auto-closed.

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

Patch: bpf: Propagate error from visit_tailcall_insn [ Upstream
Commit: 945816e63c8677cf4bfde963a0774432ce8afc85
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=945816e63c8677cf4bfde963a0774432ce8afc85

Changed files:
  kernel/bpf/verifier.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=945816e63c8677cf4bfde963a0774432ce8afc85

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

The original Linux kernel CVE announcement for CVE-2026-63864 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63864

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: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: 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).