[CVE-2026-64235][MODERATE REGULAR] x86/ftrace: Relocate %rip-relative percpu refs in dynamic trampolines [ Upstream

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

CVE: CVE-2026-64235
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: x86/ftrace: Relocate %rip-relative percpu refs in dynamic trampolines [ Upstream
Commit: 8093442a2d1d4b42b9340a86023ccb2afb30b93a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8093442a2d1d4b42b9340a86023ccb2afb30b93a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64235
Analysis date: Fri, 24 Jul 2026 13:01:25 -0400
ActionableScore: 4
ActionableScore lower bound: 1
Actionable bucket: Borderline manual-review Moderate. Actionable Moderate only if delegated tracing is reachable
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-64235 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64235

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 REGULAR
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-64235	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-682;*CWE-787;CWE-754;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A dynamic ftrace trampoline can contain RIP relative percpu references copied from the original ftrace caller code. If the displacement is not relocated for the new trampoline address, the call_depth accounting instruction can write to the wrong kernel address and trigger a supervisor page fault and kernel panic. The practical trigger requires a specific affected x86 configuration with CONFIG_CALL_DEPTH_TRACKING and retbleed stuffing enabled, plus the ability to register or enable dynamic tracing such as kprobe based tracing. For the CVSS the PR:L is used for the paranoid score because a local user with delegated tracing or reduced administration rights may be able to reach the interface, while typical systems require administrator level tracing access. The issue is not network reachable. Impact is at least local denial of service, and in the worst still defensible interpretation may include limited confidentiality or integrity impact due to an invalid kernel memory write candidate.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual-review Moderate. Actionable Moderate only if delegated tracing is reachable (with actual score 3)	YES	OOB TRACE BPF KERNEL_PANIC_PLUS_UAF HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=1

1. ActionableScore
* Conservative score: 1
* Paranoid score: 4
* Final recommended bucket: **Borderline manual-review Moderate. Actionable Moderate only if delegated tracing is reachable**::

2. Signal breakdown
* Weak or indirect corruption candidate: +1. The bad RIP-relative relocation can make the generated ftrace trampoline write to an unintended kernel address via the call-depth accounting instruction. This is not an arbitrary write because target and payload control are not demonstrated.
* Reliable kernel crash / strong DoS: +1. The commit provides a simple reproducer that reaches a supervisor write page fault and kernel panic.
* Availability impact realistic: +1 in paranoid scoring only. The crash is host-wide and reproducible after the tracing setup.
* Local trigger / delegated local trigger: +1 in paranoid scoring only. Typical access is privileged, but delegated tracing or reduced-admin environments can make this closer to PR:L.
* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2 for conservative scoring. Writing kprobe events and enabling tracing normally requires root or tracing privileges.
* Rare or configuration-dependent condition: -1. The issue requires CONFIG_CALL_DEPTH_TRACKING, a retbleed-affected x86 platform, retbleed stuffing, and dynamic ftrace trampoline registration.
* No strong LPE signal. There is no shown attacker-controlled reclaim, arbitrary write, type confusion, callback control, or controlled target selection.

3. Reachability analysis
The practical trigger is local and normally requires privileged access to tracingfs or equivalent tracing control. A reduced-capability service, container setup, or delegated tracing policy could lower practical privileges, but this is not the common default. The path is not network reachable. The bug is configuration-specific to affected x86 retbleed mitigation and call-depth tracking setups. Call-site confidence: high, because the patch and commit directly identify create_trampoline(), the copied CALL_DEPTH_ACCOUNT instruction, and the resulting faulting write.

4. Severity interpretation
This behaves primarily as a reliable local DoS on a specific x86 tracing configuration. The theoretical concern is an invalid kernel write caused by an unrelocated RIP-relative percpu reference, but the patch does not show enough control over the destination or written value to treat it as a strong privilege-escalation primitive. Conservative handling is ordinary Moderate / Low-like due to privileged reachability. Paranoid handling is borderline manual-review Moderate because invalid kernel writes in generated trampoline code can be under-triaged.

5. One-sentence report phrase
A missing relocation for RIP-relative percpu references in x86 dynamic ftrace trampolines can cause the call-depth accounting instruction to access the wrong kernel address, leading to a reproducible local kernel panic on affected retbleed call-depth-tracking configurations.

6. Manual review recommendation
MANUAL CHECK REQUIRED. The observed impact is DoS, but the failure mode is an unintended kernel write from generated executable trampoline code, so it should not be auto-closed without confirming whether target control or mapped-address corruption is possible.

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

Patch: x86/ftrace: Relocate %rip-relative percpu refs in dynamic trampolines [ Upstream
Commit: 8093442a2d1d4b42b9340a86023ccb2afb30b93a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8093442a2d1d4b42b9340a86023ccb2afb30b93a

Changed files:
  arch/x86/kernel/ftrace.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=8093442a2d1d4b42b9340a86023ccb2afb30b93a

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

The original Linux kernel CVE announcement for CVE-2026-64235 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64235

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:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 1
  Paranoid ActionableScore: 4

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 REGULAR

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).