[CVE-2026-97944][LOW] x86/cfi: Fix FineIBT hash offset in cfi_get_func_hash()

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

CVE: CVE-2026-97944
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: x86/cfi: Fix FineIBT hash offset in cfi_get_func_hash()
Commit: cd9b4595d6283f9b8a35d7ae607508de45a351ce
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cd9b4595d6283f9b8a35d7ae607508de45a351ce
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97944
Analysis date: Sat, 26 Sep 2026 15:57:08 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-97944 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97944

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: NO

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

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

CVE-2026-97944	LOW	https://www.kernelcve.org/list/?q=CVE-2026-97944 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:N/I:N/A:H';CWE-682;CWE-693;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 'An incorrect open coded offset in cfi_get_func_hash() makes the function read the FineIBT hash from byte 7 even though the immediate value starts at byte 5. The BPF JIT can therefore assign a corrupted CFI hash to a struct_ops trampoline. The first indirect kernel call into the affected BPF struct_ops program can fail the FineIBT check and terminate the kernel with a fatal CFI failure. For the CVSS the PR:L value is used for the paranoid score because BPF struct_ops access may be delegated through CAP_BPF, CAP_NET_ADMIN, or a restricted service account and does not always imply full host administration. PR:N is not appropriate because a local process with access to the BPF control interface is required. The issue is not directly network reachable. Network activity may reach an affected congestion control callback only after the required BPF program has been locally loaded and attached. Impact is denial of service through a kernel crash. There is no supported UAF, out of bounds write, information disclosure, or privilege escalation primitive in this code path.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)  SKIP CVE-2026-97944 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	BPF HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* No floor or cap changed either score.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel crash / strong DoS. The malformed trampoline hash causes the first affected indirect call to fail FineIBT validation and terminate the kernel.
* +0 Weak corruption concern. Reading the hash at offset 7 produces an incorrect value, but the read remains within mapped instruction bytes and no invalid memory access is established.
* +0 Remote reachability. Network activity can invoke an already attached congestion-control callback, but it cannot independently load and attach the required BPF struct_ops program.
* +0 Local unprivileged trigger. Complete below-host-admin access to BPF loading, JIT compilation, struct_ops attachment, and activation is not established.
* +0 Strong semantic reinterpretation. The malformed hash affects active CFI validation, but it is not derived from attacker-controlled bytes and only a fail-closed CFI crash is demonstrated.
* +0 Confidentiality and integrity. No UAF, OOB write, disclosure, controlled reclaim, arbitrary write, callback redirection, or CFI bypass is established.
* +0 Additional Availability. The fatal CFI failure is already counted as the single crash signal.
* No privilege penalty was applied because host-admin-equivalent privilege is not positively established.
* No configuration penalty was applied merely because CONFIG_FINEIBT and BPF struct_ops are required.

3. Reachability analysis

The direct actor is a local process that can load and attach a BPF struct_ops program. The supplied example uses BPF congestion control and reaches the failure when the kernel invokes the attached init callback.

The complete privilege boundary is unresolved. BPF program loading may require CAP_BPF, CAP_SYS_ADMIN, or product-specific policy, while congestion-control attachment may additionally involve CAP_NET_ADMIN. The evidence does not establish whether all required checks can be satisfied by a reduced service account, an attacker-created namespace, or only a host administrator. Therefore HostAdminRequired and NamespaceScopedPrivilege remain UNKNOWN.

Network traffic alone is insufficient. It may activate the callback only after local BPF loading and attachment. No race or special timing is required. The failure is deterministic on an affected kernel with FineIBT enabled and the vulnerable BPF struct_ops path exercised. CONFIG_FINEIBT and an affected kernel version are prerequisites, but no rare physical hardware requirement is established.

4. Primitive and severity interpretation

The exact defect is an incorrect instruction-layout offset. cfi_get_func_hash reads two bytes from the real immediate and two bytes from the following instruction, then assigns the resulting invalid CFI hash to a BPF struct_ops trampoline.

This is not a UAF, lifetime violation, repeated release, active-state retirement bypass, or writable memory-corruption primitive. It is fail-closed control-flow validation behavior. The demonstrated consequence is a fatal CFI failure and kernel DoS.

No useful primitive remains after excluding the crash. In particular, the patch does not establish bypass of FineIBT, attacker-directed execution, disclosure, or modification of protected state. PrimitiveClass is therefore CLASS_C. ImportantCandidate is NO because there is no unresolved corruption or security-boundary primitive. AutoDowngradeAllowed is YES because the issue is a configuration-dependent crash with no demonstrated exploitation leverage beyond availability.

5. One-sentence report phrase

An incorrect FineIBT hash offset causes BPF struct_ops trampolines to receive a malformed CFI hash, resulting in a deterministic kernel crash when an affected callback is invoked, without evidence of memory corruption, CFI bypass, information disclosure, or privilege escalation.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a fail-closed CFI crash only, with no UAF, write primitive, authorization bypass, or unresolved lifetime corruption.

7. Call-site confidence

Call-site confidence: high

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

Patch: x86/cfi: Fix FineIBT hash offset in cfi_get_func_hash()
Commit: cd9b4595d6283f9b8a35d7ae607508de45a351ce
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cd9b4595d6283f9b8a35d7ae607508de45a351ce

Commit description:

The switch of the FineIBT preamble from "subl $hash, %r10d" to the
shorter "subl $hash, %eax" moved the hash immediate from offset 7 to
offset 5 of the preamble. fineibt_preamble_hash was updated to match,
but the open-coded offset in cfi_get_func_hash() was missed and it
still reads the hash at offset 7.

cfi_get_func_hash() is used by the BPF JIT to give a struct_ops
trampoline the CFI hash of the stub function it stands in for. With
FineIBT the trampoline now gets the upper half of the real hash
followed by the first two bytes of the next instruction, so the first
indirect call from the kernel into a struct_ops program,
tcp_init_congestion_control() calling ->init() of a BPF congestion
control for example, fails the FineIBT check and the kernel dies with
a CFI failure.

Move the FineIBT preamble template and its offset defines above
cfi_get_func_hash() and use fineibt_preamble_hash there, so every
reader of the preamble shares one definition of its layout. The
CFI_FINEIBT arm is only built with CONFIG_FINEIBT, the only
configuration in which cfi_mode can take that value.
cfi_get_func_arity() does not need the same treatment: the __bhi_args
call whose displacement it reads still ends at the function address.

Fixes: 85a2d4a ("x86,ibt: Use UDB instead of 0xEA")
Assisted-by: LLM
Signed-off-by: Soheil Hassas Yeganeh <soheil.kdev@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org # 6.18+
Link: https://patch.msgid.link/20260831-b4-x86-cfi-fineibt-func-hash-v1-1-6ffc0af5c4ec@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kernel/alternative.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=cd9b4595d6283f9b8a35d7ae607508de45a351ce

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

The original Linux kernel CVE announcement for CVE-2026-97944 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97944

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: unknown
  Paranoid ActionableScore: 1

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).