[CVE-2026-98551][MODERATE REGULAR] perf/core: Fill branch entries with a single assignment [ Upstream

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

CVE: CVE-2026-98551
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: perf/core: Fill branch entries with a single assignment [ Upstream
Commit: c7de8521947a521b766979ff5c09f66fe905a5bd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c7de8521947a521b766979ff5c09f66fe905a5bd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98551
Analysis date: Fri, 23 Oct 2026 21:25:15 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
ARM64 BRBE perf sampling can disclose limited uninitialized kernel heap bits through uncleared new_type and priv fields in perf_branch_entry, requiring local perf access but providing no demonstrated write, privilege escalation or denial-of-service primitive.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98551 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98551

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-98551	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98551 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N';*CWE-457;*CWE-200;CWE-908;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '3.3';DESCR 'perf branch entries on ARM64 can expose uninitialized kernel data because perf_clear_branch_entry_bitfields() does not clear the newer new_type and priv fields. On the first pass over a kmalloc allocated branch stack these fields can contain residual heap bits, while later records can retain metadata from the previous entry. The issue is locally reachable through permitted perf branch sampling on an ARM64 system with BRBE support. For the CVSS the PR is PR:L because a caller must execute locally and open a suitable perf event, although system perf restrictions may require CAP_PERFMON or may block the trigger entirely. The primitive is a limited information leak from stale bitfields with C:L impact. There is no evidence of a write primitive, memory corruption, kernel pointer disclosure, privilege escalation or remote reachability. Manual review is warranted because the exact number and sensitivity of leaked bits depend on the current perf_branch_entry layout, ARM64 BRBE record type and the data previously held by the per-CPU allocation. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 3.3 and the production ActionableScore returned 4 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage IMPORTANT to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. KP and FD specialist checks stage MODERATE_KPANIC to MODERATE because KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 3.3 and ActionableScore 2 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	MAYBE	INIT SPECTRE HARDWARE LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	guess

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::
* Conservative CVSS:3.1 vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N (2.5).
* Highest defensible paranoid CVSS:3.1 vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N (3.3).
* No floor or cap changed the scores.

2. Signal breakdown

* +2. A confirmed kernel-to-userspace information disclosure exists. ARM64 BRBE can export uninitialized new_type and priv bits from a kmalloc allocated branch-stack entry.
* +1. The disclosure is observable through perf branch samples and can recur across branch-stack slots. Later samples can also expose values retained from previous records.
* +1 paranoid only. First-use entries contain allocator-origin residual bits rather than only stale branch metadata. Their exact sensitivity and usefulness require inspection of the structure layout and allocation behavior.
* No point for memory corruption. The defect does not write outside an object or alter kernel control state.
* No point for substantial disclosure. Only the omitted new_type and priv bitfields are established as uninitialized.
* No point for privilege escalation, arbitrary read, pointer disclosure, integrity impact, availability impact or remote reachability.
* No privilege penalty was applied because host-administrator privilege is not positively established. Ordinary-user access is also configuration-dependent and therefore not treated as confirmed.
* CWE-457. Use of Uninitialized Variable.
* CWE-200. Exposure of Sensitive Information to an Unauthorized Actor.
* CWE-908. Use of Uninitialized Resource.

3. Reachability analysis

The affected path is ARM64 BRBE branch sampling. arm_pmuv3 allocates a per-CPU branch stack with kmalloc(). BRBE records are converted by perf_entry_from_brbe_regset(). The obsolete perf_clear_branch_entry_bitfields() helper clears older metadata fields but leaves new_type and priv unchanged. brbe_set_perf_entry_type() does not assign new_type for an unrecognized branch type, and source-only records do not assign priv. The resulting perf_branch_entry can then reach userspace through normal perf sample output.

A caller must execute locally and obtain permission to create an appropriate perf branch-sampling event through perf_event_open(). For the CVSS the PR is PR:L because the trigger requires a local process and a device-like kernel performance interface. On a concrete system, perf_event_paranoid, CAP_PERFMON policy, PMU access controls or other perf restrictions may prevent an ordinary user from reaching the path.

The supplied evidence does not establish whether the complete trigger is available below host-admin privilege on every affected deployment. HostAdminRequired and RequiresHostUID0 therefore remain UNKNOWN. No user-namespace or network-namespace capability path is established. NamespaceScopedPrivilege is also UNKNOWN.

Required prerequisites are ARM64, BRBE-capable hardware, enabled perf PMU support and permission to collect relevant branch samples. There is no network trigger, race requirement, retry sequence, shared-backing write, authorization bypass or prior object-lifetime violation.

4. Primitive and severity interpretation

The exact primitive is a limited disclosure of uninitialized kernel allocation bits through the new_type and priv fields of struct perf_branch_entry. On the first pass, those fields can contain residual kmalloc data. On subsequent use, they can retain metadata from a previous record in the same slot.

This is not an OOB read, OOB write, UAF, stale post-lifetime access, arbitrary read or memory-corruption primitive. No address, pointer, credential, key material or other specifically sensitive value is shown to fit within or be reconstructible from the exposed fields. Integrity and availability impact are not established.

PrimitiveClass is CLASS_B because a real kernel information-disclosure primitive reaches userspace and its exact sensitivity requires review. It is not CLASS_A because the disclosure is narrow and no substantial kernel disclosure or exploitation chain is demonstrated.

ImportantCandidate is NO because the patch supports only a limited confidentiality impact. There is no strong corruption, authorization, ownership, cross-boundary or privilege-escalation primitive. AutoDowngradeAllowed is YES because no unresolved Important-preserving primitive exists. Manual review remains required to confirm the exact field widths, allocation reuse behavior, sample permissions and whether repeated observations provide useful kernel-state information.

The patch replaces partial bitfield clearing with whole-structure assignments. Unnamed fields are consequently initialized to zero, and the obsolete helper is removed.

5. One-sentence report phrase

ARM64 BRBE perf sampling can disclose limited uninitialized kernel heap bits through uncleared new_type and priv fields in perf_branch_entry, requiring local perf access but providing no demonstrated write, privilege escalation or denial-of-service primitive.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The disclosure is confirmed, but its practical sensitivity depends on the exact bitfield layout, ARM64 BRBE record forms, kmalloc reuse and effective perf_event_open permission policy.

7. Call-site confidence

Call-site confidence: high

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

Patch: perf/core: Fill branch entries with a single assignment [ Upstream
Commit: c7de8521947a521b766979ff5c09f66fe905a5bd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c7de8521947a521b766979ff5c09f66fe905a5bd

Changed files:
  arch/x86/events/amd/brs.c
  arch/x86/events/amd/lbr.c
  arch/x86/events/intel/lbr.c
  drivers/perf/arm_brbe.c
  include/linux/perf_event.h

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=c7de8521947a521b766979ff5c09f66fe905a5bd

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

The original Linux kernel CVE announcement for CVE-2026-98551 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98551

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:L/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N
  The Best / paranoid CVSS score: 3.3

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: 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).