[CVE-2026-64402][MODERATE 7.0] coresight: ultrasoc-smb: Fix OOB write in smb_sync_perf_buffer()

[CVE-2026-64402][MODERATE 7.0] coresight: ultrasoc-smb: Fix OOB write in smb_sync_perf_buffer()

[AL-KERNEL]

CVE: CVE-2026-64402
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: coresight: ultrasoc-smb: Fix OOB write in smb_sync_perf_buffer()
Commit: 38dbc8db8341ccdf8e1e1a067453d33ad751864b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=38dbc8db8341ccdf8e1e1a067453d33ad751864b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64402
Analysis date: Sat, 25 Jul 2026 12:26:13 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local OOB write in the CoreSight UltraSoc SMB perf AUX sink can occur because smb_sync_perf_buffer derives the first dst_pages index before normalizing head to the AUX ring size, allowing kernel memory corruption or crash on affected hardware.

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

The original Linux kernel CVE announcement for CVE-2026-64402 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64402

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-2026-64402	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-787;CWE-129;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'An out of bounds write exists in the CoreSight UltraSoc SMB perf AUX sink path because smb_sync_perf_buffer derives the first dst_pages index from handle head before normalizing it to the AUX ring buffer size. A local attacker with access to perf AUX tracing and the affected SMB sink may drive head beyond the AUX buffer size and cause the initial page index to address memory outside dst_pages. For the CVSS the PR:L is used because reliable triggering requires local access to perf or tracing interfaces, but not necessarily full administrator control in deployments where such access is delegated or available to low privileged users. The issue is not network reachable and depends on affected CoreSight hardware and driver use. Impact is at least local denial of service via kernel memory corruption and in the paranoid case may allow confidentiality and integrity impact or privilege escalation. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-64402 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	WRITE OOB SIMPLEFIX HARDWARE KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	-	-	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: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative scoring:

* Local unprivileged or low privileged trigger: +1
  Triggering requires local access to perf AUX tracing with the affected CoreSight UltraSoc SMB sink. This is not remote, but it may be available to non root or delegated tracing users in some deployments.

* Memory corruption, strong corruption primitive: +2
  The commit explicitly describes an out of bounds write past dst_pages[] caused by deriving pg_idx from an unnormalized head value.

* Weak LPE concern: +1
  This is a kernel OOB write, so privilege escalation cannot be dismissed. However, the target and payload appear constrained because the write copies hardware trace data through a perf AUX buffer path rather than providing an obvious arbitrary write primitive.

* Reliable kernel crash / strong DoS: +1
  Writing outside the perf AUX page array is a credible kernel crash or memory corruption path.

* Privileged kernel/device-management memory corruption path: +1
  The bug is in a trusted hardware tracing driver path, with data movement between CoreSight SMB trace hardware and perf AUX ring buffer pages.

* Corruption primitive highly constrained: -1
  The patch does not show attacker chosen target selection or fully attacker controlled overwrite contents.

* Rare AND difficult-to-reach subsystem/configuration: -1
  The issue depends on UltraSoc SMB CoreSight hardware and use as a perf AUX sink.

Paranoid delta:

* Stronger LPE plausibility instead of weak LPE concern: +1 net increase
  Because the primitive is an actual OOB write in kernel memory handling, a paranoid interpretation treats exploitation beyond DoS as plausible enough for manual review, even though no concrete reclaim or arbitrary write primitive is shown.

Call-site confidence: high. The commit message describes the caller smb_update_buffer(), the sink mode, the affected helper smb_sync_perf_buffer(), and the exact dst_pages indexing issue.

## 3. Reachability analysis

The bug is locally reachable through perf AUX tracing when the CoreSight UltraSoc SMB sink is present and used. It is not network reachable. Typical systems may restrict perf and hardware tracing via perf_event_paranoid, CAP_PERFMON, CAP_SYS_ADMIN, tracefs policy, or device access controls, so a fully unprivileged trigger is configuration dependent. Namespaces or containers matter only if tracing or perf access is delegated into that environment. The affected hardware path is not broadly present on all Linux systems, but on systems with this CoreSight device the vulnerable code is a real data movement path rather than dead code.

## 4. Severity interpretation

This is more than an ordinary Moderate correctness issue because the patch fixes an explicit OOB write in kernel tracing code. The realistic impact is local DoS or memory corruption on systems with the affected hardware and perf AUX usage. The theoretical risk includes privilege escalation, but the patch does not demonstrate attacker controlled target selection, arbitrary write, UAF reclaim, callback control, or type confusion. Therefore the conservative result is borderline manual review, while the paranoid result is Actionable Moderate at minimum.

## 5. One-sentence report phrase

A local OOB write in the CoreSight UltraSoc SMB perf AUX sink can occur because smb_sync_perf_buffer derives the first dst_pages index before normalizing head to the AUX ring size, allowing kernel memory corruption or crash on affected hardware.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch explicitly fixes a kernel OOB write. Even though reachability is hardware and perf access dependent, memory corruption in a privileged tracing path should not be auto-closed.

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

Patch: coresight: ultrasoc-smb: Fix OOB write in smb_sync_perf_buffer()
Commit: 38dbc8db8341ccdf8e1e1a067453d33ad751864b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=38dbc8db8341ccdf8e1e1a067453d33ad751864b

Commit description:

When the SMB sink is used as a perf AUX sink, smb_update_buffer() calls
smb_sync_perf_buffer() to copy hardware trace data into the perf AUX ring
buffer pages. It derives pg_idx = head >> PAGE_SHIFT from @Head , which is
handle->head, and indexes dst_pages[pg_idx]. The pg_idx %= nr_pages
normalization is only applied after the first loop iteration.

This leaves the initial page index underived from the buffer size, which
can result in an out-of-bounds write past dst_pages[] when head exceeds
the AUX buffer size.

Normalize head modulo the AUX buffer size before deriving the page index
and offset, mirroring tmc_etr_sync_perf_buffer().

Fixes: 06f5c29 ("drivers/coresight: Add UltraSoc System Memory Buffer driver")
Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Junrui Luo <moonafterrain@outlook.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/SYBPR01MB788156B3380A36835DB22290AF102@SYBPR01MB7881.ausprd01.prod.outlook.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/hwtracing/coresight/ultrasoc-smb.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=38dbc8db8341ccdf8e1e1a067453d33ad751864b

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

The original Linux kernel CVE announcement for CVE-2026-64402 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64402

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:L/UI:N/S:U/C:L/I:L/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: 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).