[CVE-2026-72237][MODERATE REGULAR] perf/x86/amd/brs: Fix kernel address leakage

[CVE-2026-72237][MODERATE REGULAR] perf/x86/amd/brs: Fix kernel address leakage

[AL-KERNEL]

CVE: CVE-2026-72237
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: perf/x86/amd/brs: Fix kernel address leakage
Commit: ac44b4a3d6137489f8fa2e794b12e849c6b22eaa
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ac44b4a3d6137489f8fa2e794b12e849c6b22eaa
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72237
Analysis date: Sun, 16 Aug 2026 07:09:32 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline manual review recommended / Ordinary Moderate with KASLR leakage impact, not an Important candidate
Manual review required: YES

Summary:
AMD BRS perf branch sampling may expose kernel branch source addresses to local user space when user-only branch sampling is requested, weakening KASLR but not directly causing memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-72237 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72237

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-2026-72237	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:N/A:N';*CWE-200;CWE-668;CWE-284;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.5';DESCR 'AMD BRS branch sampling can expose kernel branch source addresses to user space when a user only branch stack is requested. The software privilege filter checked only the branch target address, so kernel to user transitions such as SYSRET or interrupt returns could leave a kernel branch from address visible in perf output. For the CVSS the PR:L is used because a local user or process must be able to access perf branch sampling, while hardened systems may further restrict this through perf_event_paranoid or capabilities. The issue is not network reachable and does not by itself provide memory corruption or code execution. Impact is information disclosure that can weaken KASLR and may help exploit a separate local kernel vulnerability.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Ordinary Moderate with KASLR leakage impact, not an Important candidate (with actual score 3)	YES	OOB LEAK HARDWARE  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline manual review recommended / Ordinary Moderate with KASLR leakage impact, not an Important candidate**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local user or process with access to perf branch sampling can request user-only branch stacks and observe leaked kernel branch-from addresses.

* Confidentiality impact plausible: +1
  The bug leaks kernel text addresses to user space and can weaken KASLR. It does not disclose arbitrary kernel memory.

Paranoid-only signal:

* Broad/default/common subsystem or broadly deployed exposure: +1
  perf is a common kernel observability interface, and affected AMD Zen 3 systems are realistic deployments. This is counted only in the paranoid score because access is often restricted by perf_event_paranoid or capabilities.

Signals not awarded:

* No remote/network trigger.
* No generic memory corruption.
* No UAF, OOB write, double-free, refcount abuse, or type confusion.
* No direct privilege escalation primitive.
* No reliable kernel crash or availability impact.

## 3. Reachability analysis

The issue is locally reachable through perf branch sampling on AMD processors supporting X86_FEATURE_BRS, described as Zen 3 only. A user must be able to create the relevant perf event and read branch stack output. On hardened systems this may require relaxed perf_event_paranoid settings or capabilities such as CAP_PERFMON, CAP_SYS_ADMIN, or equivalent policy allowance.

Namespaces do not automatically make this remotely reachable. Container impact depends on whether the container can access perf events for the host or relevant CPU PMU. In typical locked-down container deployments, this path is restricted.

## 4. Severity interpretation

This behaves like an ordinary Moderate information disclosure issue with manual-review relevance. The realistic impact is kernel address disclosure and KASLR weakening. There is no evidence in the patch of memory corruption, code execution, arbitrary read, arbitrary write, or direct LPE.

The paranoid score is higher only because kernel address leaks can materially assist exploitation of a separate local kernel memory corruption vulnerability.

## 5. One-sentence report phrase

AMD BRS perf branch sampling may expose kernel branch source addresses to local user space when user-only branch sampling is requested, weakening KASLR but not directly causing memory corruption or privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because kernel address disclosure can assist later local exploitation chains, but this patch alone supports only confidentiality impact and does not show a direct LPE or memory corruption primitive.

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

Patch: perf/x86/amd/brs: Fix kernel address leakage
Commit: ac44b4a3d6137489f8fa2e794b12e849c6b22eaa
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ac44b4a3d6137489f8fa2e794b12e849c6b22eaa

Commit description:

A user-only branch stack can contain branches that originate from
the kernel. As a result, kernel addresses are exposed to user space
even when PERF_SAMPLE_BRANCH_USER is requested. On AMD processors
supporting X86_FEATURE_BRS (Zen 3 only), perf can still report entries
such as SYSRET/interrupt returns for which the branch-from addresses
are in the kernel.

E.g.

  $ perf record -j any,u -c 4000 -e branch-brs -o - -- \
        perf bench syscall basic --loop 1000 | \
        perf script -i - -F brstack|tr ' ' '\n'| \
        grep -E '0x[89a-f][0-9a-f]{15}'

  ...
  0xffffffff810001c4/0x72e2e32955eb/-/-/-/0//-
  0xffffffff810001c4/0x72e2d94a9821/-/-/-/0//-
  0xffffffff810001c4/0x72e2d94ffa1b/-/-/-/0//-
  ...

BRS provides no hardware branch filtering, so privilege level
filtering is performed entirely in software. However, amd_brs_match_plm()
only validates the branch-to address against the requested privilege
levels. For branches from the kernel to user space, the branch-from
address is left unchecked and is leaked. Extend the software filter to
also validate the branch-from address, so that any branch record whose
branch-from address is in the kernel is dropped when
PERF_SAMPLE_BRANCH_USER is requested.

Fixes: 8910075 ("perf/x86/amd: Enable branch sampling priv level filtering")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Sandipan Das <sandipan.das@amd.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: stable@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://patch.msgid.link/f05931c4f89a146c364bd5dc6b8170b1ac611c65.1783701239.git.sandipan.das@amd.com
Closes: https://lore.kernel.org/all/20260710110235.F3FD81F000E9@smtp.kernel.org/
[sandipan: backport to linux-6.6.y]
Signed-off-by: Sandipan Das <sandipan.das@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/events/amd/brs.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=ac44b4a3d6137489f8fa2e794b12e849c6b22eaa

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

The original Linux kernel CVE announcement for CVE-2026-72237 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72237

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:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
  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: 2
  Paranoid ActionableScore: 3

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).