[CVE-2026-64300][IMPORTANT] perf/aux: Fix page UAF in map_range()

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

CVE: CVE-2026-64300
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: perf/aux: Fix page UAF in map_range()
Commit: c8b7e113f7b61eef2f017e6329c27c2331058c5a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c8b7e113f7b61eef2f017e6329c27c2331058c5a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64300
Analysis date: Sat, 25 Jul 2026 10:52:19 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in perf AUX mapping can leave a `VM_PFNMAP` user mapping pointing to a freed physical frame, creating a page UAF with plausible confidentiality, integrity, and local privilege escalation impact when local perf access is available.

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

The original Linux kernel CVE announcement for CVE-2026-64300 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64300

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: IMPORTANT
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-2026-64300	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A page use-after-free exists in the perf AUX mapping path because map_range reads rb aux_pages, rb aux_nr_pages, and rb aux_pgoff while only holding event mmap_mutex, while those AUX fields are protected by rb aux_mutex. Two perf events sharing the same ring buffer through PERF_EVENT_IOC_SET_OUTPUT can race rb_alloc_aux with map_range, allowing a VM_PFNMAP mapping to reference a physical frame after the page was freed. Pages mapped as VM_PFNMAP do not get normal refcount protection, so the stale user mapping can become a real freed physical page exposure rather than only a warning or NULL dereference. For the CVSS the PR:L is used because reliable triggering requires local code execution with access to perf_event_open, AUX mmap, and shared output setup, but not full administrator privileges when perf access is allowed by system policy. The issue is not network reachable. Impact is at least local denial of service via kernel crash, and the freed PFN mapping makes confidentiality and integrity impact plausible enough for manual review and possible local privilege escalation assessment.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-64300 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: b709eb872e19a19607bbb6d2975bc264d59735cf YES  NO NO unknown 	MAYBE	READ RACE UAF HARDWARE LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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

ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process may trigger this through perf event setup, AUX mmap, and `PERF_EVENT_IOC_SET_OUTPUT`, assuming perf access is allowed by system policy.

* Memory corruption, strong corruption primitive: +2
  The patch describes a real page UAF where a `VM_PFNMAP` VMA can retain a mapping to a freed physical frame.

* Real lifetime corruption: +1
  The bug is a lifetime race between AUX page allocation/free and `map_range()` walking `rb->aux_pages`.

* Privilege escalation plausible: +1 conservative, +2 paranoid
  Conservative scoring treats this as plausible but not demonstrated LPE because there is no provided reclaim or object replacement primitive. Paranoid scoring raises this because a stale user mapping to a freed physical frame can become a serious confidentiality/integrity primitive if the frame is reused for sensitive kernel or user data.

* Confidentiality impact plausible: +1
  A stale PFN mapping may expose data from a reused physical page.

* Integrity impact plausible: +1
  If the mapping is writable or can affect reused page contents, integrity impact is plausible.

* Hard or unreliable race: -1
  The trigger requires racing `rb_alloc_aux()` with `map_range()` across shared perf ring buffer state.

Not counted:

* Remote reachable: +0
  This is not network-triggerable.

* Broad/default exposure: +0
  Perf is common, but practical reachability depends heavily on `perf_event_paranoid`, `CAP_PERFMON`, `CAP_SYS_ADMIN`, and distro policy.

Call-site confidence: high. The commit provides the affected function, the relevant shared fields, the missing lock, and the exact race sequence.

## 3. Reachability analysis

The bug is locally reachable through perf AUX buffer mapping when two events share one ring buffer via `PERF_EVENT_IOC_SET_OUTPUT`. A fully unprivileged trigger depends on system policy. Many production systems restrict perf via `kernel.perf_event_paranoid` or require `CAP_PERFMON` / `CAP_SYS_ADMIN`, but permissive configurations or delegated perf access can make this reachable by a non-root local user.

Namespaces and containers usually reduce reachability because perf access is commonly blocked or capability-gated. However, if a container or reduced-privilege service is granted perf access, this should not be treated as full host-root only. The realistic exploitation condition is local code execution with sufficient perf access and the ability to create the shared AUX mapping race.

## 4. Severity interpretation

This is stronger than an ordinary Moderate race or crash-only bug. The important detail is that `VM_PFNMAP` pages do not have normal refcount protection, so the stale mapping can point to a freed physical frame after the AUX page is released. That creates a real page lifetime violation, not merely a warning or NULL dereference.

Realistic exploitation is still not proven by the patch alone because there is no demonstrated controlled reclaim, overwrite target, or stable LPE chain. Conservative handling should classify it as Actionable Moderate. Paranoid handling is a Strong Important candidate because stale physical-frame exposure has historically been a high-risk primitive and should not be auto-closed.

## 5. One-sentence report phrase

A race in perf AUX mapping can leave a `VM_PFNMAP` user mapping pointing to a freed physical frame, creating a page UAF with plausible confidentiality, integrity, and local privilege escalation impact when local perf access is available.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because this is a real page UAF involving a stale PFN mapping to freed physical memory. Even though exploitation requires a race and perf access, the primitive is strong enough to warrant manual assessment for possible local privilege escalation or sensitive data exposure.

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

Patch: perf/aux: Fix page UAF in map_range()
Commit: c8b7e113f7b61eef2f017e6329c27c2331058c5a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c8b7e113f7b61eef2f017e6329c27c2331058c5a

Commit description:

map_range() reads rb->aux_pages[], rb->aux_nr_pages and rb->aux_pgoff via
perf_mmap_to_page() while holding only event->mmap_mutex. Those fields are
serialized by rb->aux_mutex, and mmap_mutex is per event.

Thus, two events sharing one rb via PERF_EVENT_IOC_SET_OUTPUT can race
rb_alloc_aux() with map_range(), leading to a page-UAF scenario as follows:

  CPU 0                           CPU 1
  =====                           =====
  rb_alloc_aux()                  map_range()
  [1]: allocate rb->aux_pages[0]
  [2]: rb->aux_nr_pages++
                                  [3]: perf_mmap_to_page()
                                         returns rb->aux_pages[0]
                                  [4]: map it as VM_PFNMAP
  [5]: rb->aux_pgoff = 1

  munmap the page
  [6]: free rb->aux_pages[0]

Pages mapped as VM_PFNMAP have no refcount protection, so CPU 1 holds a
mapping to a freed physical frame.

Fix this by taking rb->aux_mutex across the page walk in map_range().

Fixes: b709eb8 ("perf: map pages in advance")
Signed-off-by: Lee Jia Jie <jiajie.lee@starlabs.sg>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: stable@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/events/core.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=c8b7e113f7b61eef2f017e6329c27c2331058c5a

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

The original Linux kernel CVE announcement for CVE-2026-64300 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64300

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:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 6
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).