[CVE-2026-74637][MODERATE 7.0] perf/core: Fix group leader use-after-free after sibling detach

[CVE-2026-74637][MODERATE 7.0] perf/core: Fix group leader use-after-free after sibling detach

[AL-KERNEL]

CVE: CVE-2026-74637
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: perf/core: Fix group leader use-after-free after sibling detach
Commit: a979a642402d0b1f856c7a729b4cb2d92de4cf2f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a979a642402d0b1f856c7a729b4cb2d92de4cf2f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74637
Analysis date: Sat, 22 Aug 2026 17:51:26 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local perf_events use-after-free can occur when a detached sibling event keeps a stale group_leader pointer after CPU hotplug, allowing PERF_IOC_FLAG_GROUP to dereference a freed leader and crash the kernel, with LPE remaining a manual-review concern but not demonstrated by the patch.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74637 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74637

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-2026-74637	MODERATE	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-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use after free in perf event group handling can occur when a sibling event is detached with DETACH_GROUP and kept alive while its group_leader pointer still references the old leader. If the old leader is later closed and freed, a local process can issue a PERF_IOC_FLAG_GROUP ioctl on the remaining sibling and perf_event_for_each can follow the stale group_leader pointer into freed memory. For the CVSS the PR:L is used because the attacker needs local access to perf event file descriptors and ioctl access, while full administrator privileges are not inherent to the vulnerable operation in permissive perf configurations. The issue is not network reachable. Impact is at least local denial of service via kernel crash and in the paranoid case may include privilege escalation due to a concrete kernel use after free race.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	OOB DANGER RACE UAF HARDWARE KPANIC 	NO	NO	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: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative scoring:

* Local unprivileged trigger: +1
  A local process with access to perf event file descriptors can later call PERF_IOC_FLAG_GROUP on the detached sibling. Actual reachability depends on perf_event_paranoid and local perf access policy.

* Memory corruption, weak or indirect corruption candidate: +1
  The bug is a concrete UAF, but the demonstrated primitive is stale pointer dereference of a freed group leader, not controlled reclaim, arbitrary write, type confusion, or callback control.

* Real lifetime corruption: +1
  The stale sibling->group_leader pointer survives after the old leader can be closed and freed.

* Reliable kernel crash / strong DoS: +1
  The commit includes a reproduced kernel paging fault in perf_ioctl.

* Broad/default/common subsystem exposure: +1
  perf_events is a common kernel subsystem, though user access is often policy restricted.

* Hard or unreliable race / special timing required: -1
  The reproducer needs perf fuzzing, CPU hotplug, and stress workload concurrently. CPU hotplug is a significant triggering condition.

Conservative total: 4

Paranoid adjustment:

* Do not subtract hard timing: +1 relative to conservative
  The bug was reproduced with a concrete workload, and CPU hotplug is realistic in some environments. However, the race-UAF downgrade rule still applies because no attacker-controlled reclaim or write primitive is shown.

Paranoid total: 5

## 3. Reachability analysis

The direct attacker-controlled operation is local perf_event usage followed by ioctl on a surviving sibling event. In permissive configurations, a local unprivileged user may obtain relevant perf file descriptors. In hardened deployments, perf_event_open is commonly restricted by perf_event_paranoid or capabilities, reducing reachability.

The DETACH_GROUP path is associated with CPU hotplug. CPU hotplug itself is typically administrator controlled, so a fully reliable trigger may require either administrator-induced CPU hotplug, system hotplug activity, or a test environment where hotplug is available. Containers and namespaces do not automatically make this reachable, but reduced-capability service or container contexts with perf access should not be treated as full host-root.

The issue is not network reachable.

Call-site confidence: high. The commit message explicitly identifies perf_ioctl, PERF_IOC_FLAG_GROUP, perf_event_for_each, and dereference of the freed leader context.

## 4. Severity interpretation

This is more than an ordinary Moderate because it is a real kernel UAF reachable from a local perf ioctl path and has a demonstrated crash. It should not be auto-closed.

At the same time, the demonstrated impact is a local kernel crash. The patch and commit do not show attacker-controlled reclaim, write-after-free, callback dispatch, type confusion, refcount takeover, or arbitrary write. Therefore the conservative score stays in the manual-review borderline range, while the paranoid score reaches Actionable Moderate.

Realistic impact: local DoS.
Theoretical impact: possible LPE concern due to kernel UAF class, but not strongly demonstrated by this patch alone.

## 5. One-sentence report phrase

A local perf_events use-after-free can occur when a detached sibling event keeps a stale group_leader pointer after CPU hotplug, allowing PERF_IOC_FLAG_GROUP to dereference a freed leader and crash the kernel, with LPE remaining a manual-review concern but not demonstrated by the patch.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a concrete kernel UAF in a common subsystem with a reproduced crash and a local ioctl trigger path. Even though the exploitability evidence currently supports DoS more strongly than privilege escalation, the lifetime-corruption class and perf object context warrant manual review.

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

Patch: perf/core: Fix group leader use-after-free after sibling detach
Commit: a979a642402d0b1f856c7a729b4cb2d92de4cf2f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a979a642402d0b1f856c7a729b4cb2d92de4cf2f

Commit description:

perf_group_detach() handles leader and sibling detach differently. When the
group leader is detached, all siblings are promoted to singleton events and
their group_leader pointer is reset to themselves. When a sibling is
detached, it is removed from the leader's sibling_list, but its
group_leader pointer is left pointing at the old leader.

That is harmless when the sibling is being closed and freed immediately, as
in the DETACH_DEAD path. It is not safe when the sibling is detached but
kept alive, such as during CPU hotplug with DETACH_GROUP. In that case the
sibling is removed from the context, while its file descriptor can still
keep it alive.

A typical failing sequence is:

  - A group contains leader L and sibling S.
  - CPU hot-unplug detaches S with DETACH_GROUP, removing it from
    L->sibling_list but leaving S->group_leader == L.
  - L is later closed and freed.
  - A PERF_IOC_FLAG_GROUP ioctl on S follows S->group_leader and
    dereferences the freed leader.

This was reproduced by running the perf event fuzzer, CPU hotplug, and a
stress workload concurrently:

  Unable to handle kernel paging request at virtual address 006b6b6b6b6b6cdb
  CPU: 2 PID: 12489 Comm: perf_fuzzer 6.18.7 PREEMPT
  pc : perf_ioctl+0x34c/0xc68
  x20: ffffff89a3fa2c70 x8 : 6b6b6b6b6b6b6b6b
  Code: 943c4a0e 340047a0 f9404a94 f9411e88 (f940b908)
  Call trace:
  perf_ioctl+0x34c/0xc68 (P)
  __arm64_sys_ioctl+0xa0/0xf4
  invoke_syscall+0x58/0xe4
  el0_svc_common+0xa8/0xdc
  do_el0_svc+0x1c/0x28
  el0_svc+0x40/0xc0
  el0t_64_sync_handler+0x68/0xdc
  el0t_64_sync+0x1c4/0x1c8

The fault happened in perf_ioctl(), where perf_event_for_each() follows
the stale group_leader pointer and perf_event_for_each_child() then
dereferences the freed leader's context.

Fix the use-after-free by promoting the detached sibling to a singleton.
Also fix __event_disable() cgroup accounting and event state change.

Fixes: 8a49542 ("perf_events: Fix races in group composition")
Assisted-by: PatchWise:gpt-5.5
Signed-off-by: Aditya Chillara <aditya.chillara@oss.qualcomm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260807-fix-group-leader-uaf-v3-1-b0c2310c9a0d@oss.qualcomm.com
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=a979a642402d0b1f856c7a729b4cb2d92de4cf2f

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

The original Linux kernel CVE announcement for CVE-2026-74637 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74637

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: 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).