[CVE-2026-80778][IMPORTANT] futex/pi: Reject cross-mm private futex owners

[CVE-2026-80778][IMPORTANT] futex/pi: Reject cross-mm private futex owners

[AL-KERNEL]

CVE: CVE-2026-80778
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: futex/pi: Reject cross-mm private futex owners
Commit: 2b92e5562653b5293529f63b0300837d9dcedbd7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2b92e5562653b5293529f63b0300837d9dcedbd7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80778
Analysis date: Fri, 04 Sep 2026 14:10:13 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local race in futex PI handling can attach a private futex key across different mm address spaces, allowing owner exit cleanup to access a freed private futex hash and potentially causing a kernel crash or UAF-sensitive corruption.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80778 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80778

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-80778	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-80778 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:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in futex PI handling can attach a private futex key from one mm to an owner task in another mm even though the private key only borrows the waiter mm without an mm_users reference. During owner exit, exit_pi_state_list can later use the saved key to access the waiters private futex hash while the last user of that mm concurrently exits and frees the hash, creating a use-after-free condition in kernel futex cleanup. For the CVSS the PR:L is used because a local unprivileged process can use futex PI operations, but reliable triggering still requires local code execution and race timing. The issue is not network reachable. Impact is at least local denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because the bug class is kernel use-after-free with a plausible privilege escalation review path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	OOB LOCK DANGER INIT HARDWARE LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	YES	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 signals:

* Local unprivileged trigger: +1. Futex PI operations are reachable through normal futex syscalls by local user processes.
* Memory corruption, weak primitive: +1. The patch describes a race where a private futex hash may be freed while still used through a saved private key. This is a UAF candidate, but no controlled reclaim or arbitrary write primitive is shown.
* Real lifetime corruption: +1. The bug is explicitly about borrowing waiter mm without mm_users reference and later using a freed private hash during owner exit cleanup.
* Reliable kernel crash / strong DoS: +1. A freed hash bucket/reference use in futex cleanup is a credible kernel crash path.
* Broad/default/common subsystem: +1. Futex is a core Linux userspace synchronization API.
* Hard or unreliable race / special timing required: -1. Triggering depends on a cross-mm PI futex owner setup plus concurrent mm teardown and owner exit cleanup.

Conservative total: 4.

Paranoid additional signal:

* Weak LPE concern: +1. The freed private hash may still be referenced and have lifetime/reference state manipulated during PI cleanup, which is enough for manual-review sensitivity, but not enough for strong LPE scoring.

Paranoid total: 5.

## 3. Reachability analysis

A local unprivileged process can normally invoke futex operations, including PI futex paths, without root privileges. The affected condition requires private futex state to be attached across different mm address spaces and then raced against task exit or mm teardown. This makes the bug local and timing-dependent, not network reachable.

Namespaces do not remove concern because futex syscalls are broadly available to containerized or sandboxed workloads unless specifically filtered by seccomp or other policy. The path is part of the core kernel futex subsystem and is not a rare hardware or driver-only configuration.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is a real kernel lifetime bug in a broad syscall path. Conservative handling should treat it as local DoS with UAF risk, not as a simple validation cleanup.

Theoretical privilege escalation is plausible enough for manual review because the bug involves stale lifetime state in futex PI cleanup, but the supplied patch does not demonstrate attacker-controlled reclaim, object replacement, callback control, arbitrary write, or a reliable LPE primitive. Therefore the practical bucket is Actionable Moderate at minimum, with Important-candidate review justified only if further analysis finds controlled reuse or refcount/object corruption.

## 5. One-sentence report phrase

A local race in futex PI handling can attach a private futex key across different mm address spaces, allowing owner exit cleanup to access a freed private futex hash and potentially causing a kernel crash or UAF-sensitive corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

Reason: this is a local kernel race UAF in the core futex PI path, reachable by local users and involving mm lifetime plus futex hash lifetime. The current evidence supports Actionable Moderate, while privilege escalation remains plausible enough to require human review.

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

Patch: futex/pi: Reject cross-mm private futex owners
Commit: 2b92e5562653b5293529f63b0300837d9dcedbd7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2b92e5562653b5293529f63b0300837d9dcedbd7

Commit description:

A private futex key borrows the waiter's mm without taking an mm_users
reference. Nevertheless, attach_to_pi_owner() currently accepts an owner
from a different address space and copies the private key into the owner's
PI state.

When that owner exits, exit_pi_state_list() uses the saved key to find the
hash bucket and acquires a reference to the waiter's private hash. If the
last user of the waiter's mm exits concurrently, futex_hash_free() frees
the hash while the owner still uses its bucket and reference.

Prevent this by validating in attach_to_pi_owner() that, for private
futexes, the owner mm and waiter mm are the same. Perform the check with
the owner's pi_lock held and after validating owner::futex::state to
serialize against a concurrent PI-state exit cleanup.

[ tglx: Amended comment ]

Fixes: 80367ad ("futex: Add basic infrastructure for local task local hash")
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Assisted-by: Codex:gpt-5.6-sol
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/futex/core.c
  kernel/futex/futex.h
  kernel/futex/pi.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=2b92e5562653b5293529f63b0300837d9dcedbd7

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

The original Linux kernel CVE announcement for CVE-2026-80778 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80778

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:N/I:N/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: 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).