[CVE-2026-89766][MODERATE 7.0] pidfd: hold exec_update_lock around namespace ioctl [ Upstream

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

CVE: CVE-2026-89766
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: pidfd: hold exec_update_lock around namespace ioctl [ Upstream
Commit: cf24ce48d60225fdbee68ed2058763a1fd172ff3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cf24ce48d60225fdbee68ed2058763a1fd172ff3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89766
Analysis date: Fri, 11 Sep 2026 22:28:59 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89766 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89766

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89766	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89766 CHECK WITH IMPACT FROM ORIG NN LOW	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:L/A:N';CWE-367;CWE-863;CWE-362;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '5.3';DESCR 'A TOCTOU race in pidfd namespace ioctls can allow the caller to pass a filesystem credential ptrace check against the targets old credentials and then obtain a namespace file descriptor after the target completes execve with new setuid credentials. This can bypass the intended access decision for PIDFD_GET_*_NAMESPACE and expose namespace information or a namespace handle that should have been denied. For the CVSS the PR:L is used because a local user or local process context is required to use pidfd ioctls and race the target execve path. AC:H is used because reliable triggering depends on a timing window between the ptrace access check and the namespace lookup. The issue is not network reachable and does not indicate memory corruption. Impact is mainly an access control bypass with confidentiality impact and possible limited integrity policy impact through unauthorized namespace fd acquisition.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	DISK IMPROVEONLY LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5. No floor or cap applied.
* Raw paranoid score: 6
* Paranoid score: 6. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive conservative signals:

* +2 Concrete authorization decision bypass. `PIDFD_GET_*_NAMESPACE` can pass `ptrace_may_access(..., PTRACE_MODE_READ_FSCREDS)` against old target credentials and then return a namespace fd after the target has completed `execve()` with new setuid credentials.
* +1 Security-state lifetime mismatch. The credentials used for the access decision can diverge from the credentials associated with the namespace lookup because `exec_update_lock` was not held across both operations.
* +1 Local unprivileged trigger. The relevant interface is a local pidfd ioctl path, and the described attack does not require host-admin-equivalent privilege to the caller when racing a target process through setuid `execve()`.
* +1 Confidentiality plausible. The successful bypass exposes namespace information or a namespace file descriptor that should have been denied by the ptrace-style filesystem-credentials check.

Additional paranoid signal:

* +1 Practical exploit chain strongly implied. The commit describes the concrete chain: old-credential ptrace check succeeds, target commits new setuid credentials, namespace lookup happens afterward, and the caller obtains access that should have been denied.

Non-awarded material signals:

* RealUAF not awarded. No use-after-free or stale memory object use is established.
* Kernel memory corruption not awarded. The patch fixes locking around authorization and lookup, not heap, refcount, OOB, or object lifetime corruption.
* Privileged FD theft not awarded. The obtained object is a namespace fd, not evidence of stealing an arbitrary privileged process file descriptor such as via `pidfd_getfd`.
* Root privilege escalation not awarded. No supplied exploit evidence shows root acquisition or arbitrary privileged command execution.
* Integrity impact not awarded as an ActionableScore signal. A namespace fd can be security-relevant, but the supplied evidence does not establish unauthorized modification of protected state.
* Remote reachability not awarded. The path is local pidfd ioctl control-plane behavior.
* Race penalty not applied. Although this is a TOCTOU race, the prompt forbids subtracting merely because a race exists, and no evidence proves a rare or one-shot uncontrollable race.
* Host-admin penalty not applied. Host-admin-equivalent privilege is not positively established.

3. Reachability analysis

The attacker is a local process using pidfd namespace ioctls. The core reachable operation is `pidfd_ioctl()` handling `PIDFD_GET_*_NAMESPACE`, which performs a ptrace filesystem-credentials check and then obtains a namespace reference.

The supplied commit explicitly describes a caller passing the access check against the target task old credentials and then reading the namespace after the target has executed a setuid binary and committed new credentials. That supports below-host-admin reachability for the vulnerable authorization decision.

This is not a namespace-scoped capability case. The relevant gate is `ptrace_may_access()` with filesystem credentials, not a CAP_NET_ADMIN or CAP_SYS_ADMIN check inside an attacker-created namespace. Therefore `NamespaceScopedPrivilege=NO`.

No prior host setup beyond the presence of pidfd namespace ioctl support and a target lifecycle involving setuid `execve()` is established. No special hardware, boot option, or rare subsystem is required. The race is timing-sensitive, but the patch and commit do not establish that it is practically unrepeatable.

4. Primitive and severity interpretation

The exact primitive is a TOCTOU authorization bypass between ptrace-style credential validation and namespace lookup. The access decision can be made using stale pre-`execve()` credentials while the returned namespace fd corresponds to the target after a credential-changing `execve()`.

This is not active structured UAF, fixed stale storage, repeated release, or refcount mutation. There is no first lifetime release, no surviving dangling reference, and no second lifetime/refcount operation.

PrimitiveClass is `CLASS_A` because the patch directly establishes an intended mandatory authorization condition changing from deny to allow for an externally meaningful kernel resource. The demonstrated consequence is unauthorized namespace information or namespace fd access. The speculative consequence is broader policy impact depending on namespace type and surrounding permissions, but root LPE or arbitrary privileged resource theft is not established.

ImportantCandidate is `YES` because this is a concrete security-boundary authorization race in a pidfd access path. AutoDowngradeAllowed is `NO` because automatically closing it as ordinary Moderate would lose a real authorization bypass involving credential lifetime and protected namespace fd access.

Likely operational severity is actionable Moderate with Important review warranted, especially for environments relying on pidfd namespace access checks across setuid transitions.

5. One-sentence report phrase

A TOCTOU race in pidfd namespace ioctls lets a local caller pass a ptrace filesystem-credentials check against old target credentials and then obtain a namespace file descriptor after setuid execve changes the target credentials, bypassing the intended access decision.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a concrete authorization boundary bypass caused by credential lifetime mismatch across `execve()`, with externally meaningful namespace fd access and no safe basis for automatic downgrade.

7. Call-site confidence

Call-site confidence: high

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

Patch: pidfd: hold exec_update_lock around namespace ioctl [ Upstream
Commit: cf24ce48d60225fdbee68ed2058763a1fd172ff3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cf24ce48d60225fdbee68ed2058763a1fd172ff3

Changed files:
  fs/pidfs.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=cf24ce48d60225fdbee68ed2058763a1fd172ff3

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

The original Linux kernel CVE announcement for CVE-2026-89766 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89766

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:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:L/A:N
  The Best / paranoid CVSS score: 5.3

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: 5
  Paranoid ActionableScore: 6

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).