[CVE-2026-46063][LOW] x86/shstk: Prevent deadlock during shstk sigreturn [ Upstream

[CVE-2026-46063][LOW] x86/shstk: Prevent deadlock during shstk sigreturn [ Upstream

[AL-KERNEL]

CVE: CVE-2026-46063
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: x86/shstk: Prevent deadlock during shstk sigreturn [ Upstream
Commit: e2c2b044458cbf22da05264fa707308e8d4f86f9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e2c2b044458cbf22da05264fa707308e8d4f86f9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46063
Analysis date: Wed, 27 May 2026 11:44:00 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A local x86 shadow-stack sigreturn path can deadlock if reading the shadow stack faults while mmap locking is already contended, causing DoS without memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-46063 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46063

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-46063	LOW	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-667;*CWE-833;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A local denial of service can occur during x86 shadow stack sigreturn because the kernel could read the user shadow stack while holding mmap read lock. If that read faults, the fault handler may try to acquire mmap read lock again while a writer is pending, creating a deadlock. For the CVSS the PR:L is used because triggering requires local code execution in a shadow stack enabled process and control over signal return, page fault behavior, or concurrent mmap activity. AC:H is used because the issue depends on a timing window involving a page fault and a competing mmap writer. The issue is not network reachable and does not involve UAF, out of bounds access, arbitrary write, information disclosure, or privilege escalation. Impact is denial of service only through a stuck sigreturn or mmap lock deadlock.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	READ OOB LOCK DEADLOCK DANGER HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

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

ActionableScore=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process can exercise signal return paths in a shadow-stack-enabled process and influence user memory fault behavior.

* Reliable DoS plausible: +1
  The described failure mode is a deadlock involving recursive mmap read locking while a writer is pending.

* Availability impact realistic: +1
  The impact is a stuck sigreturn or mmap lock deadlock affecting the process and potentially related mm operations.

Subtracting signals:

* Hard race / special timing required: -1
  Requires a user shadow stack read fault and a competing mmap writer at the wrong time.

* Configuration-dependent exposure: -1
  Requires X86_USER_SHADOW_STACK and PER_VMA_LOCK enabled, and a shadow-stack-enabled process.

No signals apply for memory corruption, UAF, OOB access, arbitrary write, information disclosure, privilege escalation, remote reachability, or security-boundary bypass.

## 3. Reachability analysis

The trigger is local code execution in a process using x86 user shadow stacks. The process must reach sigreturn with a shadow stack signal frame state that causes the kernel to validate and read shadow stack memory, and that read must fault while another thread or CPU contends for mmap write locking.

Namespaces and containers do not materially change impact except where untrusted workloads can run CET/shadow-stack-enabled binaries. The issue is not network reachable.

## 4. Severity interpretation

This behaves like a local DoS / deadlock bug, not an Important vulnerability. The patch changes lock sequencing and speculative VMA validation to avoid recursive mmap lock deadlock.

Theoretical memory corruption or privilege escalation is not supported by the patch. Realistic impact is availability only, under a narrow timing and configuration condition.

## 5. One-sentence report phrase

A local x86 shadow-stack sigreturn path can deadlock if reading the shadow stack faults while mmap locking is already contended, causing DoS without memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a local configuration-dependent mmap-lock deadlock with DoS-only impact and no supported memory corruption, remote exposure, or security-boundary bypass.

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

Patch: x86/shstk: Prevent deadlock during shstk sigreturn [ Upstream
Commit: e2c2b044458cbf22da05264fa707308e8d4f86f9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e2c2b044458cbf22da05264fa707308e8d4f86f9

Changed files:
  arch/x86/kernel/shstk.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=e2c2b044458cbf22da05264fa707308e8d4f86f9

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

The original Linux kernel CVE announcement for CVE-2026-46063 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46063

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: unknown
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).