[CVE-2026-68171][IMPORTANT] arm64: syscall: Ensure saved x0 is kept in-sync with tracer updates

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

CVE: CVE-2026-68171
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: arm64: syscall: Ensure saved x0 is kept in-sync with tracer updates
Commit: 8000a5f4d1d192f5bb3e4f29e7606a9460d376df
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8000a5f4d1d192f5bb3e4f29e7606a9460d376df
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68171
Analysis date: Mon, 10 Aug 2026 15:26:11 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
On arm64, stale orig_x0 after ptrace or SECCOMP_RET_TRACE updates can cause seccomp, audit, and tracepoints to inspect a different first syscall argument than the one actually executed, creating a local seccomp policy bypass risk.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-68171 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68171

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-2026-68171	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N';CWE-693;CWE-367;CWE-284;CWE-807;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '7.1';DESCR 'On arm64, orig_x0 can stay stale after a tracer changes x0 during a syscall-enter-stop or SECCOMP_RET_TRACE path. Seccomp, audit, and syscall tracepoints can then inspect the old first syscall argument while the actual syscall is dispatched with the updated x0 value. This is a security policy bypass risk rather than memory corruption. For the CVSS the PR:L is used because a local user or local process context is needed to use ptrace or seccomp trace behavior, but full administrative privileges are not necessarily required in practical sandbox setups. The issue is not network reachable directly. Impact can include bypassing seccomp argument filtering and reducing audit correctness, with possible confidentiality or integrity impact if the sandbox policy relied on x0 based checks.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	INIT TRACE HARDWARE  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: **5**
* Paranoid score: **7**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* **Local unprivileged trigger: +1**
  A local user/process can participate in ptrace or seccomp trace scenarios under normal same-user or sandbox arrangements. This does not require full host-root in all practical deployments.

* **Security-boundary bypass with practical sensitive-object access: +2**
  The bug can make seccomp, audit, and tracepoints observe a stale first syscall argument while the actual syscall executes with the tracer-modified x0 value. This is a policy enforcement mismatch and can bypass seccomp argument filtering.

* **Integrity impact plausible: +1**
  If a seccomp policy allows or denies operations based on the first syscall argument, the executed operation may differ from the checked one.

* **Confidentiality impact plausible: +1**
  A sandbox escape or seccomp argument-filter bypass may allow access to files, descriptors, or process resources that the policy intended to block.

Paranoid additional signals:

* **Practical exploit chain demonstrated or strongly implied: +1**
  The commit explicitly describes the historical seccomp ordering issue and the incomplete fix, with the concrete mismatch between syscall_get_arguments and the actual dispatched syscall argument.

* **Cross-boundary isolation or policy bypass: +1**
  This affects seccomp-based isolation, audit correctness, and syscall tracing assumptions. It is especially relevant for sandboxed services, container runtimes, and broker-style syscall filtering.

No memory-corruption signals are counted. This is not UAF, OOB write, double-free, refcount corruption, or arbitrary write.

## 3. Reachability analysis

The issue is **local only**, not network reachable directly. Triggering requires an arm64 native task and a ptrace or SECCOMP_RET_TRACE path where x0 is changed after the original syscall argument has been saved into orig_x0.

Privileges are best treated as **PR:L**, not PR:H, because same-user ptrace or sandbox-controlled seccomp tracing can be available without full administrative privileges. Namespace and container setups matter because seccomp is commonly used as a security boundary in containerized and sandboxed workloads. The issue is not relevant to compat tasks in the same way because compat exposes orig_r0 differently.

The path is in the arm64 syscall entry path and seccomp/ptrace handling, so the affected mechanism is broad on arm64 systems that use seccomp filtering or syscall tracing.

## 4. Severity interpretation

This behaves more like an **Important-class policy bypass candidate** than an ordinary Moderate kernel bug. It is not memory corruption and does not imply kernel code execution by itself. However, it can undermine seccomp argument filtering by letting the policy engine inspect one first argument while the syscall executes with another.

The realistic impact depends heavily on the sandbox policy. If no seccomp argument filtering or ptrace/seccomp tracing is used, practical impact may be limited. If a sandbox relies on x0-based syscall argument checks, the bug can become a meaningful isolation bypass with confidentiality and integrity consequences.

## 5. One-sentence report phrase

On arm64, stale orig_x0 after ptrace or SECCOMP_RET_TRACE updates can cause seccomp, audit, and tracepoints to inspect a different first syscall argument than the one actually executed, creating a local seccomp policy bypass risk.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

This should not be auto-closed because it is a security-boundary bypass in seccomp/ptrace syscall handling. The final impact is policy-dependent, but sandbox and container use cases make the issue actionable even without memory corruption.

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

Patch: arm64: syscall: Ensure saved x0 is kept in-sync with tracer updates
Commit: 8000a5f4d1d192f5bb3e4f29e7606a9460d376df
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8000a5f4d1d192f5bb3e4f29e7606a9460d376df

Commit description:

When seccomp support was originally added to arm64 in a1ae65b
("arm64: add seccomp support"), seccomp was erroneously called _before_
the ptrace syscall-enter-stop and therefore the tracer could trivially
manipulate the syscall register state after the seccomp check had
passed. This was subsequently fixed in a5cd110 ("arm64/ptrace: run
seccomp after ptrace") by moving the seccomp check after the tracer has
run. Unfortunately, a decade later, that fix has been reported to be
incomplete.

On arm64, both the first argument to a syscall and its eventual return
value are allocated to register x0. In order to facilitate syscall
restarting and querying of syscall arguments on the syscall exit path,
the original value of x0 is stashed in 'struct pt_regs::orig_x0' early
during the syscall entry path and is returned for the first argument by
syscall_get_arguments(). Unlike 32-bit Arm, this stashed value is not
directly exposed via ptrace() and so changes to register x0 made by the
tracer on a syscall-enter-stop are not reflected in 'orig_x0'. This
means that seccomp, syscall tracepoints and audit can observe a stale
value for the register compared to the argument that will be observed by
the actual syscall.

Re-sync 'orig_x0' from x0 on the syscall entry path following a
potential ptrace stop (i.e. PTRACE_EVENTMSG_SYSCALL_ENTRY or
SECCOMP_RET_TRACE). This behaviour is limited to native tasks (because
compat tasks expose 'orig_r0' to ptrace) where the syscall is not being
skipped (because x0 is updated to hold the return value of -ENOSYS in
that case).

Cc: Kees Cook <kees@kernel.org>
Cc: Jinjie Ruan <ruanjinjie@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: stable@vger.kernel.org
Reported-by: Yiqi Sun <sunyiqixm@gmail.com>
Link: https://lore.kernel.org/all/20260529065444.1336608-1-sunyiqixm@gmail.com/
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Fixes: a5cd110 ("arm64/ptrace: run seccomp after ptrace")
Reviewed-by: Jinjie Ruan <ruanjinjie@huawei.com>
Tested-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/arm64/kernel/ptrace.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=8000a5f4d1d192f5bb3e4f29e7606a9460d376df

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

The original Linux kernel CVE announcement for CVE-2026-68171 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68171

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:L/PR:L/UI:N/S:U/C:H/I:H/A:N
  The Best / paranoid CVSS score: 7.1

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: 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).