[CVE-2026-68174][LOW] tracing: Fix union collision of module and refcnt for dynamic events

[CVE-2026-68174][LOW] tracing: Fix union collision of module and refcnt for dynamic events

[AL-KERNEL]

CVE: CVE-2026-68174
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: tracing: Fix union collision of module and refcnt for dynamic events
Commit: b6a4575f22925da7e6aa00171e9fc0e5029c0bc9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b6a4575f22925da7e6aa00171e9fc0e5029c0bc9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68174
Analysis date: Mon, 10 Aug 2026 17:51:43 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like. Not an Actionable Moderate candidate
Manual review required: NO

Summary:
A dynamic tracing event can expose its refcnt through the union field used for call module, causing `:mod:` event filtering to treat a small refcnt value as a struct module pointer and trigger a local kernel Oops, with typical access requiring privileged tracing control.

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

The original Linux kernel CVE announcement for CVE-2026-68174 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68174

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-68174	LOW	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:N/I:N/A:H';CWE-843;**CWE-476;CWE-704;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A dynamic tracing event can store an active refcnt in the same union field that is used as call module for non dynamic events. When a user applies a :mod: filter through set_event, the tracing code may treat that small non zero refcnt value as a struct module pointer and pass it to module_name, causing a NULL/invalid pointer dereference and kernel Oops. For the CVSS the PR:L is used for the paranoid view because delegated tracing access, container admin access, or a restricted service account with tracing control may be enough in some deployments. In typical host deployments PR:H is more realistic because tracefs event control usually requires root or CAP_SYS_ADMIN. The issue is not network reachable and has no clear evidence of information disclosure or privilege escalation. Impact is denial of service via kernel Oops or crash.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like. Not an Actionable Moderate candidate (with actual score 1)  SKIP CVE-2026-68174 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 4c86bc531e60900053384867c082675bba82c29f YES  NO NO unknown 	MAYBE	TRACE LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like. Not an Actionable Moderate candidate**

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The commit explicitly describes NULL/invalid pointer dereference and kernel Oops when `module_name(call->module)` is called on a refcnt value.
* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2. Writing to tracing control files such as `set_event` and configuring dynamic tracing events usually requires root or CAP_SYS_ADMIN.
* Availability impact realistic: +1 is not added separately because the crash impact is already counted as strong DoS and there is no persistent data corruption or broader system state damage shown.

Paranoid additional interpretation:

* Memory corruption, weak/indirect corruption candidate: +1. The bug is a bogus object pointer interpretation where an atomic refcnt stored in a union is interpreted as `struct module *`. This is not a strong corruption primitive because the value is a small integer and the demonstrated effect is invalid dereference only.
* Privilege penalty reduced in paranoid view: 0 instead of -2. Some deployments may delegate tracing control to container admin, service accounts, or restricted operational users, so the attacker may not be equivalent to full host root.

Signals not applied:

* No remote reachable signal. This is a local tracefs control-plane path.
* No local unprivileged trigger. The patch does not show ordinary unprivileged access.
* No strong memory corruption. No UAF, double free, OOB write, controlled overwrite, refcount takeover, or callback control is shown.
* No privilege escalation plausible. The dereferenced pointer is a small refcnt value, not attacker-controlled replacement of a freed object.
* No confidentiality or integrity impact. The demonstrated primitive is Oops/DoS only.

Call-site confidence: high for the shown `__ftrace_set_clr_event_nolock()` path because the faulty dereference and fix are visible. Medium for `update_event_fields()` because it is mentioned in the commit text but not shown in the provided diff excerpt.

## 3. Reachability analysis

The bug is triggered by local tracing configuration operations, specifically using a module filter such as `:mod:<module>` while dynamic events have a non-zero reference count from triggers or perf attachments. In typical host deployments this requires root or CAP_SYS_ADMIN access to tracefs, so conservative scoring treats it as privileged local. Namespace or container setups may lower the practical privilege boundary if tracing controls are delegated to a reduced-capability admin or service account, but that is configuration-dependent. The affected subsystem is common in kernels, but the vulnerable operation is not default-exposed to unprivileged users and is not reachable through network traffic.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like local DoS issue, not an Important-class vulnerability. The theoretical concern is bogus struct interpretation through a union collision, but the practical evidence points to a small integer being treated as a module pointer and causing an invalid dereference. There is no supported path to attacker-controlled memory corruption, information disclosure, integrity violation, or privilege escalation.

## 5. One-sentence report phrase

A dynamic tracing event can expose its refcnt through the union field used for call module, causing `:mod:` event filtering to treat a small refcnt value as a struct module pointer and trigger a local kernel Oops, with typical access requiring privileged tracing control.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed for urgent triage purposes because the patch supports invalid pointer dereference and DoS only, with privileged local reachability in typical deployments and no concrete LPE or memory corruption primitive beyond bogus pointer interpretation.

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

Patch: tracing: Fix union collision of module and refcnt for dynamic events
Commit: b6a4575f22925da7e6aa00171e9fc0e5029c0bc9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b6a4575f22925da7e6aa00171e9fc0e5029c0bc9

Commit description:

In 'struct trace_event_call', the 'module' pointer and the 'refcnt'
atomic variable share the same memory space in a union. For dynamic
events, the union member is 'refcnt', which acts as an active
reference counter.

When a dynamic event (such as kprobe, uprobe, fprobe, eprobe, or
wprobe) has a non-zero reference count (e.g. due to active event
triggers or perf attachments), its 'call->module' evaluates to a
small non-zero integer instead of NULL.

When filtering or setting events for a specific module (e.g., writing
':mod:<module>' to 'set_event'), the code in
'__ftrace_set_clr_event_nolock()' and 'update_event_fields()' reads
'call->module' directly without checking whether the event is dynamic.
This causes the kernel to treat the small integer (refcnt) as a
'struct module' pointer, leading to a NULL/invalid pointer dereference
(Oops) when dereferencing the module name.

Fix this by ensuring that the 'TRACE_EVENT_FL_DYNAMIC' flag is checked
before treating 'call->module' as a valid pointer in these code paths.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/178425670947.84440.11344393611899824907.stgit@devnote2
Fixes: 4c86bc5 ("tracing: Add :mod: command to enabled module events")
Assisted-by: Antigravity:gemini-3.5-flash
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/trace/trace_events.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=b6a4575f22925da7e6aa00171e9fc0e5029c0bc9

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

The original Linux kernel CVE announcement for CVE-2026-68174 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68174

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:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 0
  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).