[CVE-2026-74633][LOW] tracing: Fix NULL pointer dereference in module event cache removal

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

CVE: CVE-2026-74633
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: tracing: Fix NULL pointer dereference in module event cache removal
Commit: 152a00440dc6ef62c6e4cf9bd881a7e6bb81fda9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=152a00440dc6ef62c6e4cf9bd881a7e6bb81fda9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74633
Analysis date: Sat, 22 Aug 2026 12:31:53 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like. Not Actionable Moderate
Manual review required: NO

Summary:
A local user with write access to tracefs set_event can trigger a NULL pointer dereference in tracing module event cache removal, causing a kernel crash, but the issue does not show memory corruption or privilege escalation primitives.

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

The original Linux kernel CVE announcement for CVE-2026-74633 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74633

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-2026-74633	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-476;*CWE-20;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'Tracing module event cache removal can dereference a NULL event_mod match pointer when a cached module only filter is followed by removal of a specific event match for the same not yet loaded module. The crash happens in strcmp from the ftrace event write path and can be triggered from userspace by writing crafted set_event filters to tracefs. For the CVSS the PR:L is used for the paranoid score because some deployments may delegate tracefs write access to a tracing group or a constrained service account, although default systems usually require administrator privileges. The issue is not network reachable and requires local access to the tracing control plane. Impact is denial of service through a kernel NULL pointer dereference and crash. There is no evidence of UAF, OOB access, arbitrary write, information disclosure, or privilege escalation in the code path.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like. Not Actionable Moderate (with actual score 1)  SKIP CVE-2026-74633 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	NULLPTR TRACE IMPROVEONLY LINUS SYZBOT  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
======================================================================

ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

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

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1
  The patch and trace show a reproducible NULL pointer dereference in strcmp from the tracefs set_event write path.

* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2
  Writing to /sys/kernel/tracing/set_event normally requires administrator level tracing privileges or equivalent tracefs write access.

Conservative total: 0 after floor at zero.

Paranoid signals:

* Local low-privileged trigger: +1
  In some deployments tracefs write access may be delegated to a tracing group, service account, test harness, or constrained local operator.

* Reliable kernel crash / strong DoS: +1
  The failure mode is a direct NULL dereference and kernel crash/Oops.

Paranoid total: 2.

Not awarded:

* No generic memory corruption. This is a NULL pointer dereference, not UAF, OOB write, double free, type confusion, or refcount corruption.
* No privilege escalation plausible signal. There is no reclaim, stale object reuse, write-after-free, callback control, or arbitrary write.
* No confidentiality or integrity impact. The code path crashes before exposing data or modifying attacker-selected kernel objects.
* No remote/network reachability. The trigger is tracefs control-plane access, not packet input.

## 3. Reachability analysis

The bug is triggered locally through writes to the tracing set_event control file. On default systems this is normally restricted to root or processes with tracing-related administrative access, so the conservative interpretation treats it as privileged local DoS. Namespaces and containers usually do not make host tracefs safely writable to untrusted users, but some observability or CI environments may delegate tracefs access to non-root or reduced-privilege accounts. That justifies a paranoid local low-privilege interpretation, but not an Important-class score.

The affected subsystem is common in kernels, but the vulnerable interface is not normally exposed to arbitrary users and is not network reachable.

## 4. Severity interpretation

This behaves like an ordinary Moderate / Low-like kernel DoS issue. The demonstrated primitive is a NULL pointer dereference caused by passing a cached NULL match pointer to strcmp. There is no evidence of UAF, attacker-controlled object reuse, arbitrary write, information disclosure, or privilege escalation. The paranoid score accounts for delegated tracefs access, but the impact remains availability-only.

## 5. One-sentence report phrase

A local user with write access to tracefs set_event can trigger a NULL pointer dereference in tracing module event cache removal, causing a kernel crash, but the issue does not show memory corruption or privilege escalation primitives.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed for practical triage unless the evaluated product intentionally grants tracefs write access to untrusted users.

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

Patch: tracing: Fix NULL pointer dereference in module event cache removal
Commit: 152a00440dc6ef62c6e4cf9bd881a7e6bb81fda9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=152a00440dc6ef62c6e4cf9bd881a7e6bb81fda9

Commit description:

A module-only event filter such as ":mod:foo" is cached with a NULL
event_mod->match when foo has not been loaded. If a later write tries to
remove a specific match from the same module, remove_cache_mod() passes
the NULL cached match to strcmp(), causing a NULL pointer dereference.

The issue can be reproduced from userspace:

  echo ':mod:trace_events_kunit_missing' > /sys/kernel/tracing/set_event
  echo '!foo_bar:mod:trace_events_kunit_missing' >> /sys/kernel/tracing/set_event

The second write must be a concatenation (">>") to not include O_TRUNC as
that would cause ftrace_clear_events() to clear the cached modules lines.

The crash was reproduced on x86_64 QEMU while KUnit workers contended on
the event tracing path:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  #PF: supervisor read access in kernel mode
  RIP: 0010:strcmp+0x10/0x30
  Call Trace:
   __ftrace_set_clr_event_nolock+0x373/0x4a0
   ftrace_set_clr_event+0xf0/0x180
   ftrace_event_write+0xdf/0x110
   vfs_write+0xf6/0x440
   ksys_write+0x68/0xe0
   do_syscall_64+0xf9/0x540
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

Check event_mod->match before comparing it, consistent with the existing
NULL checks for the cached system and event fields. The mismatched removal
continues to return -EINVAL; a broad cached module filter is removed with
"!:mod:<module>".

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260811173902.1927376-2-sh_def@163.com
Fixes: b355247 ("tracing: Cache \":mod:\" events for modules not loaded yet")
Reported-by: syzbot+4d3143c8e28f6266c636@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/lkml/6a7a6b7f.9c11d2ce.289b96.00f8.GAE@google.com/
Signed-off-by: Hui Su <sh_def@163.com>
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=152a00440dc6ef62c6e4cf9bd881a7e6bb81fda9

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

The original Linux kernel CVE announcement for CVE-2026-74633 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74633

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).