[CVE-2026-64451][LOW] tracing: Fix NULL pointer dereference in func_set_flag()

[CVE-2026-64451][LOW] tracing: Fix NULL pointer dereference in func_set_flag()

[AL-KERNEL]

CVE: CVE-2026-64451
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: tracing: Fix NULL pointer dereference in func_set_flag()
Commit: 69f17ac132a38974cf1defb480cef6b79d1ab768
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=69f17ac132a38974cf1defb480cef6b79d1ab768
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64451
Analysis date: Sat, 25 Jul 2026 13:40:33 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A local NULL pointer dereference in the tracing function tracer option path can crash the kernel when tracing options are written after switching away from the function tracer, typically requiring tracing control privileges and providing DoS impact only.

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

The original Linux kernel CVE announcement for CVE-2026-64451 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64451

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-2026-64451	LOW	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-703;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 'A NULL pointer dereference in the tracing function tracer option path can occur because func_set_flag reads tr current_trace_flags before checking that the active tracer is function. A local user who can write tracing options can open a function tracer option file, switch the active tracer to another tracer such as wakeup_rt, and then write the option so current_trace_flags is NULL and the kernel crashes. For the CVSS the PR:L is used for the paranoid score because some systems delegate tracefs or tracing control to non root service accounts or containers, although typical default deployments require administrator privileges. The issue is not network reachable and is triggered through tracefs control plane writes. Impact is denial of service via kernel crash, with no supported confidentiality or integrity primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)  SKIP CVE-2026-64451 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 76680d0d2825900f23bf35290ab2b80bdf3a8e4a YES  NO NO unknown 	MAYBE	WRITE OOB NULLPTR TRACE IMPROVEONLY KERNEL_PANIC_PLUS_UAF LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  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**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1
  The patch and crash dump show a real NULL pointer dereference in `func_set_flag()` leading to kernel crash.
* Requires admin/root/CAP_* in typical deployments: -2
  Writing tracing options through tracefs/debugfs normally requires root or equivalent tracing privileges.
* No memory corruption bonus: +0
  This is a NULL pointer dereference at address 0, not UAF, OOB write, double free, type confusion, or attacker-controlled overwrite.
* No confidentiality or integrity impact: +0
  No read-back, information leak, write primitive, or privilege escalation primitive is shown.

Paranoid signals:

* Local unprivileged or reduced-privilege trigger in delegated environments: +1
  Some systems may delegate tracefs/tracing control to service accounts, containers, or reduced-privilege administrators.
* Reliable kernel crash / strong DoS: +1
  The crash path is direct and supported by the provided call trace.
* No LPE concern: +0
  The primitive remains a NULL dereference, with no controlled reclaim, callback control, writable stale object, or object confusion.

## 3. Reachability analysis

The bug is triggered locally through tracing control-plane writes, specifically by opening a function tracer option file, switching the active tracer away from `function`, and then writing to the old option file. In default deployments this normally requires administrative tracing privileges, so the conservative score applies the admin privilege penalty. Namespaces or containers may matter only if tracefs or tracing controls are delegated to a less privileged user or service account. The path is not network reachable and cannot be triggered by packet traffic.

Call-site confidence: high. The commit provides the direct call chain `trace_options_write()` to `__set_tracer_option()` to `func_set_flag()`, plus the exact NULL dereference and the fixing condition reorder.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like local DoS issue. The realistic impact is a kernel crash caused by a NULL pointer dereference. The theoretical memory corruption potential is low because the faulting pointer is NULL and the patch does not show UAF, stale object reuse, arbitrary write, refcount abuse, callback dispatch, or type confusion. The paranoid score only accounts for possible delegated tracing access, not for privilege escalation.

## 5. One-sentence report phrase

A local NULL pointer dereference in the tracing function tracer option path can crash the kernel when tracing options are written after switching away from the function tracer, typically requiring tracing control privileges and providing DoS impact only.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed for most triage workflows because it is a local privileged or delegated-tracing NULL dereference DoS with no evidence of memory corruption, confidentiality impact, integrity impact, or privilege escalation.

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

Patch: tracing: Fix NULL pointer dereference in func_set_flag()
Commit: 69f17ac132a38974cf1defb480cef6b79d1ab768
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=69f17ac132a38974cf1defb480cef6b79d1ab768

Commit description:

func_set_flag() dereferences tr->current_trace_flags before verifying
that the current tracer is actually the function tracer. When the active
tracer has been switched away from "function" (e.g., to "wakeup_rt"),
tr->current_trace_flags can be NULL, leading to a NULL pointer
dereference and kernel crash.

The call chain that triggers this is:

  trace_options_write()
    -> __set_tracer_option()
      -> trace->set_flag()          /* func_set_flag */

In func_set_flag(), the first operation is:

  if (!!set == !!(tr->current_trace_flags->val & bit))

This dereferences tr->current_trace_flags unconditionally. The safety
check that guards against a non-function tracer:

  if (tr->current_trace != &function_trace)
      return 0;

is placed *after* the dereference, which is too late.

This was observed with the following crash dump:

  BUG: unable to handle page fault at 0000000000000000
  RIP: func_set_flag+0xd

  Call Trace:
   __set_tracer_option+0x27
   trace_options_write+0x75
   vfs_write+0x12a
   ksys_write+0x66
   do_syscall_64+0x5b

  RIP: ffffffff914c973d  RSP: ff67ec88b01dfdf0  RFLAGS: 00010202
  RAX: 0000000000000000  RBX: ff3a826e80354580  RCX: 0000000000000001
  RDX: 0000000000000001  RSI: 0000000000000000  RDI: ffffffff93918080

The disassembly confirms the fault:

  func_set_flag+0:   mov 0x1f08(%rdi), %rax  ; RAX = tr->current_trace_flags = NULL
  func_set_flag+13:  mov (%rax), %eax        ; page fault: dereference NULL

At the time of the crash:
  tr->current_trace_flags = 0x0 (NULL)
  tr->current_trace = wakeup_rt_tracer (not function_trace)

The scenario is that a process opens a function tracer option file (such
as "func_stack_trace"), then the current tracer is switched to another
tracer (e.g., "wakeup_rt"), which sets current_trace_flags to NULL. When
the process subsequently writes to the option file, func_set_flag() is
invoked and crashes on the NULL dereference.

Fix this by moving the current_trace check before the
current_trace_flags dereference, so that func_set_flag() returns early
when the function tracer is not active.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260624061715.1445655-1-xiangzao@linux.alibaba.com
Fixes: 76680d0 ("tracing: Have function tracer define options per instance")
Signed-off-by: Yuanhe Shu <xiangzao@linux.alibaba.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/trace/trace_functions.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=69f17ac132a38974cf1defb480cef6b79d1ab768

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

The original Linux kernel CVE announcement for CVE-2026-64451 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64451

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).