[CVE-2026-99141][LOW] tracing: Fix crash in print_graph_function_event()

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

CVE: CVE-2026-99141
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: tracing: Fix crash in print_graph_function_event()
Commit: b1547c6ba93b8863010aa324cd20c441f886f23e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b1547c6ba93b8863010aa324cd20c441f886f23e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99141
Analysis date: Thu, 29 Oct 2026 00:06:09 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
Switching tracers while retained function-graph snapshot records exist can cause a deterministic NULL-pointer dereference during trace output, with availability impact only.

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

The original Linux kernel CVE announcement for CVE-2026-99141 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99141

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-2026-99141	LOW	https://www.kernelcve.org/list/?q=CVE-2026-99141 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=0 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both 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-665;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'Switching from function_graph to a tracer lacking static and default flags can leave current_trace_flags NULL while the trace or snapshot buffer still contains function graph records. Reading such a record reaches print_graph_function_event and dereferences the NULL flags pointer, causing a kernel oops and potentially a host crash when panic on oops is enabled. The snapshot sequence provides a deterministic trigger and does not require a race. For the CVSS the PR:L value in the paranoid vector reflects deployments that delegate access to a tracefs instance, while PR:H is typical because host tracing controls are normally restricted to administrators. The demonstrated primitive is a NULL pointer read with availability impact only. There is no evidence of UAF, OOB access, arbitrary write, information disclosure, or privilege escalation. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE_KPANIC to LOW because the ActionableScore threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3. The independent weaker Aegis shadow evaluated CVSS 2.7 and ActionableScore 0 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	NULLPTR TRACE RACE LEAK LINUS TEST  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 1
* Conservative score: 1; no floor or cap applied.
* Raw paranoid score: 1
* Paranoid score: 1; no separate technically supported escalation.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel crash/strong DoS: retained function-graph records are printed after `current_trace_flags` becomes NULL, producing the demonstrated supervisor-mode NULL dereference. Host panic depends on oops policy.
* +0 Deterministic trigger: the snapshot sequence avoids a race, but repeatability is not a separate availability point.
* No UAF, OOB access, write primitive, disclosure, authorization bypass, or attacker-controlled corruption is shown.
* No extra availability point: it would duplicate the same NULL-dereference crash.
* No local-unprivileged bonus: delegated tracefs access is possible in some deployments, but complete below-host-admin access is not established.
* NEW52/B5-R and NEW53/B3-PENDING do not apply: there is neither a heap OOB write nor a shared-backing write.

3. Reachability analysis

The trigger requires selecting tracers, creating a snapshot, switching to a tracer without flags, and reading the retained snapshot. It is deterministic and requires no race or special hardware. Tracefs control is commonly restricted, while the supplied hints mention possible delegation without proving the permissions needed for the complete sequence. Therefore both effective host-admin requirement and namespace-scoped reachability remain unknown; no privilege penalty or unprivileged bonus is applied.

4. Primitive and severity reading

The exact primitive is invalid active tracing state: `current_trace_flags` becomes NULL while old function-graph records remain consumable. `print_graph_function_event()` then performs a NULL-pointer read. This is CLASS_C crash behavior, not memory corruption or a lifetime violation. The patch installs a static empty flags object, preserving the non-NULL invariant. No useful effect remains after excluding the fault, so this is not an Important candidate and automatic downgrade is allowed. B4 authorization-state preservation is inapplicable because no DENY-to-ALLOW decision or protected resource is involved.

5. One-sentence report phrase

Switching tracers while retained function-graph snapshot records exist can cause a deterministic NULL-pointer dereference during trace output, with availability impact only.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch and call trace establish a constrained NULL-dereference crash with no unresolved corruption or security-boundary primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: tracing: Fix crash in print_graph_function_event()
Commit: b1547c6ba93b8863010aa324cd20c441f886f23e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b1547c6ba93b8863010aa324cd20c441f886f23e

Commit description:

tracing_set_tracer() assigns the flags of the newly selected tracer
with:

	tr->current_trace_flags = t->flags ? : t->tracer->flags;

If the new tracer defines neither static flags nor default flags, like
hwlat, irqsoff or wakeup, add_tracer() has no per instance flags copy
for it, and both operands are NULL, so tr->current_trace_flags ends up
being NULL as well.

The ring buffer is not cleared when switching tracers: it is reset
inside tracer_init(), after current_trace_flags was already assigned.
If the buffer still holds function_graph entries of the previous
tracer, printing one of them dereferences the NULL pointer. This was
hit by cat trace during an ftrace stress test:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  #PF: supervisor read access in kernel mode
  RIP: 0010:print_graph_function_event+0xe/0x20
  Call Trace:
   s_show+0x4d/0x110
   seq_read_iter+0x2d4/0x490
   seq_read+0x130/0x170
   vfs_read+0xb5/0x370
   ksys_read+0x6d/0xe0
   do_syscall_64+0xbc/0x450

The snapshot buffer is not reset at all, so this can be reproduced
without a race:

  echo function_graph > /sys/kernel/tracing/current_tracer
  echo 1 > /sys/kernel/tracing/snapshot
  echo hwlat > /sys/kernel/tracing/current_tracer
  cat /sys/kernel/tracing/snapshot

Rather than teaching every user of tr->current_trace_flags about this
corner case, make sure it is never NULL: when the newly selected
tracer has no flags at all, tracing_set_tracer() falls back to a
static empty no_tracer_flags. run_tracer_selftest() can hit the same
case, so the same fallback is added there too. The existing users
already handle opts == NULL, so these tracers keep behaving as before.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20261015130658.221198-1-hao.ge@linux.dev
Fixes: e29aa91 ("tracing: Have function graph tracer define options per instance")
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Hao Ge <hao.ge@linux.dev>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/trace/trace.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=b1547c6ba93b8863010aa324cd20c441f886f23e

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

The original Linux kernel CVE announcement for CVE-2026-99141 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99141

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: unknown
  Paranoid ActionableScore: 1

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).