From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-99141][LOW] tracing: Fix crash in print_graph_function_event() Date: Thu, 29 Oct 2026 00:06:09 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-99141 X-AL-KERNEL-Priority: LOW X-AL-KERNEL-Severity: LOW X-AL-KERNEL-Base-Severity: LOW X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 1 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: b1547c6ba93b8863010aa324cd20c441f886f23e List-Id: 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. ====================================================================== 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 ====================================================================== 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 Signed-off-by: Hao Ge Signed-off-by: Steven Rostedt Signed-off-by: Greg Kroah-Hartman 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).