* [CVE-2026-98273][MODERATE REGULAR] x86/kprobes: Fix crash when probing CS CALL instructions [ Upstream
@ 2026-10-10 20:57 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-10 20:57 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98273
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: x86/kprobes: Fix crash when probing CS CALL instructions [ Upstream
Commit: cfc1af3d054aaabee08f1f77c441b3533b082869
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cfc1af3d054aaabee08f1f77c441b3533b082869
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98273
Analysis date: Sat, 10 Oct 2026 16:57:40 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98273 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98273
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: MODERATE REGULAR
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-2026-98273 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98273 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (independent Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-682;CWE-670;CWE-193;*CWE-787;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A kprobes instruction length error treats a CS prefixed six byte CALL as a normal five byte CALL when constructing the exception return address. After the probe handler returns, execution resumes at the final byte of the original CALL and the processor decodes an unintended instruction stream. The demonstrated result is a supervisor write page fault and a kernel crash. For the CVSS the PR:L value in the paranoid score reflects environments where BPF and tracing capabilities are delegated to a restricted service account, container root, or another partially privileged local user. The base score uses PR:H because installing arbitrary kernel probes normally requires CAP_BPF with CAP_PERFMON or CAP_SYS_ADMIN. The issue is not network reachable. Impact is at least a local denial of service. Because execution can continue from the middle of an instruction and has already produced a kernel mode write attempt, a valid target write, kernel memory corruption, and privilege escalation cannot be excluded under carefully selected probe and register conditions. Reliable exploitation beyond a crash would have high complexity and is not demonstrated by the supplied patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) MAYBE OOB DANGER KERNEL_PANIC_PLUS_UAF HARDWARE DEBUGFS LINUS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap changed either score.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
Positive signals:
* +1 Reliable kernel crash / strong DoS. The incorrect return address resumes execution at the final byte of a six-byte CS-prefixed CALL, producing an unintended instruction and a demonstrated supervisor write page fault.
* +1 Weak or indirect corruption candidate. The kernel instruction pointer is redirected into the middle of an instruction, and the resulting unintended instruction attempts a kernel-mode write. The supplied evidence does not establish a useful destination, payload, or surviving corruption.
Negative signals:
* No privilege penalty applied. The path is BPF and kprobe privilege-gated, but the supplied evidence does not establish whether the complete trigger requires host-admin-equivalent privilege or a reduced combination such as CAP_BPF and CAP_PERFMON.
* No timing penalty. This is a deterministic instruction-length calculation error, not a race.
* No subsystem rarity or hardware penalty. No special physical hardware is required.
Important non-awarded signals:
* No confirmed heap or slab OOB write exists. The fault is an attempted write to a non-present address after unintended instruction decoding.
* B5-R does not apply. There is no retry-accumulated index, missing retry reset, or later consumption of a corrupted heap descriptor.
* Strong semantic reinterpretation was not awarded because attacker control over the unintended instruction stream, register state, write target, or payload is not established.
* Strong or weak LPE leverage was not separately awarded. Selecting a probe site establishes the trigger but not a useful corruption primitive beyond the demonstrated crash.
* Arbitrary or directed kernel write is not demonstrated.
* Controlled code bytes, controlled reclaim, persistent corruption, information disclosure, and privilege escalation are not demonstrated.
* Remote reachability is not established.
* No extra Availability point was awarded because the page fault and kernel crash are the same consequence.
* No authorization bypass, protected-resource access, B3-PENDING shared-backing write, or B4-PENDING condition exists.
3. Reachability analysis
The trigger installs an eBPF-backed kprobe on a CS-prefixed CALL instruction. Loading the BPF program and creating or attaching the probe are privilege-gated operations, but the supplied patch does not show the applicable capability checks or product configuration.
On modern systems, the operation may involve CAP_BPF and CAP_PERFMON, while older or more restrictive configurations may require CAP_SYS_ADMIN or equivalent host authority. Because the complete privilege chain is not visible, HostAdminRequired and RequiresHostUID0 remain UNKNOWN rather than being inferred from generic BPF behavior.
The evidence does not establish that all required operations work inside an attacker-created user namespace. NamespaceScopedPrivilege is therefore UNKNOWN. Unprivileged BPF availability, relevant sysctls, lockdown policy, perf-event restrictions, and kprobe policy are also not supplied.
No network trigger, prior device configuration, special hardware, retry sequence, or race is involved. Once an authorized actor installs the probe at an affected instruction, the failure appears deterministic.
4. Primitive and severity interpretation
The exact defect is an incorrect exception return address caused by assuming a five-byte CALL length for a six-byte CS-prefixed CALL. Execution resumes one byte too early and decodes an unintended instruction stream.
This is not a UAF, lifetime violation, repeated release, heap OOB write, or shared-page ownership violation. The demonstrated primitive is invalid kernel control flow ending in a supervisor write page fault. Although a different probe location could theoretically decode a different instruction, the supplied evidence does not establish control over the resulting write target, payload, or register state sufficient to preserve a useful corruption primitive after excluding the crash.
The issue is therefore CLASS_C rather than CLASS_A or CLASS_B. The demonstrated consequence is local kernel crash and DoS. A useful kernel write or privilege escalation remains speculative and is not scored.
ImportantCandidate is NO because no independently useful corruption or security-boundary primitive is established. AutoDowngradeAllowed is YES because the evidence supports a privilege-gated crash path without UAF, persistent corruption, controlled write, disclosure, or authorization bypass. Manual review remains recommended due to unintended kernel instruction execution and the observed write attempt.
5. One-sentence report phrase
A hardcoded CALL length in x86 kprobe emulation can resume execution inside a CS-prefixed CALL instruction and trigger an unintended kernel-mode write followed by a kernel crash, but no controlled or persistent corruption primitive is established.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Confirm the effective BPF and kprobe privilege boundary and whether any allowed CS-prefixed probe sites produce a surviving or attacker-influenced write rather than an immediate fault.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: x86/kprobes: Fix crash when probing CS CALL instructions [ Upstream
Commit: cfc1af3d054aaabee08f1f77c441b3533b082869
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cfc1af3d054aaabee08f1f77c441b3533b082869
Changed files:
arch/x86/include/asm/text-patching.h
arch/x86/kernel/alternative.c
arch/x86/kernel/kprobes/core.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=cfc1af3d054aaabee08f1f77c441b3533b082869
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98273 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98273
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:H/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7
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: 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: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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