[CVE-2026-98158][MODERATE REGULAR] ppp_async: drop the errored frame instead of resetting its headroom [ Upstream

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

CVE: CVE-2026-98158
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ppp_async: drop the errored frame instead of resetting its headroom [ Upstream
Commit: d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98158
Analysis date: Fri, 25 Sep 2026 12:09:02 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98158 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98158

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: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-98158	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98158 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';*CWE-787;CWE-125;CWE-119;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.4';DESCR 'A flaw in the PPP asynchronous receive path can reuse an skb after an errored frame with its required headroom removed. A peer controlling an existing PPP link can then cause skb_push to underflow while PPP filters add an inbound direction tag, resulting in a kernel panic. CCP processing can also read before the skb buffer in the same state. For the CVSS the PR:N is used because a remote PPP peer does not need local access or credentials on the victim. AV:A is used because the attacker must be able to communicate over the affected PPP link, not merely reach a public Internet service. The base impact is denial of service. The paranoid score includes limited confidentiality and integrity impact because the patch context identifies an out of bounds read and a constrained write before the skb buffer. PPP links are commonly used for dial up, serial, tunnel, or internal point to point deployments, so practical exploitation normally requires access to that specific link or its transport.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	WRITE OOB BPF ERRORPATH NETWORK SKB KERNEL_PANIC_PLUS_UAF LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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

ActionableScore=3
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +1 Constrained kernel buffer overwrite. The direction tag write can land below skb->head after an errored frame is reused with headroom reset to zero. The write is fixed size and fixed content, and no attacker-controlled target selection or controlled replacement is established.
* +1 Reliable kernel crash or strong DoS. The supplied trace demonstrates skb_under_panic followed by a kernel panic in interrupt context.
* +1 Reachability through an established PPP peer connection. The patch explicitly describes a peer supplying the frame sequence and calls the result a remote DoS. This is limited to an already configured PPP link and is not scored as fully direct network reachability plus two.
* +0 CCP out of bounds read. The patch establishes that CCP decompression can receive skb data minus two, but no read back, disclosure, sensitive value influence, or attacker-visible output is shown.
* +0 No controlled reclaim, active object replacement, arbitrary write, directed write, callback control, credential manipulation, or practical privilege-escalation chain is established.
* +0 No separate Confidentiality or Integrity signal. A bounded read before skb head does not itself establish disclosure. The constrained underflow write is already counted once and does not demonstrate modification of a selected security-sensitive target.
* +0 No UAF or repeated release/refcount signal. The skb is incorrectly reused while still referenced by ap rpkt. It is not freed and later used, and the fix frees it specifically to prevent reuse.
* +0 No hard race penalty. This is an ordered malformed-frame sequence, not an externally uncontrollable timing race.
* +0 No niche subsystem penalty. PPP setup by an administrator is a real deployment prerequisite, but the supplied evidence does not establish a rare hardware or exceptionally difficult execution mode.

3. Reachability analysis

The attacker model is a PPP peer that can send malformed frames over an already established asynchronous PPP transport. The supplied commit states that root configures PPP and that the peer supplies the two crashing frames.

The attacker does not need host administrator privileges once the vulnerable PPP link exists. Therefore HostAdminRequired is NO. However, activation of the PPP endpoint on the victim is prior administrative setup and materially limits exposure compared with an Internet-facing service.

This is not a namespace-capability issue. No local CAP_NET_ADMIN, user namespace capability, BPF loading privilege, or attacker-created network namespace is required for the peer-side trigger. NamespaceScopedPrivilege is NO.

The vulnerable path is not generally reachable from arbitrary Internet traffic. The attacker must access the particular PPP link or its underlying transport, such as a serial, dial-up, tunnel, or internal point-to-point deployment.

4. Primitive and severity interpretation

The exact primitive is reuse of a live skb whose required NET_SKB_PAD headroom was removed after a receive error. A subsequent valid-looking PPP frame can make skb_push move skb data below skb head before the inbound filter direction tag is written.

This is not a UAF. The problematic skb remains reachable through ap rpkt and is reused in an invalid state. The patch changes this behavior by freeing the errored skb and clearing ap rpkt, forcing later frames to use a newly allocated skb with valid headroom.

The demonstrated consequence is a kernel panic and denial of service. A constrained out of bounds write is directly implied by the tag store below skb head when the debug underflow path does not stop execution first. CCP also has an out of bounds read path through skb data minus two. Neither path demonstrates controlled target selection, controlled payload beyond the fixed tag, disclosure, arbitrary kernel write, or privilege escalation.

PrimitiveClass is CLASS_B because a real but constrained kernel memory-boundary violation exists. It is not CLASS_A because no attacker-controlled overwrite target, controlled reclaim, type confusion, callback dispatch, arbitrary read/write, or security-boundary bypass is established.

ImportantCandidate is NO. The corruption primitive is limited and the patch-supported result is crash or DoS. Manual review remains recommended because this is attacker-triggerable kernel memory-boundary corruption over an established PPP peer connection. AutoDowngradeAllowed is YES because the currently established leverage is constrained and no meaningful post-crash exploitation primitive remains unresolved.

Likely operational severity is a configuration-dependent adjacent-peer kernel DoS. It should be prioritized higher when PPP links are exposed to untrusted peers or used across tenant, dial-up, serial-console, VPN, or tunnel boundaries.

5. One-sentence report phrase

An attacker controlling an established PPP peer link can trigger a constrained skb headroom underflow after a malformed frame, causing a kernel panic and denial of service, while no controlled memory-corruption or privilege-escalation primitive is currently demonstrated.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The patch establishes a bounded write below skb head and an additional CCP out of bounds read, but the supplied evidence demonstrates a kernel crash rather than controlled corruption, disclosure, or privilege escalation.

7. Call-site confidence

Call-site confidence: high

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

Patch: ppp_async: drop the errored frame instead of resetting its headroom [ Upstream
Commit: d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da

Changed files:
  net/core/skbuff.c
  drivers/net/ppp/ppp_async.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=d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da

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

The original Linux kernel CVE announcement for CVE-2026-98158 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98158

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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.4

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: 3

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).