[CVE-2026-80793][MODERATE REGULAR] ipv4: reject undersized MTUs in ip_do_fragment()

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

CVE: CVE-2026-80793
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipv4: reject undersized MTUs in ip_do_fragment()
Commit: a716a64a4ba68cd46f2745fba2b1099fe8e0aa59
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a716a64a4ba68cd46f2745fba2b1099fe8e0aa59
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80793
Analysis date: Fri, 04 Sep 2026 15:58:33 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
IPv4 fragmentation can enter a non progressing loop when an effective MTU is smaller than the IPv4 header length plus 8 bytes, allowing packets hitting a vulnerable undersized MTU forwarding path to cause CPU softlockup and denial of service.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80793 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80793

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-80793	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-80793 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-835;**CWE-400;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.9';DESCR 'IPv4 fragmentation can enter a non progressing loop when ip_do_fragment() receives an effective MTU smaller than the IPv4 header length plus 8 bytes. In that case ip_frag_next() can compute a zero length fragment payload, while the fragmentation state does not advance and the kernel keeps allocating and transmitting header only fragments until a CPU soft lockup occurs. For the CVSS the PR:N is used in the paranoid network scenario because a remote sender does not need authenticated privileges if the vulnerable forwarding route or undersized device MTU already exists. The issue is network reachable only under specific routing or device MTU conditions, such as an MTU around 20, so AC:H is appropriate. Impact is denial of service via CPU soft lockup and possible kernel panic depending on watchdog policy. No confidentiality or integrity impact is indicated because the primitive is a non progressing loop rather than memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 2)  SKIP CVE-2026-80793 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	REMOTE DEADLOCK SIMPLEFIX NETWORK HARDWARE IPV6 PACKET  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

## 1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1. No floor or cap applied.
* Raw paranoid score: 3
* Paranoid score: 3. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

## 2. Signal breakdown

Conservative score:

* +1 Reliable kernel crash / strong DoS. The patch description establishes a non progressing fragmentation loop that continues allocating and transmitting header only fragments until the softlockup detector fires.
* +1 Common networking core path. The affected code is IPv4 fragmentation in `ip_do_fragment()`, a core packet processing path.
* -1 Rare and difficult to reach vulnerable condition. The trigger requires an effective MTU smaller than IPv4 header length plus 8, for example MTU 20, which is unusual and normally depends on special route or device configuration.

Paranoid score:

* +1 Reliable kernel crash / strong DoS. Same softlockup behavior.
* +1 Common networking core path. Same IPv4 fragmentation path.
* +2 Network trigger in a preexisting vulnerable forwarding configuration. If a host already forwards packets toward an undersized MTU route or device, attacker controlled packets may reach the vulnerable fragmentation path remotely.
* -1 Rare and difficult to reach vulnerable condition. The undersized MTU condition remains non standard and materially limits exposure.

Non awarded signals:

* No UAF or stale lifetime primitive is established.
* No OOB write, arbitrary write, controlled reclaim, or object replacement is established.
* No confidentiality or integrity signal is awarded because the primitive is a non progressing loop, not memory corruption or disclosure.
* No extra Availability point is awarded because the softlockup and DoS are the same consequence already counted.
* No ImportantCandidate signal is awarded because this is DoS only with no unresolved corruption or boundary bypass primitive.

## 3. Reachability analysis

The local administrative trigger is clear: a route such as `mtu lock 20` or a device with MTU 20 can cause the vulnerable state. Creating such host routing or device configuration normally requires privileged network administration, but the supplied evidence also says the bug can be triggered by forwarding to an already undersized device MTU.

For that reason, `HostAdminRequired=UNKNOWN`: host admin privilege is likely required to create the vulnerable configuration, but may not be required by the packet sender once such a configuration already exists.

Namespace scoped reachability is unresolved. CAP_NET_ADMIN may be namespace scoped for some routing/device operations, but the supplied context does not prove that the complete trigger chain can be built inside attacker controlled namespaces.

Remote reachability is conditional. A remote packet sender can be relevant only if the target system already has a forwarding path or route/device MTU small enough to make `mtu < hlen + 8`. This is not a normal Internet exposed IPv4 condition.

No race is involved. No special hardware is required. The main exposure limiter is the unusual MTU condition.

## 4. Primitive and severity interpretation

The exact primitive is an algorithmic denial of service: `ip_do_fragment()` subtracts the IPv4 header length from an already too small effective MTU, and `ip_frag_next()` can produce zero payload progress. The fragmentation state remains unchanged, so the loop can continue until CPU softlockup.

This is not an active structured object UAF, not fixed stale storage, and not a repeated release/refcount case.

Primitive class is `CLASS_C` because the demonstrated behavior is crash or DoS through an invalid non progressing state. There is no independently useful corruption primitive.

The demonstrated consequence is CPU softlockup and possible kernel panic depending on watchdog policy. The speculative consequence does not extend beyond DoS based on the supplied patch.

ImportantCandidate is NO because there is no memory corruption, security boundary bypass, ownership violation, or privilege escalation primitive. AutoDowngradeAllowed is YES because the issue is DoS only and does not contain unresolved Important grade exploitation leverage.

Operational severity is borderline Moderate: low in normal configurations, but worth manual review for routers or appliances that may forward through unusual tiny MTU links or test/tunnel configurations.

## 5. One-sentence report phrase

IPv4 fragmentation can enter a non progressing loop when an effective MTU is smaller than the IPv4 header length plus 8 bytes, allowing packets hitting a vulnerable undersized MTU forwarding path to cause CPU softlockup and denial of service.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the primitive is DoS only and auto downgrade is allowed, but the path can be network triggered in specific preexisting forwarding configurations, so exposed router or appliance deployments should be checked manually.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: ipv4: reject undersized MTUs in ip_do_fragment()
Commit: a716a64a4ba68cd46f2745fba2b1099fe8e0aa59
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a716a64a4ba68cd46f2745fba2b1099fe8e0aa59

Commit description:

ip_do_fragment() subtracts the IPv4 header length from the effective
MTU and passes the resulting payload MTU to ip_frag_next().

If the effective MTU is smaller than hlen + 8, ip_frag_next() rounds
the fragment payload length down to zero. The fragmentation state then
never makes forward progress: state->left, state->ptr and state->offset
stay unchanged while ip_do_fragment() keeps allocating and transmitting
header-only fragments until the softlockup detector fires.

This is reproducible with a route installed using "mtu lock 20", but it
is also reproducible without route MTU lock, for example by forwarding a
packet to a device whose MTU is 20.

Fix it in ip_do_fragment() by rejecting mtu < hlen + 8 with -EMSGSIZE,
matching the existing IPv6 fragmentation check.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Yong Wang <edragain@163.com>
Signed-off-by: Ren Wei <weir@nebusec.ai>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/8809ef6314b98913681b0b370a05a85c2b6cd579.1786599079.git.edragain@163.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv4/ip_output.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=a716a64a4ba68cd46f2745fba2b1099fe8e0aa59

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

The original Linux kernel CVE announcement for CVE-2026-80793 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80793

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 1
  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).