[CVE-2026-88881][MODERATE 7.0] Merge branch 'ipv4-ipv6-account-for-fraggap-on-paged-allocation-paths'

[CVE-2026-88881][MODERATE 7.0] Merge branch 'ipv4-ipv6-account-for-fraggap-on-paged-allocation-paths'

[AL-KERNEL]

CVE: CVE-2026-88881
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: Merge branch 'ipv4-ipv6-account-for-fraggap-on-paged-allocation-paths'
Commit: 38becddc332c1dbee6ab8dc8f13a860c6280b905
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=38becddc332c1dbee6ab8dc8f13a860c6280b905
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-88881
Analysis date: Wed, 01 Jul 2026 11:50:32 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
Incorrect fraggap accounting in IPv4 and IPv6 paged skb allocation paths can make skb copy length calculations negative and leave packet buffer layout inconsistent, allowing a local user to potentially trigger a kernel crash or networking malfunction through crafted outbound fragmented traffic.

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

The original Linux kernel CVE announcement for CVE-2026-88881 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-88881

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 7.0
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-2026-88881	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-682;CWE-131;*CWE-20;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'IPv4 and IPv6 output paths can misaccount fraggap on paged skb allocation paths, causing copy length calculation to become negative and leaving skb data length and offset handling inconsistent. A local process may trigger this through crafted outbound fragmented traffic that reaches the paged allocation path, for example with scatter gather capable output and packet layouts that require fraggap handling. For the CVSS the PR:L is used because local code execution as an unprivileged process is still required to drive the send path and reliably shape the traffic. The issue is not directly network reachable from a remote peer because it is in the local packet output path. Impact is at least local denial of service via kernel crash or networking stack malfunction. For the paranoid score, limited confidentiality or integrity impact is kept because incorrect skb length accounting can be a memory safety candidate, but the patch does not show a strong UAF or OOB write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	MAYBE	REMOTE NETWORK IPV6 KPANIC 	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local process can plausibly drive IPv4 or IPv6 output paths by sending crafted outbound traffic.

* Memory layout invariant restoration: +1
  The patch restores consistency between interdependent skb construction variables: alloclen, pagedlen, copy, fraggap, datalen, and transhdrlen.

* Memory corruption, weak or indirect corruption candidate: +1
  The bug can make copy negative and leave skb length or offset accounting inconsistent in paged allocation paths. This is a memory safety candidate, but the patch does not show a concrete OOB write, UAF, double free, or attacker-controlled overwrite primitive.

* Common networking core path or packet-processing path: +1
  The affected code is in IPv4 and IPv6 packet output and fragmentation logic, which is core networking code.

Paranoid additional signal:

* Availability impact realistic: +1
  Incorrect skb construction in core output paths can plausibly cause kernel crash, packet processing failure, or networking stack malfunction under crafted local traffic. This is enough to raise the paranoid score, but not enough to claim strong memory corruption.

Not counted:

* Remote reachable: +0
  This is packet output construction, not a remote receive parser.

* Strong LPE plausibility: +0
  No reclaim, arbitrary write, type confusion, refcount takeover, callback corruption, or controlled overwrite is shown.

* Strong corruption primitive: +0
  The patch fixes accounting, but does not demonstrate a direct OOB write or UAF.

* Dirty-Pipe-like primitive: +0
  No page-cache, pipe, COW, or shared file-backed page ownership bypass is indicated.

## 3. Reachability analysis

A local unprivileged user may be able to trigger the path by generating outbound IPv4 or IPv6 traffic that enters fragmentation and the paged skb allocation path. Practical triggering likely requires specific packet layout conditions involving fraggap handling and scatter-gather capable output paths. This is not directly network-triggerable by a remote peer because the vulnerable code is in the local output path.

Namespaces may matter if an untrusted user can create or use network namespaces and send crafted traffic through configured interfaces. However, the bug does not appear to require CAP_NET_ADMIN merely to send traffic, so PR:L is the practical CVSS interpretation. The path is in common networking code, but the exact paged allocation plus fraggap condition is more specific than ordinary packet send.

Call-site confidence: medium. The changed code is in the actual skb allocation path, so the local consequence of inconsistent length accounting is visible, but the patch does not include a crash trace or a downstream concrete corruption primitive.

## 4. Severity interpretation

This behaves stronger than an ordinary low-risk correctness bug because it restores size and layout accounting in core IPv4 and IPv6 skb construction. It should not be auto-closed as a trivial Moderate.

Realistically, the evidence supports local DoS or networking stack malfunction under crafted outbound traffic. Theoretical memory corruption potential exists because negative copy and inconsistent skb length accounting can affect how data is split between linear and paged skb regions. However, the patch does not demonstrate a strong primitive such as UAF, OOB write, attacker-controlled overwrite target, or privilege escalation path.

Recommended handling is Actionable Moderate at minimum, with manual review recommended to check whether the negative length accounting can become an actual OOB access or skb memory corruption primitive.

## 5. One-sentence report phrase

Incorrect fraggap accounting in IPv4 and IPv6 paged skb allocation paths can make skb copy length calculations negative and leave packet buffer layout inconsistent, allowing a local user to potentially trigger a kernel crash or networking malfunction through crafted outbound fragmented traffic.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the patch touches core skb allocation and fragmentation accounting, and the bug involves inconsistent length and offset calculations. There is no proven strong memory corruption primitive in the provided patch, but this class is risky enough that it should not be auto-closed without reviewing the exact skb construction consequences.

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

Patch: Merge branch 'ipv4-ipv6-account-for-fraggap-on-paged-allocation-paths'
Commit: 38becddc332c1dbee6ab8dc8f13a860c6280b905
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=38becddc332c1dbee6ab8dc8f13a860c6280b905

Commit description:

Wongi Lee says:

====================
<title>ipv4/ipv6: account for fraggap on paged allocation paths</title>

Fix fraggap accounting in the paged-allocation paths of IPv4 and IPv6.

The IPv6 patch is the v4 update of the previously posted patch. The IPv4
patch handles the same code pattern (by Ido).

v3: https://lore.kernel.org/aiq3f7UZGFp0F3MV@DESKTOP-19IMU7U.localdomain
v2: https://lore.kernel.org/aigx83czv+UJZA0d@DESKTOP-19IMU7U.localdomain
v1: https://lore.kernel.org/aibiIYMAwUErTw5U@DESKTOP-19IMU7U.localdomain
====================

Link: https://patch.msgid.link/ajFQn6yh43eDeQm9@DESKTOP-19IMU7U.localdomain
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Changed files:
  net/ipv4/ip_output.c
  net/ipv6/ip6_output.c

Diff excerpt:

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 3b4e9b8af044f..e6dd1e5b8c327 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1116,8 +1116,8 @@ alloc_new_skb:
 				  !(rt->dst.dev->features & NETIF_F_SG)))
 				alloclen = fraglen;
 			else {
-				alloclen = fragheaderlen + transhdrlen;
-				pagedlen = datalen - transhdrlen;
+				alloclen = fragheaderlen + transhdrlen + fraggap;
+				pagedlen = datalen - transhdrlen - fraggap;
 			}

 			alloclen += alloc_extra;
@@ -1164,9 +1164,6 @@ alloc_new_skb:
 			}

 			copy = datalen - transhdrlen - fraggap - pagedlen;
-			/* [!] NOTE: copy will be negative if pagedlen>0
-			 * because then the equation reduces to -fraggap.
-			 */
 			if (copy > 0 &&
 			    INDIRECT_CALL_1(getfrag, ip_generic_getfrag,
 					    from, data + transhdrlen, offset,
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 9f1e0e4f74641..368e4fa3b43ca 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1667,8 +1667,8 @@ alloc_new_skb:
 				  !(rt->dst.dev->features & NETIF_F_SG)))
 				alloclen = fraglen;
 			else {
-				alloclen = fragheaderlen + transhdrlen;
-				pagedlen = datalen - transhdrlen;
+				alloclen = fragheaderlen + transhdrlen + fraggap;
+				pagedlen = datalen - transhdrlen - fraggap;
 			}
 			alloclen += alloc_extra;

@@ -1683,10 +1683,7 @@ alloc_new_skb:
 			fraglen = datalen + fragheaderlen;

 			copy = datalen - transhdrlen - fraggap - pagedlen;
-			/* [!] NOTE: copy may be negative if pagedlen>0
-			 * because then the equation may reduces to -fraggap.
-			 */
-			if (copy < 0 && !(flags & MSG_SPLICE_PAGES)) {
+			if (copy < 0) {
 				err = -EINVAL;
 				goto error;
 			}
generated by cgit 1.3-korg (git 2.53.0) at 2026-07-01 14:56:35 +0000

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=38becddc332c1dbee6ab8dc8f13a860c6280b905

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

The original Linux kernel CVE announcement for CVE-2026-88881 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-88881

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:H/PR:L/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:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 4
  Paranoid ActionableScore: 5

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).