[CVE-2026-43501][IMPORTANT] ipv6: rpl: reserve mac_len headroom when recompressed SRH grows

[CVE-2026-43501][IMPORTANT] ipv6: rpl: reserve mac_len headroom when recompressed SRH grows

[AL-KERNEL]

CVE: CVE-2026-43501
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ipv6: rpl: reserve mac_len headroom when recompressed SRH grows
Commit: 8e8be63465a5e80394c70324603dfea1bfdad48f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8e8be63465a5e80394c70324603dfea1bfdad48f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43501
Analysis date: Thu, 21 May 2026 08:52:12 -0400
ActionableScore: 9
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An IPv6 RPL SRH recompression bug can leave insufficient skb headroom, causing skb_mac_header_rebuild to wrap mac_header and perform an out-of-bounds kernel write, with local raw-socket triggering demonstrated and adjacent-network exposure possible when RPL SRH packets are accepted.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-43501 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43501

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: IMPORTANT
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-2026-43501	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-119;CWE-131;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.8';DESCR 'An out-of-bounds write in the IPv6 RPL SRH receive path can occur when recompression makes the routing header larger than the received header and the skb headroom is not expanded for both the new header and mac_len. After skb_push consumes too much headroom, skb_mac_header_rebuild can make the u16 mac_header field wrap and memmove can write mac_len bytes about 64KiB past skb head. The demonstrated trigger is a local AF_INET6 SOCK_RAW IPV6_HDRINCL packet over lo, but the vulnerable code is in IPv6 receive processing and may be reachable from an adjacent network when RPL SRH packets are accepted on an interface. For the CVSS the PR:N is used for the paranoid score because a network peer may not need an account on the target if the RPL enabled receive path is exposed. Impact is at least denial of service via kernel crash and in worst case may allow confidentiality or integrity impact because the primitive is an out-of-bounds kernel write.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 9)	YES	REMOTE WRITE KASAN OOB NETWORK SKB LINUS IPV6 KPANIC PACKET 	YES	NO	guess

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ACTIONABLESCORE ANALYSIS
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ActionableScore=9
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: 7
* Paranoid score: 9
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. The described reproducer uses a local IPv6 raw socket with IPV6_HDRINCL over loopback. This may require CAP_NET_RAW, but user and network namespaces can make this reachable by an otherwise unprivileged local user on many systems.
* Memory corruption, strong corruption primitive: +2. The patch description explicitly states a KASAN confirmed OOB write from memmove about 64KiB past skb head.
* Reliable kernel crash / strong DoS: +1. KASAN reports an OOB write in ipv6_rthdr_rcv, and non KASAN systems may crash or corrupt memory.
* Weak LPE concern: +1. The primitive is an OOB kernel write, but the write size and payload are constrained by skb mac header rebuild behavior, so this is not a demonstrated arbitrary write.
* Availability impact realistic: +1. A crafted packet can cause kernel memory corruption and likely crash.
* Common networking packet-processing path: +1. The bug is in IPv6 extension header receive processing, although specifically the RPL SRH path.
* Constrained primitive uncertainty: -1. The overwrite appears to be a small memmove of MAC header bytes, not directly attacker chosen arbitrary data to an arbitrary address.

Paranoid additional interpretation:

* Remote or adjacent network triggerable: +2 instead of only local trigger. If IPv6 RPL SRH packets are accepted on an interface, a peer on the relevant network segment may be able to reach the vulnerable receive path without a local account.
* Confidentiality impact plausible: +1. Kernel OOB write can potentially corrupt objects in ways that later expose data, although no leak is demonstrated.
* Integrity impact plausible: +1. Kernel OOB write can potentially corrupt kernel metadata or adjacent objects.
* Broad manual-review sensitivity for network memory corruption: raises the practical triage priority because this is not a pure warning, stall, or resource leak.

## 3. Reachability analysis

The demonstrated trigger is local packet injection using AF_INET6 SOCK_RAW IPV6_HDRINCL over loopback, so a conservative model treats this as local with PR:L or equivalent CAP_NET_RAW access. In deployments with unprivileged user namespaces and network namespaces, a local unprivileged user may be able to obtain the needed packet crafting capability inside a namespace, so the issue should not be dismissed as admin only.

The paranoid model is adjacent-network reachable rather than generally Internet-reachable. The vulnerable code is in IPv6 receive processing for RPL Source Routing Headers, so remote practicality depends on whether RPL SRH handling is enabled and accepted on a real interface. This is not a universal IPv6 path for arbitrary Internet packets, but it is a network packet parser with a concrete OOB write when reachable.

## 4. Severity interpretation

This behaves more like an Important candidate than an ordinary Moderate issue. The realistic demonstrated impact is kernel memory corruption and crash from a crafted packet, not merely a validation failure or resource exhaustion. Practical privilege escalation is not demonstrated, and the overwrite appears constrained, so the conservative score should not assume reliable LPE. However, because the primitive is a real OOB kernel write in a packet-processing path, the paranoid score should preserve manual-review sensitivity and treat confidentiality and integrity impact as plausible.

## 5. One-sentence report phrase

An IPv6 RPL SRH recompression bug can leave insufficient skb headroom, causing skb_mac_header_rebuild to wrap mac_header and perform an out-of-bounds kernel write, with local raw-socket triggering demonstrated and adjacent-network exposure possible when RPL SRH packets are accepted.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a confirmed OOB write in the IPv6 networking receive path, with at least reliable DoS and plausible corruption-based escalation concerns even though arbitrary write or practical LPE is not demonstrated.

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

Patch: ipv6: rpl: reserve mac_len headroom when recompressed SRH grows
Commit: 8e8be63465a5e80394c70324603dfea1bfdad48f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8e8be63465a5e80394c70324603dfea1bfdad48f

Commit description:

ipv6_rpl_srh_rcv() decompresses an RFC 6554 Source Routing Header, swaps
the next segment into ipv6_hdr->daddr, recompresses, then pulls the old
header and pushes the new one plus the IPv6 header back.  The
recompressed header can be larger than the received one when the swap
reduces the common-prefix length the segments share with daddr (CmprI=0,
CmprE>0, seg[0][0] != daddr[0] gives the maximum +8 bytes).

pskb_expand_head() was gated on segments_left == 0, so on earlier
segments the push consumed unchecked headroom.  Once skb_push() leaves
fewer than skb->mac_len bytes in front of data,
skb_mac_header_rebuild()'s call to:

	skb_set_mac_header(skb, -skb->mac_len);

will store (data - head) - mac_len into the u16 mac_header field, which
wraps to ~65530, and the following memmove() writes mac_len bytes ~64KiB
past skb->head.

A single AF_INET6/SOCK_RAW/IPV6_HDRINCL packet over lo with a two
segment type-3 SRH (CmprI=0, CmprE=15) reaches headroom 8 after one
pass; KASAN reports a 14-byte OOB write in ipv6_rthdr_rcv.

Fix this by expanding the head whenever the remaining room is less than
the push size plus mac_len, and request that much extra so the rebuilt
MAC header fits afterwards.

Fixes: 8610c7c ("net: ipv6: add support for rpl sr exthdr")
Cc: stable <stable@kernel.org>
Reported-by: Anthropic
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patch.msgid.link/2026042133-gout-unvented-1bd9@gregkh
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/exthdrs.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=8e8be63465a5e80394c70324603dfea1bfdad48f

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

The original Linux kernel CVE announcement for CVE-2026-43501 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43501

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:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.8

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: 7
  Paranoid ActionableScore: 9

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).