* [CVE-2026-89565][MODERATE REGULAR] ipip: fix skb leak in collect_md mode when metadata_dst allocation fails
@ 2026-09-12 0:46 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 0:46 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89565
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ipip: fix skb leak in collect_md mode when metadata_dst allocation fails
Commit: 6d8c5b266d0035a061573843a7243be068eb6118
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6d8c5b266d0035a061573843a7243be068eb6118
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89565
Analysis date: Fri, 11 Sep 2026 20:46:50 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
An skb leak in the ipip collect_md receive path can be triggered when metadata_dst allocation fails, causing packets to be treated as consumed without freeing the skb and potentially leading to resource exhaustion on exposed ipip or mplsip tunnel endpoints.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89565 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89565
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-2026-89565 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89565 CHECK WITH IMPACT FROM ORIG NN IMPORTANT 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-401;*CWE-772;**CWE-400;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'An skb leak in the ipip receive path can occur in collect_md mode when ip_tun_rx_dst() fails to allocate metadata_dst. The function returned success without freeing the skb, and the outer tunnel receive handlers treat that return value as packet consumed, so the skb is not freed elsewhere. For the CVSS the PR:N is selected because a remote sender does not need local privileges when the configured tunnel endpoint accepts the traffic. The issue is network reachable only on systems with an ipip or mplsip tunnel configured in collect_md mode and it also requires an atomic allocation failure, so AC:H is appropriate. Impact is denial of service through memory/resource exhaustion. There is no clear evidence of privilege escalation or data exposure because the primitive is an skb leak rather than UAF or arbitrary memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) YES REMOTE SIMPLEFIX LEAK NETWORK SKB LINUS PACKET LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
## 1. ActionableScore
* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::
No floor or cap changed the score.
## 2. Signal breakdown
Conservative positive signals:
* +1 Concrete persistent kernel resource/reference leak. The patch directly establishes that an skb is consumed from the caller perspective but not freed when metadata_dst allocation fails.
* +1 Availability/resource exhaustion concern. Repeated occurrences under memory pressure can accumulate leaked skbs and worsen resource exhaustion, although immediate host-wide failure is not demonstrated.
Conservative negative signals:
* 0 No privilege penalty. The packet sender does not need host-admin privileges once a vulnerable tunnel endpoint is configured.
Paranoid additional signals:
* +2 Conditional network reachability. Attacker-controlled IPIP/MPLS-in-IP traffic may reach the vulnerable receive path when an ipip or mplsip collect_md tunnel is configured and reachable.
* -1 Concrete triggering constraint. The commit explicitly states that triggering requires collect_md mode and an atomic allocation failure, which makes the bug materially harder to exercise than an ordinary packet-triggered leak.
Important non-awarded signals:
* Real UAF not awarded. The primitive is an skb leak, not post-free use.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity not awarded. There is no disclosure path or unauthorized modification primitive.
* Strong DoS not awarded beyond the resource-exhaustion concern. A single packet or small number of packets is not shown to crash the host.
* ImportantCandidate not awarded because this is a resource leak without memory corruption, authorization bypass, COW violation, or privilege escalation evidence.
## 3. Reachability analysis
The attacker is a remote packet sender in the meaningful deployment scenario. The vulnerable path is reachable only if the target already has an ipip or mplsip tunnel configured in collect_md mode and the tunnel endpoint accepts the relevant traffic.
The attacker does not need local code execution or host privileges to send packets to an exposed endpoint, so HostAdminRequired=NO for the packet-trigger stage. However, the collect_md tunnel itself is a prior privileged network configuration, and the attacker is not shown to be able to create it.
NamespaceScopedPrivilege=NO because no namespace-scoped capability is needed for the remote packet sender model, and no complete namespace-based local trigger chain is established.
No special hardware is required. The main practical constraint is that the vulnerable path requires atomic metadata_dst allocation failure, which likely needs memory pressure or otherwise unfavorable allocation conditions.
## 4. Primitive and severity interpretation
The exact primitive is a persistent skb/resource leak on an allocation failure path. ipip_tunnel_rcv() returned 0, which tells tunnel4_rcv() or tunnelmpls4_rcv() that the packet was consumed, but the skb was not freed.
This is not an active structured UAF and not fixed stale storage after free. It is a missing cleanup/resource ownership bug.
No concrete repeated release/refcount operation exists. There is no first release that may end object lifetime followed by a second lifetime mutation through the same dangling reference.
PrimitiveClass=CLASS_C because the demonstrated behavior is resource leak and possible DoS, without an independently useful corruption primitive.
ImportantCandidate=NO because exploitation evidence does not support memory corruption, privilege escalation, protected-object access, or a security-boundary bypass.
AutoDowngradeAllowed=YES because the unresolved risk is a constrained resource exhaustion issue, not a concrete kernel-corruption primitive with meaningful exploitation leverage.
Likely operational severity is ordinary to borderline Moderate, with manual review mainly justified by conditional network exposure.
## 5. One-sentence report phrase
An skb leak in the ipip collect_md receive path can be triggered when metadata_dst allocation fails, causing packets to be treated as consumed without freeing the skb and potentially leading to resource exhaustion on exposed ipip or mplsip tunnel endpoints.
## 6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Reason: the issue is not memory corruption and can be auto-downgraded in many environments, but conditional network reachability through configured tunnel endpoints makes a brief deployment-specific review useful.
## 7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: ipip: fix skb leak in collect_md mode when metadata_dst allocation fails
Commit: 6d8c5b266d0035a061573843a7243be068eb6118
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6d8c5b266d0035a061573843a7243be068eb6118
Commit description:
In collect_md mode ipip_tunnel_rcv() returns 0 without freeing the skb
when ip_tun_rx_dst() fails to allocate the metadata_dst. ipip_rcv() and
mplsip_rcv() are registered as xfrm_tunnel handlers, so tunnel4_rcv()
and tunnelmpls4_rcv() read the zero return as "the packet has been
consumed" and do not free it either. The skb is leaked.
The other tunnel drivers all dispose of the packet at this point:
ip6_tunnel.c jumps to its drop label, ip_gre.c and ip6_gre.c return
PACKET_REJECT, which makes gre_rcv() free the skb. Only ipip returns 0.
Jump to the existing drop label instead. It frees the skb and still
returns 0, so the packet keeps being reported as consumed, which is what
we want here: the outer header has already been pulled, and neither the
remaining handlers nor an ICMP unreachable have any use for it.
Triggering this needs an ipip or mplsip tunnel in collect_md mode and an
atomic allocation failure, which is why it has gone unnoticed.
Fixes: cfc7381 ("ip_tunnel: add collect_md mode to IPIP tunnel")
Cc: [email protected]
Signed-off-by: Anton Danilov <[email protected]>
Reviewed-by: Fernando Fernandez Mancera <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
net/ipv4/ipip.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=6d8c5b266d0035a061573843a7243be068eb6118
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89565 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89565
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:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
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: 2
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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