* [CVE-2026-98541][LOW] fou: reject omitted FOU_ATTR_IPPROTO on FOU_ENCAP_DIRECT
@ 2026-10-24 13:06 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-10-24 13:06 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98541
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: fou: reject omitted FOU_ATTR_IPPROTO on FOU_ENCAP_DIRECT
Commit: fd20220f86947d66d4dba2d76f13a28dbd86ef30
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fd20220f86947d66d4dba2d76f13a28dbd86ef30
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98541
Analysis date: Sat, 24 Oct 2026 09:06:13 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
Omitting FOU_ATTR_IPPROTO when creating a direct FOU socket causes each received packet’s skb to leak, permitting sustained kernel-memory exhaustion.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98541 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98541
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: LOW
Manual review required: NO
A detailed explanation of the methodology and priority rules is included
at the end of this message.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-98541 LOW https://www.kernelcve.org/list/?q=CVE-2026-98541 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=LOW RESULT=DIFF TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: LOW, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-20;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'An omitted FOU_ATTR_IPPROTO can create a FOU_ENCAP_DIRECT socket with protocol zero. Every packet received by that socket can then be reported as consumed without freeing its skb, causing persistent kernel memory growth and a potential denial of service under sustained traffic. For the CVSS the PR:L value reflects that reliable setup requires local access to the FOU Generic Netlink control plane and normally CAP_NET_ADMIN. Once the invalid socket exists, UDP packets reaching its port can accumulate the leak, but the demonstrated test requires thousands of packets and does not establish immediate host wide exhaustion. There is no supported memory corruption, information disclosure, or integrity primitive. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage IMPORTANT to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE_KPANIC to LOW because the ActionableScore threshold rule was applied. final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic and the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 and KPANIC_UNSUPPORTED_REMOVED and V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED. The independent weaker Aegis shadow evaluated CVSS 3.3 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) MAYBE REMOTE LEAK NETWORK SKB PACKET LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 KPANIC_UNSUPPORTED_REMOVED V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 1
* Conservative score: 1
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::
No floor or cap applies.
2. Signal breakdown
* +1 Concrete persistent kernel resource leak: protocol zero makes every received packet return as consumed without freeing its skb; the test measured approximately 17 MB of unreclaimed growth from 4,000 packets.
* Paranoid +1 Reliable strong DoS: the leak occurs once per packet and sustained traffic can plausibly exhaust kernel memory. This is potential host DoS, not a demonstrated immediate panic.
* No corruption bonus: the patch shows leaked skbs, not OOB access, UAF, overwrite, or corrupted active state.
* No additional availability point: memory exhaustion is the same consequence as the scored DoS.
* No local-unprivileged bonus: creation of the malformed FOU port requires access to the administrative Generic Netlink operation; complete below-admin reachability is not established.
* No remote bonus: packets exercise an already configured socket, but an attacker must first arrange the invalid control-plane state.
* NEW52/B5-R and NEW53/B3-PENDING do not apply: there is no OOB or shared-backing write.
* No confidentiality, integrity, authorization-bypass, or protected-resource effect is shown.
3. Reachability analysis
The setup actor must issue FOU_CMD_ADD while omitting FOU_ATTR_IPPROTO, normally under CAP_NET_ADMIN. Numeric host UID 0 is not inherently required, but the evidence does not establish whether the complete operation is available through attacker-created user/net namespaces or instead requires host-equivalent administration. Thus both host-admin and namespace scope remain unknown. Once configured, UDP traffic repeatedly accumulates leaked skbs without a race, special hardware, or additional vulnerability.
4. Primitive and severity reading
The exact defect is deterministic, packet-amplified skb retention caused by accepting invalid protocol-zero configuration. It is a resource-leak/DoS condition, classified CLASS_C, with no memory-corruption or non-crash exploitation primitive. ImportantCandidate is NO and auto-downgrade is allowed because the demonstrated effect is resource growth without isolation bypass, persistent protected-state modification, or an unknown corrupt primitive.
5. One-sentence report phrase
Omitting FOU_ATTR_IPPROTO when creating a direct FOU socket causes each received packet’s skb to leak, permitting sustained kernel-memory exhaustion.
6. Manual review advice
NO MANUAL CHECK NEEDED
The patch and test establish a bounded-per-packet resource leak, with no Important-preserving primitive.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: fou: reject omitted FOU_ATTR_IPPROTO on FOU_ENCAP_DIRECT
Commit: fd20220f86947d66d4dba2d76f13a28dbd86ef30
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fd20220f86947d66d4dba2d76f13a28dbd86ef30
Commit description:
Commit 7a9bc9e ("fou: Don't allow 0 for FOU_ATTR_IPPROTO.") added
NLA_POLICY_MIN(NLA_U8, 1) to fou_nl_policy[FOU_ATTR_IPPROTO], which
rejects an explicitly supplied FOU_ATTR_IPPROTO == 0 attribute with
-ERANGE.
However, FOU_ATTR_IPPROTO is an optional netlink attribute. When a user
sends FOU_CMD_ADD with FOU_ATTR_TYPE set to FOU_ENCAP_DIRECT and omits
FOU_ATTR_IPPROTO entirely, nla_policy validation succeeds and
parse_nl_config() leaves cfg->protocol as 0 (from memset(cfg, 0,
sizeof(*cfg))). fou_create() then creates a FOU_ENCAP_DIRECT socket with
fou->protocol == 0.
In fou_udp_recv(), returning -fou->protocol to udp_queue_rcv_one_skb()
triggers IP protocol resubmission when fou->protocol > 0, whereas
returning 0 tells the UDP tunnel layer that the skb was consumed without
freeing it. When fou->protocol == 0, every packet received on the socket
returns 0 from fou_udp_recv() and leaks the sk_buff.
Reject FOU_ENCAP_DIRECT when !cfg->protocol in fou_create() so that
creating a direct encapsulation port without FOU_ATTR_IPPROTO fails with
-EINVAL while leaving FOU_CMD_DEL and FOU_CMD_GET (which share
parse_nl_config()) unaffected.
Tested in QEMU against Linux 7.3.0-rc3 by sending a FOU_CMD_ADD Generic
Netlink request with FOU_ATTR_PORT = 5555 and FOU_ATTR_TYPE =
FOU_ENCAP_DIRECT while omitting FOU_ATTR_IPPROTO. On the unfixed kernel,
FOU_CMD_ADD succeeds (err = 0), FOU_CMD_GET reports fou->type = 1 and
fou->protocol = 0, and sending 4000 UDP packets to 127.0.0.1:5555 leaks
all 4000 sk_buffs (SUnreclaim in /proc/meminfo grows from 41456 kB to
59008 kB, +17552 kB); with this patch applied, FOU_CMD_ADD is rejected
with -EINVAL (-22).
Fixes: 2346155 ("fou: Support for foo-over-udp RX path")
Fixes: 7a9bc9e ("fou: Don't allow 0 for FOU_ATTR_IPPROTO.")
Cc: [email protected]
Signed-off-by: Hui Peng <[email protected]>
Reviewed-by: Hangbin Liu <[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/fou_core.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=fd20220f86947d66d4dba2d76f13a28dbd86ef30
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98541 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98541
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:N/I:N/A:L
The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS score: 5.5
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: 2
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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW
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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