* [CVE-2026-89555][MODERATE 7.0] mpls: reload header after pskb_may_pull()
@ 2026-09-12 7:26 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-12 7:26 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89555
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: mpls: reload header after pskb_may_pull()
Commit: fed638a248116b8a249bd4202d28e5934bdc65ad
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fed638a248116b8a249bd4202d28e5934bdc65ad
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89555
Analysis date: Sat, 12 Sep 2026 03:26:05 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89555 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89555
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-2026-89555 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89555 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:L/I:L/A:H';*CWE-416;CWE-825;*CWE-664;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'An MPLS multipath hash path can dereference a stale MPLS header pointer after pskb_may_pull expands and replaces the skb head. A legal encapsulated packet can place the inner IP header in nonlinear skb data and force pskb_expand_head, leaving the cached hdr pointer referencing freed memory before the IPv4 or IPv6 header is parsed. For the CVSS the PR:N is used because a remote sender does not need privileges on the target when the MPLS multipath and tunnel endpoint are reachable. The issue is network reachable in systems that route or decapsulate traffic into a bareudp/MPLS multipath setup, but it depends on non default MPLS and encapsulation configuration. Impact is at least denial of service via kernel crash. In the paranoid interpretation the use after free read in the network receive path may justify limited confidentiality or integrity impact and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) YES READ DANGER UAF SKB IPV6 KPANIC PACKET NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
No floor or cap changed the conservative score. The paranoid score differs because it evaluates an already deployed and reachable MPLS or bareudp MPLS multipath router, without subtracting the non-default topology penalty.
2. Signal breakdown
Conservative signals:
* +2 Real UAF / lifetime violation. The cached hdr pointer can reference the old skb head after pskb_may_pull() calls pskb_expand_head() and frees that head.
* +2 Remote reachable packet-processing trigger. The supplied context states that a legal Geneve packet through a bareudp/MPLS multipath setup reproduced the KASAN UAF.
* +1 Reliable kernel crash / strong DoS. KASAN reports slab-use-after-free in mpls_select_multipath() in the RX softirq path.
* -1 Non-default and topology-dependent configuration. The trigger depends on MPLS multipath plus tunnel/encapsulation setup, which normally requires prior administrative routing configuration and is not a default host path.
Paranoid signals:
* +2 Real UAF / lifetime violation.
* +2 Remote reachable packet-processing trigger.
* +1 Reliable kernel crash / strong DoS.
* No non-default-topology penalty in the deployed-target interpretation, because a system actually using the affected MPLS/bareudp multipath forwarding path can be reached by attacker-controlled packets.
Important non-awarded signals:
* Controlled reclaim is not demonstrated.
* Arbitrary write is not demonstrated.
* Confidentiality is not awarded because no user-visible disclosure path is shown.
* Integrity is not awarded because the stale access is a read used for header parsing/hash calculation, not a demonstrated unauthorized write.
* Structured stale-object leverage is not separately awarded because the stale object is the skb head/data buffer, not a demonstrated attacker-shaped replacement object with active callbacks or security fields.
* Concrete repeated release/refcount is not awarded. There is no second put/free/refcount mutation through the same dangling reference.
* Extra Availability is not awarded because the same crash is already counted once.
* Weak LPE is not awarded merely from the UAF class.
Repeated-release/refcount check:
* First release can end lifetime: yes, pskb_expand_head() frees the old skb head.
* Exact same reference survives: yes, hdr remains cached across pskb_may_pull().
* Second operation is a real lifetime/refcount mutation: no, the second operation is a stale read/dereference, not put/free/refcount.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage awarded: no.
* FLOOR 5 applied: no. Paranoid score reaches 5 by deployed network reachability, not by repeated-release floor.
3. Reachability analysis
The evaluated attacker is a remote packet sender to a system already configured to route or decapsulate traffic into an MPLS multipath path. The supplied reproducer used a legal Geneve packet through bareudp/MPLS multipath and hit the UAF in ksoftirqd during network receive processing.
Host administrator privilege is not required for the packet sender once the vulnerable forwarding/tunnel configuration exists, so HostAdminRequired=NO for the triggering actor. However, the affected topology itself normally requires prior privileged network configuration. This is accounted for as a conservative non-default configuration penalty, not as HostAdminRequired=YES.
Namespace-scoped privilege is not relevant to the remote packet trigger, so NamespaceScopedPrivilege=NO. No local namespace capability is needed by the remote sender.
There is no evidence of special hardware. The issue is not a timing race. The trigger depends on skb layout conditions where the inner IP header is in nonlinear data and pskb_may_pull() expands/replaces the skb head, but the supplied context reports reproducible KASAN results in 2 of 2 unpatched runs.
4. Primitive and severity interpretation
The exact primitive is a stale pointer use after free. mpls_multipath_hash() caches hdr while walking the MPLS label stack, then calls pskb_may_pull() before parsing the inner IPv4 or IPv6 header. If pskb_may_pull() expands the skb head, the old head is freed and hdr still points into freed memory.
This is fixed stale skb storage rather than a proven active structured object replacement primitive. The stale use is broader than cleanup/destructor-only because it occurs in the packet forwarding datapath, but the demonstrated stale operation is a read/dereference that causes KASAN UAF and likely kernel crash.
PrimitiveClass=CLASS_B because a real kernel lifetime/UAF primitive is established. It is not CLASS_A because there is no controlled reclaim, arbitrary read/write, callback control, refcount takeover, or demonstrated privilege escalation. It is not CLASS_C because the issue is a real post-lifetime use, not merely an invalid NULL dereference or passive warning.
The demonstrated consequence is remote DoS against affected MPLS/bareudp multipath deployments. The speculative consequence is limited potential C/I concern from a network-reachable UAF read in skb processing, but the patch evidence does not establish a concrete disclosure or write primitive.
ImportantCandidate=YES because this is a network-reachable Class B UAF in a kernel packet-processing path and should not be automatically downgraded without human review. AutoDowngradeAllowed=NO because automatic closure would discard a real remotely triggerable lifetime primitive whose full exploitation leverage is not completely resolved.
Likely operational severity is Actionable Moderate, with Strong Important review justified for exposed MPLS/bareudp multipath routers.
5. One-sentence report phrase
A remote packet can trigger a stale skb header pointer use after pskb_may_pull() replaces the skb head in the MPLS multipath path, causing a kernel UAF crash on affected bareudp/MPLS multipath forwarding setups.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: network-reachable real UAF in RX packet processing, with demonstrated KASAN slab-use-after-free and kernel crash, even though controlled reclaim or LPE is not established.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: mpls: reload header after pskb_may_pull()
Commit: fed638a248116b8a249bd4202d28e5934bdc65ad
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fed638a248116b8a249bd4202d28e5934bdc65ad
Commit description:
mpls_select_multipath() calls mpls_multipath_hash() to choose a nexthop
when an MPLS route has multiple nexthops. While walking the MPLS label
stack, the hash routine caches hdr for the current label. After finding
the bottom-of-stack label, it calls pskb_may_pull() before reading the
inner IP header.
If an skb is constructed with the inner IP header in nonlinear data and
insufficient tailroom in the linear head, pskb_may_pull() calls
pskb_expand_head() to replace the skb head and free the old one. This
leaves hdr pointing to freed memory. The IPv6 path can invalidate hdr
again when it performs a second pull for the larger header.
The issue was found through static analysis. A reproducer sending a legal
Geneve packet through a bareudp/MPLS multipath setup triggered the same
KASAN report in 2 of 2 unpatched runs:
BUG: KASAN: slab-use-after-free in mpls_select_multipath
Read of size 1 at addr ffff88800ecc6e20 by task ksoftirqd/1/23
Call Trace:
mpls_select_multipath
mpls_forward
__netif_receive_skb_list_core
netif_receive_skb_list_internal
napi_complete_done
gro_cell_poll
__napi_poll
net_rx_action
Freed by task 23:
kfree
pskb_expand_head
__pskb_pull_tail
mpls_select_multipath
Reload hdr from the current skb head after each successful pull before
deriving the inner IPv4 or IPv6 header pointer.
Fixes: 9f427a0 ("net: mpls: Fix multipath selection for LSR use case")
Cc: [email protected]
Signed-off-by: Qing Ming <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
net/mpls/af_mpls.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=fed638a248116b8a249bd4202d28e5934bdc65ad
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89555 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89555
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:H
The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS score: 7
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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