UBUNTU-CVE-2026-89555: Use After Free
In the Linux kernel, the following vulnerability has been resolved: mpls: reload header after pskbmaypull() mplsselectmultipath() calls mplsmultipathhash() 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 pskbmaypull() 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, pskbmaypull() calls pskbexpandhead() 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 mplsselectmultipath Read of size 1 at addr ffff88800ecc6e20 by task ksoftirqd/1/23 Call Trace: mplsselectmultipath mplsforward netifreceiveskblistcore netifreceiveskblistinternal napicompletedone grocellpoll napipoll netrxaction Freed by task 23: kfree pskbexpandhead pskbpulltail mplsselectmultipath Reload hdr from the current skb head after each successful pull before deriving the inner IPv4 or IPv6 header pointer.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
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ubuntu/linux-aws-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1119.126~20.04.1 - Upgrade
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ubuntu/linux-gcp-fipsto a version that resolves this vulnerability.Fixed in 5.15.0-1120.130+fips1 - Upgrade
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ubuntu/linux-hwe-7.0to a version that resolves this vulnerability.Fixed in 7.0.0-42.42~24.04.1 - Upgrade
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ubuntu/linux-riscv-7.0to a version that resolves this vulnerability.Fixed in 7.0.0-42.42.1~24.04.1 - Upgrade
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ubuntu/linuxto a version that resolves this vulnerability.Fixed in 7.0.0-42.42 - Upgrade
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ubuntu/linux-awsto a version that resolves this vulnerability.Fixed in 7.0.0-1016.17 - Upgrade
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ubuntu/linux-riscvto a version that resolves this vulnerability.Fixed in 7.0.0-42.42.1 - Compensating control
In mpls_select_multipath(), reload the MPLS header pointer from the current skb head after each successful pskb_may_pull() before deriving or reading the inner IPv4/IPv6 header; repeat the reload on the IPv6 path after its second pull.
Event History
Frequently Asked Questions
Which systems are most exposed to this issue?
Systems that forward MPLS traffic using routes with multiple nexthops are exposed in the affected kernel code path. The reported reproducer used legal Geneve traffic through a bareudp/MPLS multipath setup.
What does an attacker need to do to trigger the flaw?
The issue is reachable remotely with low attack complexity and requires no privileges or user interaction, according to the provided CVSS vector. Triggering it involves traffic that causes MPLS multipath selection to inspect an inner IP header stored in nonlinear packet data.
How can I identify a possible occurrence?
A kernel built with KASAN may report a slab use-after-free in mpls_select_multipath, with a call trace including mpls_forward and network receive processing functions. The issue was reproduced as a KASAN report on unpatched kernels.