UBUNTU-CVE-2026-90075: Medium severity ubuntu/linux vulnerability
In the Linux kernel, the following vulnerability has been resolved: net/sched: fqcodel: clamp default quantum and mtu fqcodelinit() sets q->quantum = pschedmtu(qdiscdev(sch)) without clamping. A device with a huge MTU (e.g. dummy with maxmtu == 0 accepting MTU 2147483634) makes pschedmtu() return 0x80000000, which overflows the signed flow->deficit to INTMIN in fqcodeldequeue(), causing an infinite loop and soft lockup. Emulate fqcodelchange() and constrain to [256, FQCODELQUANTUMMAX]. The same unclamped pschedmtu() is assigned to q->cparams.mtu a bit below, and fqcodelchange() never updates it. codelshoulddrop() tests "backlog <= params->mtu"; with mtu == 0x80000000 (~2 GiB) and the default 32 MiB memorylimit, the test is always true, so CoDel is silently and completely disabled (no drops, no ECN). Declare a single clamped mtu and assign both q->quantum and q->cparams.mtu from it, which also removes the double pschedmtu() call. Conditions to recreate the bug: a device whose MTU (plus hardheaderlen) wraps pschedmtu() into the sign bit (e.g. a dummy device with maxmtu == 0 accepting MTU 2147483634). Requires CAPNETADMIN in a user namespace.
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-azure-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1125.134~20.04.1 - Upgrade
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ubuntu/linux-azure-fde-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1124.133~20.04.1 - Upgrade
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ubuntu/linux-gcp-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1120.130~20.04.1 - Upgrade
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ubuntu/linux-hwe-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-202.212~20.04.1 - Upgrade
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ubuntu/linux-ibm-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1113.117~20.04.1 - Upgrade
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ubuntu/linux-intel-iotg-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1114.120~20.04.1 - Upgrade
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ubuntu/linux-lowlatency-hwe-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-197.207~20.04.1 - Upgrade
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ubuntu/linux-nvidia-tegra-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1070.72~20.04.1 - Upgrade
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ubuntu/linux-oracle-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1116.122~20.04.1 - Upgrade
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ubuntu/linux-riscv-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1113.117~20.04.2 - Upgrade
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ubuntu/linuxto a version that resolves this vulnerability.Fixed in 5.15.0-202.212 - Upgrade
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ubuntu/linux-awsto a version that resolves this vulnerability.Fixed in 5.15.0-1119.126 - Upgrade
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ubuntu/linux-azureto a version that resolves this vulnerability.Fixed in 5.15.0-1125.134 - Upgrade
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ubuntu/linux-gcpto a version that resolves this vulnerability.Fixed in 5.15.0-1120.130 - Upgrade
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ubuntu/linux-kvmto a version that resolves this vulnerability.Fixed in 5.15.0-1111.116 - Upgrade
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ubuntu/linux-lowlatencyto a version that resolves this vulnerability.Fixed in 5.15.0-197.207 - Upgrade
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ubuntu/linux-nvidiato a version that resolves this vulnerability.Fixed in 5.15.0-1114.115 - Upgrade
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ubuntu/linux-aws-fipsto a version that resolves this vulnerability.Fixed in 5.15.0-1119.126+fips1 - Upgrade
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ubuntu/linux-azure-fipsto a version that resolves this vulnerability.Fixed in 5.15.0-1125.134+fips1 - Upgrade
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ubuntu/linux-fipsto a version that resolves this vulnerability.Fixed in 5.15.0-202.212+fips1 - 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-gcpto a version that resolves this vulnerability.Fixed in 7.0.0-1016.18 - Upgrade
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ubuntu/linux-ibmto a version that resolves this vulnerability.Fixed in 7.0.0-1017.17 - Upgrade
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ubuntu/linux-oem-7.0to a version that resolves this vulnerability.Fixed in 7.0.0-1017.17 - Upgrade
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ubuntu/linux-oracleto a version that resolves this vulnerability.Fixed in 7.0.0-1015.16 - Upgrade
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ubuntu/linux-riscvto a version that resolves this vulnerability.Fixed in 7.0.0-42.42.1 - Configuration
Declare a single clamped mtu, constrain it to [256, FQ_CODEL_QUANTUM_MAX] as in fq_codel_change(), and assign the clamped value to both q->quantum and q->cparams.mtu.
Linux kernel fq_codel quantum and q->cparams.mtu = clamped to [256, FQ_CODEL_QUANTUM_MAX]
Event History
Frequently Asked Questions
What access does an attacker need to trigger this issue?
The attacker needs CAP_NET_ADMIN in a user namespace. They must be able to configure a network device with an extremely large MTU such that the MTU plus hard-header length causes psched_mtu() to enter the signed range.
Which configurations are realistically exposed?
The issue can be reproduced with a device that accepts an enormous MTU, such as a dummy device with max_mtu set to 0 and an MTU of 2147483634. Normal MTU configurations do not meet the described trigger condition.
What is the impact after triggering the vulnerable configuration?
The fq_codel deficit can overflow to INT_MIN during dequeue, causing an infinite loop and soft lockup. The same oversized MTU can also effectively disable CoDel dropping and ECN because the default 32 MiB memory limit remains below the configured MTU value.
What can be done before a patched kernel is available?
Prevent untrusted users or workloads from obtaining CAP_NET_ADMIN in user namespaces, and restrict creation or configuration of network devices with extremely large MTUs. Avoid assigning MTU values that could cause psched_mtu() to wrap into the sign bit.