UBUNTU-CVE-2026-63836: Medium severity ubuntu/linux-azure vulnerability
In the Linux kernel, the following vulnerability has been resolved: batman-adv: tpmeter: avoid divide-by-zero for deccwnd The cwnd is always MSS <= cwnd <= 0x20000000. But the calculation in batadvtpupdatecwnd() assumes unsigned 32 bit arithmetics. ((mss 8) 2) / (cwnd 8) In case cwnd is actually 0x20000000, it will be shifted by 3 bit to the left end up at 0x100000000 or U32MAX + 1. It will therefore wrap around and be 0 - resulting in: ((mss 8) 2) / 0 This is of course invalid and cannot be calculated. The calculation should must be simplified to avoid this overflow: (mss 2) 8 / cwnd It will keep the precision enhancement from the scaling (by 8) but avoid the overflow in the divisor. In theory, there could still be an overflow in the dividend. It is at the moment fixed to BATADVTPPLEN in batadvtprecvack() - so it is not an imminent problem. But allowing it to use the whole u32 bit range, would mean that it can still use up to 67 bits. To keep this calculation safe for 32 bit arithmetic, mss must never use more than floor((32 - 3) / 2) bits - or in other words: must never be larger than 16383.
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 5.15.0-1117.124~20.04.1 - Upgrade
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ubuntu/linux-azure-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1123.132~20.04.1 - Upgrade
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ubuntu/linux-azure-fde-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1122.131~20.04.1 - Upgrade
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ubuntu/linux-gcp-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1118.128~20.04.1 - Upgrade
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ubuntu/linux-hwe-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-198.208~20.04.1 - Upgrade
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ubuntu/linux-ibm-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1111.115~20.04.1 - Upgrade
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ubuntu/linux-intel-iotg-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1112.118~20.04.1 - Upgrade
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ubuntu/linux-lowlatency-hwe-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-195.205~20.04.1 - Upgrade
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ubuntu/linux-oracle-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-riscv-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1111.115~20.04.1 - Upgrade
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ubuntu/linuxto a version that resolves this vulnerability.Fixed in 5.15.0-198.208 - Upgrade
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ubuntu/linux-awsto a version that resolves this vulnerability.Fixed in 5.15.0-1117.124 - Upgrade
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ubuntu/linux-azureto a version that resolves this vulnerability.Fixed in 5.15.0-1123.132 - Upgrade
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ubuntu/linux-azure-fdeto a version that resolves this vulnerability.Fixed in 5.15.0-1122.131 - Upgrade
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ubuntu/linux-gcpto a version that resolves this vulnerability.Fixed in 5.15.0-1118.128 - Upgrade
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ubuntu/linux-gketo a version that resolves this vulnerability.Fixed in 5.15.0-1113.119 - Upgrade
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ubuntu/linux-gkeopto a version that resolves this vulnerability.Fixed in 5.15.0-1100.108 - Upgrade
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ubuntu/linux-ibmto a version that resolves this vulnerability.Fixed in 5.15.0-1111.115 - Upgrade
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ubuntu/linux-intel-iotgto a version that resolves this vulnerability.Fixed in 5.15.0-1112.118 - Upgrade
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ubuntu/linux-kvmto a version that resolves this vulnerability.Fixed in 5.15.0-1109.114 - Upgrade
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ubuntu/linux-lowlatencyto a version that resolves this vulnerability.Fixed in 5.15.0-195.205 - Upgrade
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ubuntu/linux-nvidiato a version that resolves this vulnerability.Fixed in 5.15.0-1112.113 - Upgrade
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ubuntu/linux-oracleto a version that resolves this vulnerability.Fixed in 5.15.0-1114.120 - Upgrade
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ubuntu/linux-xilinx-zynqmpto a version that resolves this vulnerability.Fixed in 5.15.0-1080.84 - Upgrade
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ubuntu/linux-bluefieldto a version that resolves this vulnerability.Fixed in 5.15.0-1101.103 - Upgrade
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ubuntu/linux-intel-iot-realtimeto a version that resolves this vulnerability.Fixed in 5.15.0-1109.111 - Upgrade
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ubuntu/linux-realtimeto a version that resolves this vulnerability.Fixed in 5.15.0-1116.125 - Upgrade
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ubuntu/linux-aws-fipsto a version that resolves this vulnerability.Fixed in 5.15.0-1117.124+fips1 - Upgrade
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ubuntu/linux-azure-fipsto a version that resolves this vulnerability.Fixed in 5.15.0-1123.132+fips1 - Upgrade
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ubuntu/linux-fipsto a version that resolves this vulnerability.Fixed in 5.15.0-198.208+fips1 - Upgrade
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ubuntu/linux-gcp-fipsto a version that resolves this vulnerability.Fixed in 5.15.0-1118.128+fips1 - Upgrade
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ubuntu/linuxto a version that resolves this vulnerability.Fixed in 6.8.0-147.147 - Upgrade
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ubuntu/linux-awsto a version that resolves this vulnerability.Fixed in 6.8.0-1067.70 - Upgrade
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ubuntu/linux-aws-7.0to a version that resolves this vulnerability.Fixed in 7.0.0-1012.12~24.04.1 - Upgrade
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ubuntu/linux-azureto a version that resolves this vulnerability.Fixed in 6.8.0-1071.79 - Upgrade
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ubuntu/linux-azure-7.0to a version that resolves this vulnerability.Fixed in 7.0.0-1014.14~24.04.1 - Upgrade
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ubuntu/linux-azure-fdeto a version that resolves this vulnerability.Fixed in 6.8.0-1069.76 - Upgrade
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ubuntu/linux-azure-fde-7.0to a version that resolves this vulnerability.Fixed in 7.0.0-1013.13~24.04.1 - Upgrade
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ubuntu/linux-gcpto a version that resolves this vulnerability.Fixed in 6.8.0-1071.79 - Upgrade
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ubuntu/linux-gcp-7.0to a version that resolves this vulnerability.Fixed in 7.0.0-1011.11~24.04.1 - Upgrade
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ubuntu/linux-gketo a version that resolves this vulnerability.Fixed in 6.8.0-1066.74 - Upgrade
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ubuntu/linux-gkeopto a version that resolves this vulnerability.Fixed in 6.8.0-1052.56 - Upgrade
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ubuntu/linux-hwe-7.0to a version that resolves this vulnerability.Fixed in 7.0.0-31.31~24.04.1 - Upgrade
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ubuntu/linux-ibmto a version that resolves this vulnerability.Fixed in 6.8.0-1067.68 - Upgrade
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ubuntu/linux-lowlatencyto a version that resolves this vulnerability.Fixed in 6.8.0-147.147.1 - Upgrade
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ubuntu/linux-nvidia-7.0to a version that resolves this vulnerability.Fixed in 7.0.0-1018.18~24.04.1 - Upgrade
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ubuntu/linux-oracleto a version that resolves this vulnerability.Fixed in 6.8.0-1064.67 - Upgrade
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ubuntu/linux-oracle-7.0to a version that resolves this vulnerability.Fixed in 7.0.0-1011.11~24.04.1 - Upgrade
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ubuntu/linux-riscv-7.0to a version that resolves this vulnerability.Fixed in 7.0.0-31.31.1~24.04.1 - Upgrade
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ubuntu/linux-bluefieldto a version that resolves this vulnerability.Fixed in 6.8.0-1033.38 - Upgrade
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ubuntu/linux-realtimeto a version that resolves this vulnerability.Fixed in 6.8.1-1062.63 - Upgrade
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ubuntu/linux-aws-fipsto a version that resolves this vulnerability.Fixed in 6.8.0-1067.70+fips1 - Upgrade
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ubuntu/linux-azure-fipsto a version that resolves this vulnerability.Fixed in 6.8.0-1071.79+fips1 - Upgrade
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ubuntu/linux-fipsto a version that resolves this vulnerability.Fixed in 6.8.0-147.147+fips1 - Upgrade
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ubuntu/linux-gcp-fipsto a version that resolves this vulnerability.Fixed in 6.8.0-1071.79+fips1 - Upgrade
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ubuntu/linuxto a version that resolves this vulnerability.Fixed in 7.0.0-31.31 - Upgrade
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ubuntu/linux-awsto a version that resolves this vulnerability.Fixed in 7.0.0-1012.12 - Upgrade
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ubuntu/linux-azureto a version that resolves this vulnerability.Fixed in 7.0.0-1014.14 - Upgrade
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ubuntu/linux-azure-fdeto a version that resolves this vulnerability.Fixed in 7.0.0-1013.13 - Upgrade
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ubuntu/linux-gcpto a version that resolves this vulnerability.Fixed in 7.0.0-1011.11 - Upgrade
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ubuntu/linux-gketo a version that resolves this vulnerability.Fixed in 7.0.0-1006.7 - Upgrade
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ubuntu/linux-ibmto a version that resolves this vulnerability.Fixed in 7.0.0-1013.13 - Upgrade
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ubuntu/linux-nvidiato a version that resolves this vulnerability.Fixed in 7.0.0-1018.18 - Upgrade
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ubuntu/linux-nvidia-bosto a version that resolves this vulnerability.Fixed in 7.0.0-2018.18 - Upgrade
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ubuntu/linux-oem-7.0to a version that resolves this vulnerability.Fixed in 7.0.0-1013.13 - Upgrade
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ubuntu/linux-oracleto a version that resolves this vulnerability.Fixed in 7.0.0-1011.11 - Upgrade
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ubuntu/linux-raspito a version that resolves this vulnerability.Fixed in 7.0.0-1019.19 - Upgrade
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ubuntu/linux-realtimeto a version that resolves this vulnerability.Fixed in 7.0.0-31.31.1 - Upgrade
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ubuntu/linux-riscvto a version that resolves this vulnerability.Fixed in 7.0.0-31.31.1 - Compensating control
In batman-adv tp_meter, change the dec_cwnd calculation to `(mss ** 2) * 8 / cwnd` instead of `((mss * 8) ** 2) / (cwnd * 8)`, and ensure mss never exceeds 16383 to keep the calculation safe for 32-bit arithmetic.
Event History
Frequently Asked Questions
What conditions are needed to trigger the fault?
The congestion window must reach 0x20000000. Shifting that value left by three in the affected calculation wraps a 32-bit unsigned divisor to zero, causing a divide-by-zero operation.
Which Ubuntu kernel packages are identified as affected?
The listed packages are ubuntu/linux, ubuntu/linux-azure, ubuntu/linux-azure-fips, ubuntu/linux-aws-fips, ubuntu/linux-fips, ubuntu/linux-gcp, ubuntu/linux-gcp-fips, and ubuntu/linux-oracle.
Is there a related overflow concern after the divide-by-zero fix?
The revised calculation could theoretically overflow its dividend if MSS were allowed to span the full 32-bit range. The data states that MSS is currently fixed to BATADV_TP_PLEN in batadv_tp_recv_ack(), so this is not considered an imminent problem; safe 32-bit arithmetic requires MSS to be no larger than 16383.