CVE-2025-37799 — vmxnet3: Fix malformed packet sizing in vmxnet3_process_xdp

Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

vmxnet3: Fix malformed packet sizing in vmxnet3_process_xdp

vmxnet3 driver's XDP handling is buggy for packet sizes using ring0 (that is, packet sizes between 128 - 3k bytes).

We noticed MTU-related connectivity issues with Cilium's service load- balancing in case of vmxnet3 as NIC underneath. A simple curl to a HTTP backend service where the XDP LB was doing IPIP encap led to overly large packet sizes but only for *some* of the packets (e.g. HTTP GET request) while others (e.g. the prior TCP 3WHS) looked completely fine on the wire.

In fact, the pcap recording on the backend node actually revealed that the node with the XDP LB was leaking uninitialized kernel data onto the wire for the affected packets, for example, while the packets should have been 152 bytes their actual size was 1482 bytes, so the remainder after 152 bytes was padded with whatever other data was in that page at the time (e.g. we saw user/payload data from prior processed packets).

We only noticed this through an MTU issue, e.g. when the XDP LB node and the backend node both had the same MTU (e.g. 1500) then the curl request got dropped on the backend node's NIC given the packet was too large even though the IPIP-encapped packet normally would never even come close to the MTU limit. Lowering the MTU on the XDP LB (e.g. 1480) allowed to let the curl request succeed (which also indicates that the kernel ignored the padding, and thus th…

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CVE-2025-37799 — vmxnet3: Fix malformed packet sizing in vmxnet3_process_xdp