Linux Kernel CVE-2025-40183

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

bpf: Fix metadata_dst leak __bpf_redirect_neigh_v{4,6}

Cilium has a BPF egress gateway feature which forces outgoing K8s Pod traffic to pass through dedicated egress gateways which then SNAT the traffic in order to interact with stable IPs outside the cluster.

The traffic is directed to the gateway via vxlan tunnel in collect md mode. A recent BPF change utilized the bpf_redirect_neigh() helper to forward packets after the arrival and decap on vxlan, which turned out over time that the kmalloc-256 slab usage in kernel was ever-increasing.

The issue was that vxlan allocates the metadata_dst object and attaches it through a fake dst entry to the skb. The latter was never released though given bpf_redirect_neigh() was merely setting the new dst entry via skb_dst_set() without dropping an existing one first.

The Linux kernel is affected wherever the vulnerable bpf_redirect_neigh() metadata_dst handling is present; fixes were backported across multiple stable branches, as shown by the seven git.kernel.org stable commits (e.g., 057764172fcc, 23f3770e1a53, 2e67c2037382, 3fba965a9aac, 7404ce888a45, b6bfe44b6dbb, f36a305d30f). Distribution-packaged kernels are impacted: Ubuntu confirms affected kernels in USN-8126-1 (https://ubuntu.com/security/notices/USN-8126-1) and USN-8125-1 (https://ubuntu.com/security/notices/USN-8125-1). No CPE strings were supplied in the input, so exact version ranges should be taken from each distribution advisory and the stable-tree tags containing the listed commits.