Linux kernel (net/tls): When a BPF socket policy shrinks the plaintext after the ciphertext length was computed, kTLS

Impact

When a BPF socket policy shrinks the plaintext after the ciphertext length was computed, kTLS encrypts and transmits the stale tail - uninitialized kernel memory is appended to a complete Application Data record and sent to the peer. Kernel heap bytes leave the node on the wire, and the receiver sees a malformed record.

Who can reach it

Requires an sk_msg/BPF policy calling bpf_msg_pop_data() attached to a kTLS TX socket - the shape of a service mesh or CNI sidecar doing L7 policy over encrypted traffic. The party who receives the leak is whoever is on the far end of the connection, which for a tenant-facing proxy can be the tenant itself. Attaching the policy needs CAP_BPF; reading the leaked bytes needs nothing.

What to do

Boot a kernel carrying the linked stable commits. Interim: stop running sk_msg programs that shorten payloads (bpf_msg_pop_data) over kTLS sockets, or terminate TLS in userspace on affected nodes.

References