Affected: Linux

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

tcp: fix stale per-CPU tcp_tw_isn leak enabling ISN prediction

Blamed commit moved the TIME_WAIT-derived ISN from the skb control block to a per-CPU variable, assuming the value would always be consumed by tcp_conn_request() for the same packet that wrote it. That assumption is violated by multiple drop paths between the producer (__this_cpu_write(tcp_tw_isn, isn) in tcp_v{4,6}_rcv()) and the consumer (tcp_conn_request()):

- min_ttl / min_hopcount check - xfrm policy check - tcp_inbound_hash() MD5/AO mismatch - tcp_filter() eBPF/SO_ATTACH_FILTER drop - th->syn && th->fin discard in tcp_rcv_state_process() TCP_LISTEN - psp_sk_rx_policy_check() in tcp_v{4,6}_do_rcv() - tcp_checksum_complete() in tcp_v{4,6}_do_rcv() - tcp_v{4,6}_cookie_check() returning NULL

When a packet is dropped on any of these paths, tcp_tw_isn is left set.

The next SYN processed on the same CPU then consumes the non zero value in tcp_conn_request(), receiving a potentially predictable ISN.

This patch moves back tcp_tw_isn to skb->cb[], getting rid of the per-cpu variable.

Note that tcp_v{4,6}_fill_cb() do not set it.

Very litle impact on overall code size/complexity:

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new add/remove: 0/0 grow/shrink: 2/1 up/down: 8/-15 (-7) Function old new delta tcp_v6_rcv 3038 3042 +4 tcp_v4_rcv…