CVE-2026-64024: tcp: fix stale per-CPU tcp_tw_isn leak enabling ISN prediction

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

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

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

- minttl / minhopcount check - xfrm policy check - tcpinboundhash() MD5/AO mismatch - tcpfilter() eBPF/SOATTACHFILTER drop - th->syn && th->fin discard in tcprcvstateprocess() TCPLISTEN - pspskrxpolicycheck() in tcpv{4,6}dorcv() - tcpchecksumcomplete() in tcpv{4,6}dorcv() - tcpv{4,6}cookiecheck() returning NULL

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

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

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

Note that tcpv{4,6}fillcb() 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 tcpv6rcv 3038 3042 +4 tcpv4rcv 3035 3039 +4 tcpconnrequest 2938 2923 -15 Total: Before=24436060, After=24436053, chg -0.00%