CVE-2026-64024

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

tcp: fix stale per-CPU tcptwisn 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 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/SOATTACH_FILTER drop
• th->syn && th->fin discard in tcprcvstateprocess() TCPLISTEN
• pspskrxpolicycheck() in tcpv{4,6}do_rcv()
• tcpchecksumcomplete() in tcpv{4,6}do_rcv()
• tcpv{4,6}cookie_check() 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}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

tcpv6rcv                                  3038    3042      +4

tcpv4rcv                                  3035    3039      +4

tcpconnrequest                            2938    2923     -15

Total: Before=24436060, After=24436053, chg -0.00%