Merge branch 'tcp-fix-isn-selection-in-timewait-syn_recv'

Eric Dumazet says:

====================
tcp: fix ISN selection in TIMEWAIT -> SYN_RECV

TCP can transform a TIMEWAIT socket into a SYN_RECV one from
a SYN packet, and the ISN of the SYNACK packet is normally
generated using TIMEWAIT tw_snd_nxt.

This SYN packet also bypasses normal checks against listen queue
being full or not.

Unfortunately this has been broken almost one decade ago.

This series fixes the issue, in two patches.

First patch refactors code to add tcp_tw_isn as a parameter
to ->route_req(), to make the second patch smaller.

Second patch fixes the issue, by no longer using TCP_SKB_CB(skb)
to store the tcp_tw_isn.

Following packetdrill test passes after this series:

// Set up a server listening socket.
    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
   +0 bind(3, ..., ...) = 0
   +0 listen(3, 1) = 0

// Establish connection
   +0 < S 0:0(0) win 32792 <mss 1460,nop,nop,sackOK>
   +0 > S. 0:0(0) ack 1    <mss 1460,nop,nop,sackOK>
 +.01 < . 1:1(0) ack 1 win 32792

   +0 accept(3, ..., ...) = 4

// We close(), send a FIN, and get an ACK and FIN, in order to get into TIME_WAIT.

 +.01 close(4) = 0
   +0 > F. 1:1(0) ack 1
 +.01 < F. 1:1(0) ack 2 win 32792
   +0 > . 2:2(0) ack 2

// SYN hitting a TIME_WAIT -> should use an ISN based on TIMEWAIT tw_snd_nxt

 +.01 < S 1000:1000(0) win 65535 <mss 1460,nop,nop,sackOK>
   +0 > S. 65539:65539(0) ack 1001 <mss 1460,nop,nop,sackOK>
====================

Link: https://lore.kernel.org/r/20240407093322.3172088-1-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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