tcp: accept bare FIN packets under memory pressure

tcp: accept bare FIN packets under memory pressure

Andrew Oates reported that some macOS hosts could repeatedly
send FIN packets even if the remote peer drops them and
send back DUP ACK RWIN 0 packets.

<quoting Andrew>

 20:27:16.968254 gif0  In  IP macos > victim: Flags [SEW], seq 1950399762, win 65535, options [mss 1460,nop,wscale 6,nop,nop,TS val 501897188 ecr 0,sackOK,eol], length 0
 20:27:16.968339 gif0  Out IP victim > macos: Flags [S.E], seq 2995489058, ack 1950399763, win 1448, options [mss 1460,sackOK,TS val 3829877593 ecr 501897188,nop,wscale 0], length 0
 20:27:16.968833 gif0  In  IP macos > victim: Flags [.], ack 1, win 2058, options [nop,nop,TS val 501897188 ecr 3829877593], length 0
 20:27:16.968885 gif0  In  IP macos > victim: Flags [P.], seq 1:1449, ack 1, win 2058, options [nop,nop,TS val 501897188 ecr 3829877593], length 1448
 20:27:16.968896 gif0  Out IP victim > macos: Flags [.], ack 1449, win 0, options [nop,nop,TS val 3829877593 ecr 501897188], length 0
 20:27:19.454593 gif0  In  IP macos > victim: Flags [F.], seq 1449, ack 1, win 2058, options [nop,nop,TS val 501899674 ecr 3829877593], length 0
 20:27:19.454675 gif0  Out IP victim > macos: Flags [.], ack 1449, win 0, options [nop,nop,TS val 3829880079 ecr 501899674], length 0
 20:27:19.455116 gif0  In  IP macos > victim: Flags [F.], seq 1449, ack 1, win 2058, options [nop,nop,TS val 501899674 ecr 3829880079], length 0

 The retransmits/dup-ACKs then repeat in a tight loop.

</quoting Andrew>

RFC 9293 3.4. Sequence Numbers states :

  Note that when the receive window is zero no segments should be
  acceptable except ACK segments.  Thus, it is be possible for a TCP to
  maintain a zero receive window while transmitting data and receiving
  ACKs.  However, even when the receive window is zero, a TCP must
  process the RST and URG fields of all incoming segments.

Even if we could consider a bare FIN.ACK packet to be an ACK in RFC terms,
the retransmits should use exponential backoff.

Accepting the FIN in linux does not add extra memory costs,
because the FIN flag will simply be merged to the tail skb in
the receive queue, and incoming packet is freed.

Reported-by: Andrew Oates <aoates@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Christoph Paasch <cpaasch@apple.com>
Cc: Vidhi Goel <vidhi_goel@apple.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>