tcp: better validation of received ack sequences
Paul Fiterau Brostean reported : <quote> Linux TCP stack we analyze exhibits behavior that seems odd to me. The scenario is as follows (all packets have empty payloads, no window scaling, rcv/snd window size should not be a factor): TEST HARNESS (CLIENT) LINUX SERVER 1. - LISTEN (server listen, then accepts) 2. - --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED 3. - <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED 4. - --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED 5. - <-- <SEQ=301><ACK=101><CTL=FIN,ACK> <-- FIN WAIT-1 (server opts to close the data connection calling "close" on the connection socket) 6. - --> <SEQ=101><ACK=99999><CTL=FIN,ACK> --> CLOSING (client sends FIN,ACK with not yet sent acknowledgement number) 7. - <-- <SEQ=302><ACK=102><CTL=ACK> <-- CLOSING (ACK is 102 instead of 101, why?) ... (silence from CLIENT) 8. - <-- <SEQ=301><ACK=102><CTL=FIN,ACK> <-- CLOSING (retransmission, again ACK is 102) Now, note that packet 6 while having the expected sequence number, acknowledges something that wasn't sent by the server. So I would expect the packet to maybe prompt an ACK response from the server, and then be ignored. Yet it is not ignored and actually leads to an increase of the acknowledgement number in the server's retransmission of the FIN,ACK packet. The explanation I found is that the FIN in packet 6 was processed, despite the acknowledgement number being unacceptable. Further experiments indeed show that the server processes this FIN, transitioning to CLOSING, then on receiving an ACK for the FIN it had send in packet 5, the server (or better said connection) transitions from CLOSING to TIME_WAIT (as signaled by netstat). </quote> Indeed, tcp_rcv_state_process() calls tcp_ack() but does not exploit the @acceptable status but for TCP_SYN_RECV state. What we want here is to send a challenge ACK, if not in TCP_SYN_RECV state. TCP_FIN_WAIT1 state is not the only state we should fix. Add a FLAG_NO_CHALLENGE_ACK so that tcp_rcv_state_process() can choose to send a challenge ACK and discard the packet instead of wrongly change socket state. With help from Neal Cardwell. Signed-off-by:Eric Dumazet <edumazet@google.com> Reported-by:
Paul Fiterau Brostean <p.fiterau-brostean@science.ru.nl> Cc: Neal Cardwell <ncardwell@google.com> Cc: Yuchung Cheng <ycheng@google.com> Cc: Soheil Hassas Yeganeh <soheil@google.com> Signed-off-by:
David S. Miller <davem@davemloft.net>
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