Currently we have two different policies for orphan sockets that repeatedly stall on zero window ACKs. If a socket gets a zero window ACK when it is transmitting data, the RTO is used to probe the window. The socket is aborted after roughly tcp_orphan_retries() retries (as in tcp_write_timeout()). But if the socket was idle when it received the zero window ACK, and later wants to send more data, we use the probe timer to probe the window. If the receiver always returns zero window ACKs, icsk_probes keeps getting reset in tcp_ack() and the orphan socket can stall forever until the system reaches the orphan limit (as commented in tcp_probe_timer()). This opens up a simple attack to create lots of hanging orphan sockets to burn the memory and the CPU, as demonstrated in the recent netdev post "TCP connection will hang in FIN_WAIT1 after closing if zero window is advertised." http://www.spinics.net/lists/netdev/msg296539.html This patch follows the design in RTO-based probe: we abort an orphan socket stalling on zero window when the probe timer reaches both the maximum backoff and the maximum RTO. For example, an 100ms RTT connection will timeout after roughly 153 seconds (0.3 + 0.6 + .... + 76.8) if the receiver keeps the window shut. If the orphan socket passes this check, but the system already has too many orphans (as in tcp_out_of_resources()), we still abort it but we'll also send an RST packet as the connection may still be active. In addition, we change TCP_USER_TIMEOUT to cover (life or dead) sockets stalled on zero-window probes. This changes the semantics of TCP_USER_TIMEOUT slightly because it previously only applies when the socket has pending transmission. Signed-off-by: Yuchung Cheng <ycheng@google.com> Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: Neal Cardwell <ncardwell@google.com> Reported-by: Andrey Dmitrov <andrey.dmitrov@oktetlabs.ru> Signed-off-by: David S. Miller <davem@davemloft.net>