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ipv4: tcp: send zero IPID for RST and ACK sent in SYN-RECV and TIME-WAIT state
tcp uses per-cpu (and per namespace) sockets (net->ipv4.tcp_sk) internally to send some control packets. 1) RST packets, through tcp_v4_send_reset() 2) ACK packets in SYN-RECV and TIME-WAIT state, through tcp_v4_send_ack() These packets assert IP_DF, and also use the hashed IP ident generator to provide an IPv4 ID number. Geoff Alexander reported this could be used to build off-path attacks. These packets should not be fragmented, since their size is smaller than IPV4_MIN_MTU. Only some tunneled paths could eventually have to fragment, regardless of inner IPID. We really can use zero IPID, to address the flaw, and as a bonus, avoid a couple of atomic operations in ip_idents_reserve() Signed-off-by: Eric Dumazet <edumazet@google.com> Reported-by: Geoff Alexander <alexandg@cs.unm.edu> Tested-by: Geoff Alexander <alexandg@cs.unm.edu> Signed-off-by: David S. Miller <davem@davemloft.net>
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@ -2517,6 +2517,12 @@ static int __net_init tcp_sk_init(struct net *net)
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if (res)
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if (res)
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goto fail;
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goto fail;
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sock_set_flag(sk, SOCK_USE_WRITE_QUEUE);
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sock_set_flag(sk, SOCK_USE_WRITE_QUEUE);
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/* Please enforce IP_DF and IPID==0 for RST and
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* ACK sent in SYN-RECV and TIME-WAIT state.
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*/
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inet_sk(sk)->pmtudisc = IP_PMTUDISC_DO;
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*per_cpu_ptr(net->ipv4.tcp_sk, cpu) = sk;
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*per_cpu_ptr(net->ipv4.tcp_sk, cpu) = sk;
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}
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}
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