From 431280eebed9f5079553daf003011097763e71fd Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 22 Aug 2018 13:30:45 -0700 Subject: 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 Reported-by: Geoff Alexander Tested-by: Geoff Alexander Signed-off-by: David S. Miller --- net/ipv4/tcp_ipv4.c | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'net/ipv4/tcp_ipv4.c') diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c index 9e041fa5c545..44c09eddbb78 100644 --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -2517,6 +2517,12 @@ static int __net_init tcp_sk_init(struct net *net) if (res) goto fail; sock_set_flag(sk, SOCK_USE_WRITE_QUEUE); + + /* Please enforce IP_DF and IPID==0 for RST and + * ACK sent in SYN-RECV and TIME-WAIT state. + */ + inet_sk(sk)->pmtudisc = IP_PMTUDISC_DO; + *per_cpu_ptr(net->ipv4.tcp_sk, cpu) = sk; } -- cgit v1.2.3-59-g8ed1b