Subject: [PATCH 02/29] ipv4: tcp: send zero IPID for RST and ACK sent in SYN-RECV and TIME-WAIT state

From: Eric Dumazet <[email-masked]>
Date: Wed, 22 Aug 2018 13:30:45 -0700

[ Upstream commit 431280eebed9f5079553daf003011097763e71fd ]

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 <[email-masked]>
Reported-by: Geoff Alexander <[email-masked]>
Tested-by: Geoff Alexander <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/ipv4/tcp_ipv4.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index ef8cd0f7db89..7ac7dc54b800 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2498,6 +2498,12 @@ 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;
 	}
 
-- 
2.13.6