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 <eduma...@google.com>
Reported-by: Geoff Alexander <alexa...@cs.unm.edu>
Tested-by: Geoff Alexander <alexa...@cs.unm.edu>
---
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
9e041fa5c545367961f03fa8a9124aebbc1b6c69..44c09eddbb781c03da2417aaa925e360de01a6e9
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;
}
--
2.18.0.1017.ga543ac7ca45-goog
