[CVE-2017-14106][T/X/Z SRU][PATCH] tcp: initialize rcv_mss to TCP_MIN_MSS instead of 0

[Po-Hsu Lin]

From: Wei Wang <weiwan at google.com>

CVE-2017-14106

When tcp_disconnect() is called, inet_csk_delack_init() sets
icsk->icsk_ack.rcv_mss to 0.
This could potentially cause tcp_recvmsg() => tcp_cleanup_rbuf() =>
__tcp_select_window() call path to have division by 0 issue.
So this patch initializes rcv_mss to TCP_MIN_MSS instead of 0.

Reported-by: Andrey Konovalov  <andreyknvl at google.com>
Signed-off-by: Wei Wang <weiwan at google.com>
Signed-off-by: Eric Dumazet <edumazet at google.com>
Signed-off-by: Neal Cardwell <ncardwell at google.com>
Signed-off-by: Yuchung Cheng <ycheng at google.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit 499350a5a6e7512d9ed369ed63a4244b6536f4f8)
Signed-off-by: Po-Hsu Lin <po-hsu.lin at canonical.com>
---
 net/ipv4/tcp.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 16b5118..46c45a0 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2202,6 +2202,10 @@ int tcp_disconnect(struct sock *sk, int flags)
 	tcp_set_ca_state(sk, TCP_CA_Open);
 	tcp_clear_retrans(tp);
 	inet_csk_delack_init(sk);
+	/* Initialize rcv_mss to TCP_MIN_MSS to avoid division by 0
+	 * issue in __tcp_select_window()
+	 */
+	icsk->icsk_ack.rcv_mss = TCP_MIN_MSS;
 	tcp_init_send_head(sk);
 	memset(&tp->rx_opt, 0, sizeof(tp->rx_opt));
 	__sk_dst_reset(sk);
-- 
2.7.4