[PATCH] net: Update window_clamp if SOCK_RCVBUF is set

[Mao Wenan]

When net.ipv4.tcp_syncookies=1 and syn flood is happened,
cookie_v4_check tries to redo what tcp_v4_send_synack did,
rsk_window_clamp will be changed if SOCK_RCVBUF is set
by user, which will make rcv_wscale is different, the client
still operates with initial window scale and can overshot
granted window, the client use the initial scale but local
server use new scale to advertise window value, and session
work abnormally.

Signed-off-by: Mao Wenan <wenan.mao@xxxxxxxxxxxxxxxxx>
---
 net/ipv4/syncookies.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
index 6ac473b..57ce317 100644
--- a/net/ipv4/syncookies.c
+++ b/net/ipv4/syncookies.c
@@ -427,6 +427,10 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
 
 	/* Try to redo what tcp_v4_send_synack did. */
 	req->rsk_window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
+	/* limit the window selection if the user enforce a smaller rx buffer */
+	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
+	    (req->rsk_window_clamp > tcp_full_space(sk) || req->rsk_window_clamp == 0))
+		req->rsk_window_clamp = tcp_full_space(sk);
 
 	tcp_select_initial_window(sk, tcp_full_space(sk), req->mss,
 				  &req->rsk_rcv_wnd, &req->rsk_window_clamp,
-- 
1.8.3.1