Re: [PATCH] tcp: incoming connections might use wrong route under synflood

[Dmitry Popov <dp@highloadlab.com>]

There is a bug in cookie_v4_check (net/ipv4/syncookies.c):
	flowi4_init_output(&fl4, 0, sk->sk_mark, RT_CONN_FLAGS(sk),
			   RT_SCOPE_UNIVERSE, IPPROTO_TCP,
			   inet_sk_flowi_flags(sk),
			   (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
			   ireq->loc_addr, th->source, th->dest);

Here we do not respect sk->sk_bound_dev_if, therefore wrong dst_entry may be
taken. This dst_entry is used by new socket (get_cookie_sock -> 
tcp_v4_syn_recv_sock), so its packets may take the wrong path.

Signed-off-by: Dmitry Popov <dp@highloadlab.com>
---
 net/ipv4/syncookies.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
index ef54377..397e0f6 100644
--- a/net/ipv4/syncookies.c
+++ b/net/ipv4/syncookies.c
@@ -349,8 +349,8 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 	 * hasn't changed since we received the original syn, but I see
 	 * no easy way to do this.
 	 */
-	flowi4_init_output(&fl4, 0, sk->sk_mark, RT_CONN_FLAGS(sk),
-			   RT_SCOPE_UNIVERSE, IPPROTO_TCP,
+	flowi4_init_output(&fl4, sk->sk_bound_dev_if, sk->sk_mark,
+			   RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
 			   inet_sk_flowi_flags(sk),
 			   (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
 			   ireq->loc_addr, th->source, th->dest);

On Wed, 10 Apr 2013 23:26:12 -0400 (EDT)
David Miller <davem@davemloft.net> wrote:

> From: Dmitry Popov <dp@highloadlab.com>
> Date: Thu, 11 Apr 2013 00:09:09 +0400
> 
> > There is a bug in cookie_v4_check (net/ipv4/syncookies.c):
> > 	flowi4_init_output(&fl4, 0, sk->sk_mark, RT_CONN_FLAGS(sk),
> > 			   RT_SCOPE_UNIVERSE, IPPROTO_TCP,
> > 			   inet_sk_flowi_flags(sk),
> > 			   (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
> > 			   ireq->loc_addr, th->source, th->dest);
> > 
> > Here we do not respect sk->sk_bound_dev_if, therefore wrong dst_entry may be taken. This dst_entry is used in new socket (get_cookie_sock -> tcp_v4_syn_recv_sock), so its packets may take wrong path. There is no such bug in ipv6 code and non-cookie code (usual case). Bugfix below.
> > 
> > Signed-off-by: Dmitry Popov <dp@highloadlab.com>
> 
> Please format your commit messages properly, by not allowing lines of
> text longer than 80 columns.
> 
> Thank you.


-- 
Dmitry Popov <dp@highloadlab.com>