mptcp: avoid OOB access in setsockopt()

We can't use tcp_set_congestion_control() on an mptcp socket, as
such function can end-up accessing a tcp-specific field -
prior_ssthresh - causing an OOB access.

To allow propagating the correct ca algo on subflow, cache the ca
name at initialization time.

Additionally avoid overriding the user-selected CA (if any) at
clone time.

Closes: #182
Fixes: aa1fbd9 ("mptcp: sockopt: add TCP_CONGESTION and TCP_INFO")
Acked-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/mptcp/protocol.c

@@ -2418,13 +2418,12 @@ static int __mptcp_init_sock(struct sock *sk)
 	timer_setup(&msk->sk.icsk_retransmit_timer, mptcp_retransmit_timer, 0);
 	timer_setup(&sk->sk_timer, mptcp_timeout_timer, 0);
 
-	tcp_assign_congestion_control(sk);
-
 	return 0;
 }
 
 static int mptcp_init_sock(struct sock *sk)
 {
+	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct net *net = sock_net(sk);
 	int ret;
 
@@ -2442,6 +2441,16 @@ static int mptcp_init_sock(struct sock *sk)
 	if (ret)
 		return ret;
 
+	/* fetch the ca name; do it outside __mptcp_init_sock(), so that clone will
+	 * propagate the correct value
+	 */
+	tcp_assign_congestion_control(sk);
+	strcpy(mptcp_sk(sk)->ca_name, icsk->icsk_ca_ops->name);
+
+	/* no need to keep a reference to the ops, the name will suffice */
+	tcp_cleanup_congestion_control(sk);
+	icsk->icsk_ca_ops = NULL;
+
 	sk_sockets_allocated_inc(sk);
 	sk->sk_rcvbuf = sock_net(sk)->ipv4.sysctl_tcp_rmem[1];
 	sk->sk_sndbuf = sock_net(sk)->ipv4.sysctl_tcp_wmem[1];
@@ -2616,7 +2625,6 @@ static void __mptcp_destroy_sock(struct sock *sk)
 	sk_stream_kill_queues(sk);
 	xfrm_sk_free_policy(sk);
 
-	tcp_cleanup_congestion_control(sk);
 	sk_refcnt_debug_release(sk);
 	mptcp_dispose_initial_subflow(msk);
 	sock_put(sk);

net/mptcp/protocol.h

@@ -258,6 +258,7 @@ struct mptcp_sock {
 	} rcvq_space;
 
 	u32 setsockopt_seq;
+	char		ca_name[TCP_CA_NAME_MAX];
 };
 
 #define mptcp_lock_sock(___sk, cb) do {					\

net/mptcp/sockopt.c

@@ -547,7 +547,7 @@ static int mptcp_setsockopt_sol_tcp_congestion(struct mptcp_sock *msk, sockptr_t
 	}
 
 	if (ret == 0)
-		tcp_set_congestion_control(sk, name, false, cap_net_admin);
+		strcpy(msk->ca_name, name);
 
 	release_sock(sk);
 	return ret;
@@ -705,7 +705,7 @@ static void sync_socket_options(struct mptcp_sock *msk, struct sock *ssk)
 	sock_valbool_flag(ssk, SOCK_DBG, sock_flag(sk, SOCK_DBG));
 
 	if (inet_csk(sk)->icsk_ca_ops != inet_csk(ssk)->icsk_ca_ops)
-		tcp_set_congestion_control(ssk, inet_csk(sk)->icsk_ca_ops->name, false, true);
+		tcp_set_congestion_control(ssk, msk->ca_name, false, true);
 }
 
 static void __mptcp_sockopt_sync(struct mptcp_sock *msk, struct sock *ssk)