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)