pptp: fix illegal memory access caused by multiple bind()s

Several times already this has been reported as kasan reports caused by
syzkaller and trinity and people always looked at RCU races, but it is
much more simple. :)

In case we bind a pptp socket multiple times, we simply add it to
the callid_sock list but don't remove the old binding. Thus the old
socket stays in the bucket with unused call_id indexes and doesn't get
cleaned up. This causes various forms of kasan reports which were hard
to pinpoint.

Simply don't allow multiple binds and correct error handling in
pptp_bind. Also keep sk_state bits in place in pptp_connect.

Fixes: 00959ade ("PPTP: PPP over IPv4 (Point-to-Point Tunneling Protocol)")
Cc: Dmitry Kozlov <xeb@mail.ru>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Dave Jones <davej@codemonkey.org.uk>
Reported-by: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ppp/pptp.c

@@ -129,24 +129,27 @@ static int lookup_chan_dst(u16 call_id, __be32 d_addr)
 	return i < MAX_CALLID;
 }
 
-static int add_chan(struct pppox_sock *sock)
+static int add_chan(struct pppox_sock *sock,
+		    struct pptp_addr *sa)
 {
 	static int call_id;
 
 	spin_lock(&chan_lock);
-	if (!sock->proto.pptp.src_addr.call_id)	{
+	if (!sa->call_id)	{
 		call_id = find_next_zero_bit(callid_bitmap, MAX_CALLID, call_id + 1);
 		if (call_id == MAX_CALLID) {
 			call_id = find_next_zero_bit(callid_bitmap, MAX_CALLID, 1);
 			if (call_id == MAX_CALLID)
 				goto out_err;
 		}
-		sock->proto.pptp.src_addr.call_id = call_id;
-	} else if (test_bit(sock->proto.pptp.src_addr.call_id, callid_bitmap))
+		sa->call_id = call_id;
+	} else if (test_bit(sa->call_id, callid_bitmap)) {
 		goto out_err;
+	}
 
-	set_bit(sock->proto.pptp.src_addr.call_id, callid_bitmap);
-	rcu_assign_pointer(callid_sock[sock->proto.pptp.src_addr.call_id], sock);
+	sock->proto.pptp.src_addr = *sa;
+	set_bit(sa->call_id, callid_bitmap);
+	rcu_assign_pointer(callid_sock[sa->call_id], sock);
 	spin_unlock(&chan_lock);
 
 	return 0;
@@ -416,7 +419,6 @@ static int pptp_bind(struct socket *sock, struct sockaddr *uservaddr,
 	struct sock *sk = sock->sk;
 	struct sockaddr_pppox *sp = (struct sockaddr_pppox *) uservaddr;
 	struct pppox_sock *po = pppox_sk(sk);
-	struct pptp_opt *opt = &po->proto.pptp;
 	int error = 0;
 
 	if (sockaddr_len < sizeof(struct sockaddr_pppox))
@@ -424,10 +426,22 @@ static int pptp_bind(struct socket *sock, struct sockaddr *uservaddr,
 
 	lock_sock(sk);
 
-	opt->src_addr = sp->sa_addr.pptp;
-	if (add_chan(po))
+	if (sk->sk_state & PPPOX_DEAD) {
+		error = -EALREADY;
+		goto out;
+	}
+
+	if (sk->sk_state & PPPOX_BOUND) {
 		error = -EBUSY;
+		goto out;
+	}
+
+	if (add_chan(po, &sp->sa_addr.pptp))
+		error = -EBUSY;
+	else
+		sk->sk_state |= PPPOX_BOUND;
 
+out:
 	release_sock(sk);
 	return error;
 }
@@ -498,7 +512,7 @@ static int pptp_connect(struct socket *sock, struct sockaddr *uservaddr,
 	}
 
 	opt->dst_addr = sp->sa_addr.pptp;
-	sk->sk_state = PPPOX_CONNECTED;
+	sk->sk_state |= PPPOX_CONNECTED;
 
  end:
 	release_sock(sk);