udp: must lock the socket in udp_disconnect()

Baozeng Ding reported KASAN traces showing uses after free in
udp_lib_get_port() and other related UDP functions.

A CONFIG_DEBUG_PAGEALLOC=y kernel would eventually crash.

I could write a reproducer with two threads doing :

static int sock_fd;
static void *thr1(void *arg)
{
for (;;) {
connect(sock_fd, (const struct sockaddr *)arg,
sizeof(struct sockaddr_in));
}
}

static void *thr2(void *arg)
{
struct sockaddr_in unspec;

for (;;) {
memset(&unspec, 0, sizeof(unspec));
connect(sock_fd, (const struct sockaddr *)&unspec,
sizeof(unspec));
}
}

Problem is that udp_disconnect() could run without holding socket lock,
and this was causing list corruptions.

Signed-off-by: Eric Dumazet <[e-mail]>
Reported-by: Baozeng Ding <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/udp.h

···
 void udp4_hwcsum(struct sk_buff *skb, __be32 src, __be32 dst);
 int udp_rcv(struct sk_buff *skb);
 int udp_ioctl(struct sock *sk, int cmd, unsigned long arg);
+int __udp_disconnect(struct sock *sk, int flags);
 int udp_disconnect(struct sock *sk, int flags);
 unsigned int udp_poll(struct file *file, struct socket *sock, poll_table *wait);
 struct sk_buff *skb_udp_tunnel_segment(struct sk_buff *skb,

net/ipv4/ping.c

···
 	.init =		ping_init_sock,
 	.close =	ping_close,
 	.connect =	ip4_datagram_connect,
-	.disconnect =	udp_disconnect,
+	.disconnect =	__udp_disconnect,
 	.setsockopt =	ip_setsockopt,
 	.getsockopt =	ip_getsockopt,
 	.sendmsg =	ping_v4_sendmsg,

net/ipv4/raw.c

···
 	.close		   = raw_close,
 	.destroy	   = raw_destroy,
 	.connect	   = ip4_datagram_connect,
-	.disconnect	   = udp_disconnect,
+	.disconnect	   = __udp_disconnect,
 	.ioctl		   = raw_ioctl,
 	.init		   = raw_init,
 	.setsockopt	   = raw_setsockopt,

net/ipv4/udp.c

···
 	goto try_again;
 }
 
-int udp_disconnect(struct sock *sk, int flags)
+int __udp_disconnect(struct sock *sk, int flags)
 {
 	struct inet_sock *inet = inet_sk(sk);
 	/*
···
 		inet->inet_sport = 0;
 	}
 	sk_dst_reset(sk);
+	return 0;
+}
+EXPORT_SYMBOL(__udp_disconnect);
+
+int udp_disconnect(struct sock *sk, int flags)
+{
+	lock_sock(sk);
+	__udp_disconnect(sk, flags);
+	release_sock(sk);
 	return 0;
 }
 EXPORT_SYMBOL(udp_disconnect);
···
 
 	sk->sk_err = err;
 	sk->sk_error_report(sk);
-	udp_disconnect(sk, 0);
+	__udp_disconnect(sk, 0);
 
 	release_sock(sk);
 

net/ipv6/ping.c

···
 	.init =		ping_init_sock,
 	.close =	ping_close,
 	.connect =	ip6_datagram_connect_v6_only,
-	.disconnect =	udp_disconnect,
+	.disconnect =	__udp_disconnect,
 	.setsockopt =	ipv6_setsockopt,
 	.getsockopt =	ipv6_getsockopt,
 	.sendmsg =	ping_v6_sendmsg,

net/ipv6/raw.c

···
 	.close		   = rawv6_close,
 	.destroy	   = raw6_destroy,
 	.connect	   = ip6_datagram_connect_v6_only,
-	.disconnect	   = udp_disconnect,
+	.disconnect	   = __udp_disconnect,
 	.ioctl		   = rawv6_ioctl,
 	.init		   = rawv6_init_sk,
 	.setsockopt	   = rawv6_setsockopt,

net/l2tp/l2tp_ip.c

···
 	if (sock_flag(sk, SOCK_ZAPPED))
 		return 0;
 
-	return udp_disconnect(sk, flags);
+	return __udp_disconnect(sk, flags);
 }
 
 static int l2tp_ip_getname(struct socket *sock, struct sockaddr *uaddr,

net/l2tp/l2tp_ip6.c

···
 	if (sock_flag(sk, SOCK_ZAPPED))
 		return 0;
 
-	return udp_disconnect(sk, flags);
+	return __udp_disconnect(sk, flags);
 }
 
 static int l2tp_ip6_getname(struct socket *sock, struct sockaddr *uaddr,