udpv6: Check address length before reading address family

KMSAN will complain if valid address length passed to udpv6_pre_connect()
is shorter than sizeof("struct sockaddr"->sa_family) bytes.

(This patch is bogus if it is guaranteed that udpv6_pre_connect() is
always called after checking "struct sockaddr"->sa_family. In that case,
we want a comment why we don't need to check valid address length here.)
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/udp.c

@@ -1047,6 +1047,8 @@ static void udp_v6_flush_pending_frames(struct sock *sk)
 static int udpv6_pre_connect(struct sock *sk, struct sockaddr *uaddr,
 			     int addr_len)
 {
+	if (addr_len < offsetofend(struct sockaddr, sa_family))
+		return -EINVAL;
 	/* The following checks are replicated from __ip6_datagram_connect()
 	 * and intended to prevent BPF program called below from accessing
 	 * bytes that are out of the bound specified by user in addr_len.