vhost: Check docket sk_family instead of call getname

Doing so, we save one call to get data we already have in the struct.

Also, since there is no guarantee that getname use sockaddr_ll
parameter beyond its size, we add a little bit of security here.
It should do not do beyond MAX_ADDR_LEN, but syzbot found that
ax25_getname writes more (72 bytes, the size of full_sockaddr_ax25,
versus 20 + 32 bytes of sockaddr_ll + MAX_ADDR_LEN in syzbot repro).

Fixes: 3a4d5c94 ("vhost_net: a kernel-level virtio server")
Reported-by: syzbot+f2a62d07a5198c819c7b@syzkaller.appspotmail.com
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/vhost/net.c

@@ -1414,10 +1414,6 @@ static int vhost_net_release(struct inode *inode, struct file *f)
 
 static struct socket *get_raw_socket(int fd)
 {
-	struct {
-		struct sockaddr_ll sa;
-		char  buf[MAX_ADDR_LEN];
-	} uaddr;
 	int r;
 	struct socket *sock = sockfd_lookup(fd, &r);
 
@@ -1430,11 +1426,7 @@ static struct socket *get_raw_socket(int fd)
 		goto err;
 	}
 
-	r = sock->ops->getname(sock, (struct sockaddr *)&uaddr.sa, 0);
-	if (r < 0)
-		goto err;
-
-	if (uaddr.sa.sll_family != AF_PACKET) {
+	if (sock->sk->sk_family != AF_PACKET) {
 		r = -EPFNOSUPPORT;
 		goto err;
 	}