ieee802154: verify packet size before trying to allocate it

Currently when sending data over datagram, the send function will attempt to
allocate any size passed on from the userspace.

We should make sure that this size is checked and limited. We'll limit it
to the MTU of the device, which is checked later anyway.
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ieee802154/dgram.c

@@ -230,6 +230,12 @@ static int dgram_sendmsg(struct kiocb *iocb, struct sock *sk,
 	mtu = dev->mtu;
 	pr_debug("name = %s, mtu = %u\n", dev->name, mtu);
 
+	if (size > mtu) {
+		pr_debug("size = %Zu, mtu = %u\n", size, mtu);
+		err = -EINVAL;
+		goto out_dev;
+	}
+
 	hlen = LL_RESERVED_SPACE(dev);
 	tlen = dev->needed_tailroom;
 	skb = sock_alloc_send_skb(sk, hlen + tlen + size,
@@ -258,12 +264,6 @@ static int dgram_sendmsg(struct kiocb *iocb, struct sock *sk,
 	if (err < 0)
 		goto out_skb;
 
-	if (size > mtu) {
-		pr_debug("size = %Zu, mtu = %u\n", size, mtu);
-		err = -EINVAL;
-		goto out_skb;
-	}
-
 	skb->dev = dev;
 	skb->sk  = sk;
 	skb->protocol = htons(ETH_P_IEEE802154);