appletalk: Fix skb allocation size in loopback case

If a DDP broadcast packet is sent out to a non-gateway target, it is
also looped back. There is a potential for the loopback device to have a
longer hardware header length than the original target route's device,
which can result in the skb not being created with enough room for the
loopback device's hardware header. This patch fixes the issue by
determining that a loopback will be necessary prior to allocating the
skb, and if so, ensuring the skb has enough room.

This was discovered while testing a new driver that creates a LocalTalk
network interface (LTALK_HLEN = 1). It caused an skb_under_panic.
Signed-off-by: Doug Brown <doug@schmorgal.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/appletalk/ddp.c

@@ -1577,8 +1577,8 @@ static int atalk_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 	struct sk_buff *skb;
 	struct net_device *dev;
 	struct ddpehdr *ddp;
-	int size;
-	struct atalk_route *rt;
+	int size, hard_header_len;
+	struct atalk_route *rt, *rt_lo = NULL;
 	int err;
 
 	if (flags & ~(MSG_DONTWAIT|MSG_CMSG_COMPAT))
@@ -1641,7 +1641,22 @@ static int atalk_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 	SOCK_DEBUG(sk, "SK %p: Size needed %d, device %s\n",
 			sk, size, dev->name);
 
-	size += dev->hard_header_len;
+	hard_header_len = dev->hard_header_len;
+	/* Leave room for loopback hardware header if necessary */
+	if (usat->sat_addr.s_node == ATADDR_BCAST &&
+	    (dev->flags & IFF_LOOPBACK || !(rt->flags & RTF_GATEWAY))) {
+		struct atalk_addr at_lo;
+
+		at_lo.s_node = 0;
+		at_lo.s_net  = 0;
+
+		rt_lo = atrtr_find(&at_lo);
+
+		if (rt_lo && rt_lo->dev->hard_header_len > hard_header_len)
+			hard_header_len = rt_lo->dev->hard_header_len;
+	}
+
+	size += hard_header_len;
 	release_sock(sk);
 	skb = sock_alloc_send_skb(sk, size, (flags & MSG_DONTWAIT), &err);
 	lock_sock(sk);
@@ -1649,7 +1664,7 @@ static int atalk_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 		goto out;
 
 	skb_reserve(skb, ddp_dl->header_length);
-	skb_reserve(skb, dev->hard_header_len);
+	skb_reserve(skb, hard_header_len);
 	skb->dev = dev;
 
 	SOCK_DEBUG(sk, "SK %p: Begin build.\n", sk);
@@ -1700,18 +1715,12 @@ static int atalk_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 		/* loop back */
 		skb_orphan(skb);
 		if (ddp->deh_dnode == ATADDR_BCAST) {
-			struct atalk_addr at_lo;
-
-			at_lo.s_node = 0;
-			at_lo.s_net  = 0;
-
-			rt = atrtr_find(&at_lo);
-			if (!rt) {
+			if (!rt_lo) {
 				kfree_skb(skb);
 				err = -ENETUNREACH;
 				goto out;
 			}
-			dev = rt->dev;
+			dev = rt_lo->dev;
 			skb->dev = dev;
 		}
 		ddp_dl->request(ddp_dl, skb, dev->dev_addr);