ipv4: ip_check_defrag must not modify skb before unsharing

ip_check_defrag() might be called from af_packet within the
RX path where shared SKBs are used, so it must not modify
the input SKB before it has unshared it for defragmentation.
Use skb_copy_bits() to get the IP header and only pull in
everything later.

The same is true for the other caller in macvlan as it is
called from dev->rx_handler which can also get a shared SKB.

Reported-by: Eric Leblond <eric@regit.org>
Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_fragment.c

@@ -707,28 +707,27 @@ EXPORT_SYMBOL(ip_defrag);
 
 struct sk_buff *ip_check_defrag(struct sk_buff *skb, u32 user)
 {
-	const struct iphdr *iph;
+	struct iphdr iph;
 	u32 len;
 
 	if (skb->protocol != htons(ETH_P_IP))
 		return skb;
 
-	if (!pskb_may_pull(skb, sizeof(struct iphdr)))
+	if (!skb_copy_bits(skb, 0, &iph, sizeof(iph)))
 		return skb;
 
-	iph = ip_hdr(skb);
-	if (iph->ihl < 5 || iph->version != 4)
-		return skb;
-	if (!pskb_may_pull(skb, iph->ihl*4))
-		return skb;
-	iph = ip_hdr(skb);
-	len = ntohs(iph->tot_len);
-	if (skb->len < len || len < (iph->ihl * 4))
+	if (iph.ihl < 5 || iph.version != 4)
 		return skb;
 
-	if (ip_is_fragment(ip_hdr(skb))) {
+	len = ntohs(iph.tot_len);
+	if (skb->len < len || len < (iph.ihl * 4))
+		return skb;
+
+	if (ip_is_fragment(&iph)) {
 		skb = skb_share_check(skb, GFP_ATOMIC);
 		if (skb) {
+			if (!pskb_may_pull(skb, iph.ihl*4))
+				return skb;
 			if (pskb_trim_rcsum(skb, len))
 				return skb;
 			memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));