bridge-netfilter: don't overwrite memory outside of skb

The bridge netfilter code needs to check for space at the
front of the skb before overwriting; otherwise if skb from
device doesn't have headroom, then it will cause random
memory corruption.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/linux/netfilter_bridge.h

@@ -47,18 +47,26 @@ enum nf_br_hook_priorities {
 #define BRNF_BRIDGED			0x08
 #define BRNF_NF_BRIDGE_PREROUTING	0x10
 
-
 /* Only used in br_forward.c */
-static inline
-void nf_bridge_maybe_copy_header(struct sk_buff *skb)
+static inline int nf_bridge_maybe_copy_header(struct sk_buff *skb)
 {
+	int err;
+
 	if (skb->nf_bridge) {
 		if (skb->protocol == __constant_htons(ETH_P_8021Q)) {
+			err = skb_cow(skb, 18);
+			if (err)
+				return err;
 			memcpy(skb->data - 18, skb->nf_bridge->data, 18);
 			skb_push(skb, 4);
-		} else
+		} else {
+			err = skb_cow(skb, 16);
+			if (err)
+				return err;
 			memcpy(skb->data - 16, skb->nf_bridge->data, 16);
+		}
 	}
+	return 0;
 }
 
 /* This is called by the IP fragmenting code and it ensures there is