diff --git a/net/ipv4/ah4.c b/net/ipv4/ah4.c
index 880a5ec..86961be 100644
--- a/net/ipv4/ah4.c
+++ b/net/ipv4/ah4.c
@@ -314,14 +314,15 @@
 
 	skb->ip_summed = CHECKSUM_NONE;
 
-	ah = (struct ip_auth_hdr *)skb->data;
-	iph = ip_hdr(skb);
-	ihl = ip_hdrlen(skb);
 
 	if ((err = skb_cow_data(skb, 0, &trailer)) < 0)
 		goto out;
 	nfrags = err;
 
+	ah = (struct ip_auth_hdr *)skb->data;
+	iph = ip_hdr(skb);
+	ihl = ip_hdrlen(skb);
+
 	work_iph = ah_alloc_tmp(ahash, nfrags, ihl + ahp->icv_trunc_len);
 	if (!work_iph)
 		goto out;