arp: do not assume dev_hard_header() does not change skb->head

[ Upstream commit c92510f5e3f82ba11c95991824a41e59a9c5ed81 ]

arp_create() is the only dev_hard_header() caller
making assumption about skb->head being unchanged.

A recent commit broke this assumption.

Initialize @arp pointer after dev_hard_header() call.

Fixes: db5b4e39c4e6 ("ip6_gre: make ip6gre_header() robust")
Reported-by: syzbot+58b44a770a1585795351@syzkaller.appspotmail.com
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260107212250.384552-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: edumazet

net/ipv4/arp.c

@@ -563,20 +563,21 @@ struct sk_buff *arp_create(int type, int ptype, __be32 dest_ip,
 
 	skb_reserve(skb, hlen);
 	skb_reset_network_header(skb);
-	arp = skb_put(skb, arp_hdr_len(dev));
+	skb_put(skb, arp_hdr_len(dev));
 	skb->dev = dev;
 	skb->protocol = htons(ETH_P_ARP);
 	if (!src_hw)
 		src_hw = dev->dev_addr;
 	if (!dest_hw)
 		dest_hw = dev->broadcast;
 
-	/*
-	 *	Fill the device header for the ARP frame
+	/* Fill the device header for the ARP frame.
+	 * Note: skb->head can be changed.
 	 */
 	if (dev_hard_header(skb, dev, ptype, dest_hw, src_hw, skb->len) < 0)
 		goto out;
 
+	arp = arp_hdr(skb);
 	/*
 	 * Fill out the arp protocol part.
 	 *