From 949647e7771a4a01963fe953a96d81fba7acecf3 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 7 Jan 2026 21:22:50 +0000 Subject: 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 Link: https://patch.msgid.link/20260107212250.384552-1-edumazet@google.com Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin --- net/ipv4/arp.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c index 833f2cf97178..3ce1664e8632 100644 --- a/net/ipv4/arp.c +++ b/net/ipv4/arp.c @@ -564,7 +564,7 @@ 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) @@ -572,12 +572,13 @@ struct sk_buff *arp_create(int type, int ptype, __be32 dest_ip, 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. * -- cgit v1.2.3