From: Yiming Qian Date: Sat, 23 May 2026 12:29:10 +0000 (+0000) Subject: netfilter: bridge: make ebt_snat ARP rewrite writable X-Git-Tag: v7.1-rc7~20^2~36^2~1 X-Git-Url: http://git.ipfire.org/gitweb/index.cgi?a=commitdiff_plain;h=67ba971ae02514d85818fe0c32549ab4bfa3bf49;p=thirdparty%2Fkernel%2Flinux.git netfilter: bridge: make ebt_snat ARP rewrite writable The ebtables SNAT target keeps the Ethernet source address rewrite behind skb_ensure_writable(skb, 0). This is intentional: at the bridge ebtables hooks the Ethernet header is addressed through skb_mac_header()/eth_hdr(), while skb->data points at the Ethernet payload. Asking skb_ensure_writable() for ETH_HLEN bytes would check the payload, not the Ethernet header, and would reintroduce the small packet regression fixed by commit 63137bc5882a. However, the optional ARP sender hardware address rewrite is different. It writes through skb_store_bits() at an offset relative to skb->data: skb_store_bits(skb, sizeof(struct arphdr), info->mac, ETH_ALEN) skb_header_pointer() only safely reads the ARP header; it does not make the later sender hardware address range writable. If that range is still held in a nonlinear skb fragment backed by a splice-imported file page, skb_store_bits() maps the frag page and copies the new MAC address directly into it. Ensure the ARP SHA range is writable before reading the ARP header and before calling skb_store_bits(). Fixes: 63137bc5882a ("netfilter: ebtables: Fixes dropping of small packets in bridge nat") Reported-by: Yiming Qian Signed-off-by: Yiming Qian Signed-off-by: Florian Westphal Signed-off-by: Pablo Neira Ayuso --- diff --git a/net/bridge/netfilter/ebt_snat.c b/net/bridge/netfilter/ebt_snat.c index 7dfbcdfc30e5..c9e229af0366 100644 --- a/net/bridge/netfilter/ebt_snat.c +++ b/net/bridge/netfilter/ebt_snat.c @@ -31,6 +31,9 @@ ebt_snat_tg(struct sk_buff *skb, const struct xt_action_param *par) const struct arphdr *ap; struct arphdr _ah; + if (skb_ensure_writable(skb, sizeof(_ah) + ETH_ALEN)) + return EBT_DROP; + ap = skb_header_pointer(skb, 0, sizeof(_ah), &_ah); if (ap == NULL) return EBT_DROP;