[PATCH nf-next] netfilter: nf_nat_sip: rewind offset when NAT shrinks the packet

sashiko says:
 If map_addr() changes the packet length, such as when the public NAT IP
 string is shorter or longer than the internal IP, coff will still point to
 the offset relative to the pre-mangled packet.
 If the packet shrinks, coff could overshoot the correct position,
 potentially causing the next ct_sip_parse_header_uri() call to silently
 skip bytes and miss subsequent Contact headers. Could this lead to a
 failure to NAT those subsequent headers and leak internal network details?

Fixes: c978cd3a9371 ("[NETFILTER]: nf_nat_sip: translate all Contact headers")
Assisted-by: Claude:claude-sonnet-4-6
Signed-off-by: Florian Westphal <[email protected]>
---
 No bug reports for this, not clear how many uses are left.
 Not urgent, defer to -next.

diff --git a/net/netfilter/nf_nat_sip.c b/net/netfilter/nf_nat_sip.c
index aea02f6aff09..762d7e7bb7c7 100644
--- a/net/netfilter/nf_nat_sip.c
+++ b/net/netfilter/nf_nat_sip.c
@@ -273,12 +273,17 @@ static unsigned int nf_nat_sip(struct sk_buff *skb, unsigned int protoff,
 				       SIP_HDR_CONTACT, &in_header,
 				       &matchoff, &matchlen,
 				       &addr, &port) > 0) {
+		int old_len = skb->len, delta;
+
 		if (!map_addr(skb, protoff, dataoff, dptr, datalen,
 			      matchoff, matchlen,
 			      &addr, port)) {
 			nf_ct_helper_log(skb, ct, "cannot mangle contact");
 			return NF_DROP;
 		}
+
+		delta = (int)skb->len - old_len;
+		coff += delta;
 	}
 
 	if (!map_sip_addr(skb, protoff, dataoff, dptr, datalen, SIP_HDR_FROM) ||
-- 
2.54.0