[PATCH nf v2] netfilter: ip6tables: mark malformed IPv6 extension headers for hotdrop

[Zhixing Chen <running910@gmail.com>]

The ah, hbh and rt matches check that the fixed extension header is
present, then use the header length field to derive the advertised
extension header length for matching.

For the ah match, add the missing advertised-length check. For hbh
and rt, update the existing advertised-length checks. In all three
cases, set hotdrop to true before returning false when the advertised
extension header length exceeds the available skb data.

Returning false treats the packet as a rule mismatch. Set hotdrop to
true and drop malformed packets so they cannot bypass rules intended
to drop packets with these IPv6 extension headers.

Signed-off-by: Zhixing Chen <running910@gmail.com>
---

Changes in v2:
- Set hotdrop to true before returning false for malformed packets.
- Apply the same handling to hbh and rt matches.

v1: https://lore.kernel.org/netfilter-devel/20260618125848.93550-1-running910@gmail.com/T/

---
 net/ipv6/netfilter/ip6t_ah.c  | 5 +++++
 net/ipv6/netfilter/ip6t_hbh.c | 1 +
 net/ipv6/netfilter/ip6t_rt.c  | 1 +
 3 files changed, 7 insertions(+)

diff --git a/net/ipv6/netfilter/ip6t_ah.c b/net/ipv6/netfilter/ip6t_ah.c
index 70da2f2ce064..1258783ed876 100644
--- a/net/ipv6/netfilter/ip6t_ah.c
+++ b/net/ipv6/netfilter/ip6t_ah.c
@@ -56,6 +56,11 @@ static bool ah_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 	}
 
 	hdrlen = ipv6_authlen(ah);
+	if (skb->len - ptr < hdrlen) {
+		/* Packet smaller than its length field */
+		par->hotdrop = true;
+		return false;
+	}
 
 	pr_debug("IPv6 AH LEN %u %u ", hdrlen, ah->hdrlen);
 	pr_debug("RES %04X ", ah->reserved);
diff --git a/net/ipv6/netfilter/ip6t_hbh.c b/net/ipv6/netfilter/ip6t_hbh.c
index 450dd53846a2..6d1a5d2026a6 100644
--- a/net/ipv6/netfilter/ip6t_hbh.c
+++ b/net/ipv6/netfilter/ip6t_hbh.c
@@ -75,6 +75,7 @@ hbh_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 	hdrlen = ipv6_optlen(oh);
 	if (skb->len - ptr < hdrlen) {
 		/* Packet smaller than it's length field */
+		par->hotdrop = true;
 		return false;
 	}
 
diff --git a/net/ipv6/netfilter/ip6t_rt.c b/net/ipv6/netfilter/ip6t_rt.c
index 5561bd9cea81..e28caca759f3 100644
--- a/net/ipv6/netfilter/ip6t_rt.c
+++ b/net/ipv6/netfilter/ip6t_rt.c
@@ -57,6 +57,7 @@ static bool rt_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 	hdrlen = ipv6_optlen(rh);
 	if (skb->len - ptr < hdrlen) {
 		/* Pcket smaller than its length field */
+		par->hotdrop = true;
 		return false;
 	}
 
-- 
2.34.1