[PATCH nf v2] netfilter: ip6tables: set hotdrop for malformed extension header matches

[PATCH nf v2] netfilter: ip6tables: set hotdrop for malformed extension header matches

[Zhixing Chen]

The hbh, srh and ipv6header matches have paths that return false for
malformed IPv6 extension header packets without setting hotdrop.

For hbh, strict option parsing stops when the option type or length field
cannot be read, or when advancing to the next requested option would
exceed the available header data. Mark these packets for hotdrop instead
of treating them as a rule mismatch.

For srh, keep a missing SRH as a normal mismatch, but set hotdrop when
header lookup fails for other reasons, when the SRH fixed header is not
present, when the advertised SRH length exceeds the available skb data,
when segments_left exceeds first_segment, or when SID selector reads fail.

For ipv6header, set hotdrop when the advertised extension header length
exceeds the available skb data.

Returning false treats the packet as a rule mismatch. Set hotdrop for
these 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:
- Use hotdrop labels for hbh and srh paths.
- Mark SRH packets with segments_left greater than first_segment for
  hotdrop.
- Drop the redundant ipv6header length check before skb_header_pointer().

v1: https://lore.kernel.org/netdev/20260709063012.33160-1-running910@gmail.com/T/

---
 net/ipv6/netfilter/ip6t_hbh.c        | 27 ++++++++++---------
 net/ipv6/netfilter/ip6t_ipv6header.c |  9 +++----
 net/ipv6/netfilter/ip6t_srh.c        | 40 ++++++++++++++++++++--------
 3 files changed, 47 insertions(+), 29 deletions(-)

diff --git a/net/ipv6/netfilter/ip6t_hbh.c b/net/ipv6/netfilter/ip6t_hbh.c
index 6d1a5d2026a6..1b5dcc92b7da 100644
--- a/net/ipv6/netfilter/ip6t_hbh.c
+++ b/net/ipv6/netfilter/ip6t_hbh.c
@@ -62,21 +62,18 @@ hbh_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 			    NEXTHDR_HOP : NEXTHDR_DEST, NULL, NULL);
 	if (err < 0) {
 		if (err != -ENOENT)
-			par->hotdrop = true;
+			goto hotdrop;
 		return false;
 	}
 
 	oh = skb_header_pointer(skb, ptr, sizeof(_optsh), &_optsh);
-	if (oh == NULL) {
-		par->hotdrop = true;
-		return false;
-	}
+	if (!oh)
+		goto hotdrop;
 
 	hdrlen = ipv6_optlen(oh);
 	if (skb->len - ptr < hdrlen) {
 		/* Packet smaller than it's length field */
-		par->hotdrop = true;
-		return false;
+		goto hotdrop;
 	}
 
 	pr_debug("IPv6 OPTS LEN %u %u ", hdrlen, oh->hdrlen);
@@ -104,8 +101,8 @@ hbh_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 				break;
 			tp = skb_header_pointer(skb, ptr, sizeof(_opttype),
 						&_opttype);
-			if (tp == NULL)
-				break;
+			if (!tp)
+				goto hotdrop;
 
 			/* Type check */
 			if (*tp != (optinfo->opts[temp] & 0xFF00) >> 8) {
@@ -121,12 +118,12 @@ hbh_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 
 				/* length field exists ? */
 				if (hdrlen < 2)
-					break;
+					goto hotdrop;
 				lp = skb_header_pointer(skb, ptr + 1,
 							sizeof(_optlen),
 							&_optlen);
-				if (lp == NULL)
-					break;
+				if (!lp)
+					goto hotdrop;
 				spec_len = optinfo->opts[temp] & 0x00FF;
 
 				if (spec_len != 0x00FF && spec_len != *lp) {
@@ -147,7 +144,7 @@ hbh_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 			if ((ptr > skb->len - optlen || hdrlen < optlen) &&
 			    temp < optinfo->optsnr - 1) {
 				pr_debug("new pointer is too large!\n");
-				break;
+				goto hotdrop;
 			}
 			ptr += optlen;
 			hdrlen -= optlen;
@@ -159,6 +156,10 @@ hbh_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 	}
 
 	return false;
+
+hotdrop:
+	par->hotdrop = true;
+	return false;
 }
 
 static int hbh_mt6_check(const struct xt_mtchk_param *par)
diff --git a/net/ipv6/netfilter/ip6t_ipv6header.c b/net/ipv6/netfilter/ip6t_ipv6header.c
index c52ff929c93b..e339cefc7fff 100644
--- a/net/ipv6/netfilter/ip6t_ipv6header.c
+++ b/net/ipv6/netfilter/ip6t_ipv6header.c
@@ -52,9 +52,6 @@ ipv6header_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 			temp |= MASK_NONE;
 			break;
 		}
-		/* Is there enough space for the next ext header? */
-		if (len < (int)sizeof(struct ipv6_opt_hdr))
-			return false;
 		/* ESP -> evaluate */
 		if (nexthdr == NEXTHDR_ESP) {
 			temp |= MASK_ESP;
@@ -99,8 +96,10 @@ ipv6header_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 		nexthdr = hp->nexthdr;
 		len -= hdrlen;
 		ptr += hdrlen;
-		if (ptr > skb->len)
-			break;
+		if (ptr > skb->len) {
+			par->hotdrop = true;
+			return false;
+		}
 	}
 
 	if (nexthdr != NEXTHDR_NONE && nexthdr != NEXTHDR_ESP)
diff --git a/net/ipv6/netfilter/ip6t_srh.c b/net/ipv6/netfilter/ip6t_srh.c
index db0fd64d8986..321c7f40a24f 100644
--- a/net/ipv6/netfilter/ip6t_srh.c
+++ b/net/ipv6/netfilter/ip6t_srh.c
@@ -27,22 +27,27 @@ static bool srh_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 	struct ipv6_sr_hdr *srh;
 	struct ipv6_sr_hdr _srh;
 	int hdrlen, srhoff = 0;
+	int err;
 
-	if (ipv6_find_hdr(skb, &srhoff, IPPROTO_ROUTING, NULL, NULL) < 0)
+	err = ipv6_find_hdr(skb, &srhoff, IPPROTO_ROUTING, NULL, NULL);
+	if (err < 0) {
+		if (err != -ENOENT)
+			goto hotdrop;
 		return false;
+	}
 	srh = skb_header_pointer(skb, srhoff, sizeof(_srh), &_srh);
 	if (!srh)
-		return false;
+		goto hotdrop;
 
 	hdrlen = ipv6_optlen(srh);
 	if (skb->len - srhoff < hdrlen)
-		return false;
+		goto hotdrop;
 
 	if (srh->type != IPV6_SRCRT_TYPE_4)
 		return false;
 
 	if (srh->segments_left > srh->first_segment)
-		return false;
+		goto hotdrop;
 
 	/* Next Header matching */
 	if (srhinfo->mt_flags & IP6T_SRH_NEXTHDR)
@@ -111,6 +116,10 @@ static bool srh_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 				!(srh->tag == srhinfo->tag)))
 			return false;
 	return true;
+
+hotdrop:
+	par->hotdrop = true;
+	return false;
 }
 
 static bool srh1_mt6(const struct sk_buff *skb, struct xt_action_param *par)
@@ -121,22 +130,27 @@ static bool srh1_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 	struct in6_addr _psid, _nsid, _lsid;
 	struct ipv6_sr_hdr *srh;
 	struct ipv6_sr_hdr _srh;
+	int err;
 
-	if (ipv6_find_hdr(skb, &srhoff, IPPROTO_ROUTING, NULL, NULL) < 0)
+	err = ipv6_find_hdr(skb, &srhoff, IPPROTO_ROUTING, NULL, NULL);
+	if (err < 0) {
+		if (err != -ENOENT)
+			goto hotdrop;
 		return false;
+	}
 	srh = skb_header_pointer(skb, srhoff, sizeof(_srh), &_srh);
 	if (!srh)
-		return false;
+		goto hotdrop;
 
 	hdrlen = ipv6_optlen(srh);
 	if (skb->len - srhoff < hdrlen)
-		return false;
+		goto hotdrop;
 
 	if (srh->type != IPV6_SRCRT_TYPE_4)
 		return false;
 
 	if (srh->segments_left > srh->first_segment)
-		return false;
+		goto hotdrop;
 
 	/* Next Header matching */
 	if (srhinfo->mt_flags & IP6T_SRH_NEXTHDR)
@@ -207,7 +221,7 @@ static bool srh1_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 			  ((srh->segments_left + 1) * sizeof(struct in6_addr));
 		psid = skb_header_pointer(skb, psidoff, sizeof(_psid), &_psid);
 		if (!psid)
-			return false;
+			goto hotdrop;
 		if (NF_SRH_INVF(srhinfo, IP6T_SRH_INV_PSID,
 				ipv6_masked_addr_cmp(psid, &srhinfo->psid_msk,
 						     &srhinfo->psid_addr)))
@@ -222,7 +236,7 @@ static bool srh1_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 			  ((srh->segments_left - 1) * sizeof(struct in6_addr));
 		nsid = skb_header_pointer(skb, nsidoff, sizeof(_nsid), &_nsid);
 		if (!nsid)
-			return false;
+			goto hotdrop;
 		if (NF_SRH_INVF(srhinfo, IP6T_SRH_INV_NSID,
 				ipv6_masked_addr_cmp(nsid, &srhinfo->nsid_msk,
 						     &srhinfo->nsid_addr)))
@@ -234,13 +248,17 @@ static bool srh1_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 		lsidoff = srhoff + sizeof(struct ipv6_sr_hdr);
 		lsid = skb_header_pointer(skb, lsidoff, sizeof(_lsid), &_lsid);
 		if (!lsid)
-			return false;
+			goto hotdrop;
 		if (NF_SRH_INVF(srhinfo, IP6T_SRH_INV_LSID,
 				ipv6_masked_addr_cmp(lsid, &srhinfo->lsid_msk,
 						     &srhinfo->lsid_addr)))
 			return false;
 	}
 	return true;
+
+hotdrop:
+	par->hotdrop = true;
+	return false;
 }
 
 static int srh_mt6_check(const struct xt_mtchk_param *par)
-- 
2.34.1