[NETFILTER]: Fix ip6_tables extension header bypass bug

As reported by Mark Dowd <Mark_Dowd@McAfee.com>, ip6_tables is susceptible
to a fragmentation attack causing false negatives on extension header matches.

When extension headers occur in the non-first fragment after the fragment
header (possibly with an incorrect nexthdr value in the fragment header)
a rule looking for this extension header will never match.

Drop fragments that are at offset 0 and don't contain the final protocol
header regardless of the ruleset, since this should not happen normally.
Since all extension headers are before the protocol header this makes sure
an extension header is either not present or in the first fragment, where
we can properly parse it.

With help from Yasuyuki KOZAKAI <yasuyuki.kozakai@toshiba.co.jp>.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/netfilter/ip6_tables.c

@@ -1440,6 +1440,9 @@ static void __exit ip6_tables_fini(void)
  * If target header is found, its offset is set in *offset and return protocol
  * number. Otherwise, return -1.
  *
+ * If the first fragment doesn't contain the final protocol header or
+ * NEXTHDR_NONE it is considered invalid.
+ *
  * Note that non-1st fragment is special case that "the protocol number
  * of last header" is "next header" field in Fragment header. In this case,
  * *offset is meaningless and fragment offset is stored in *fragoff if fragoff
@@ -1463,12 +1466,12 @@ int ipv6_find_hdr(const struct sk_buff *skb, unsigned int *offset,
 		if ((!ipv6_ext_hdr(nexthdr)) || nexthdr == NEXTHDR_NONE) {
 			if (target < 0)
 				break;
-			return -1;
+			return -ENOENT;
 		}
 
 		hp = skb_header_pointer(skb, start, sizeof(_hdr), &_hdr);
 		if (hp == NULL)
-			return -1;
+			return -EBADMSG;
 		if (nexthdr == NEXTHDR_FRAGMENT) {
 			unsigned short _frag_off, *fp;
 			fp = skb_header_pointer(skb,
@@ -1477,7 +1480,7 @@ int ipv6_find_hdr(const struct sk_buff *skb, unsigned int *offset,
 						sizeof(_frag_off),
 						&_frag_off);
 			if (fp == NULL)
-				return -1;
+				return -EBADMSG;
 
 			_frag_off = ntohs(*fp) & ~0x7;
 			if (_frag_off) {
@@ -1488,7 +1491,7 @@ int ipv6_find_hdr(const struct sk_buff *skb, unsigned int *offset,
 						*fragoff = _frag_off;
 					return hp->nexthdr;
 				}
-				return -1;
+				return -ENOENT;
 			}
 			hdrlen = 8;
 		} else if (nexthdr == NEXTHDR_AUTH)

net/ipv6/netfilter/ip6t_ah.c

@@ -54,9 +54,14 @@ match(const struct sk_buff *skb,
 	const struct ip6t_ah *ahinfo = matchinfo;
 	unsigned int ptr;
 	unsigned int hdrlen = 0;
+	int err;
 
-	if (ipv6_find_hdr(skb, &ptr, NEXTHDR_AUTH, NULL) < 0)
+	err = ipv6_find_hdr(skb, &ptr, NEXTHDR_AUTH, NULL);
+	if (err < 0) {
+		if (err != -ENOENT)
+			*hotdrop = 1;
 		return 0;
+	}
 
 	ah = skb_header_pointer(skb, ptr, sizeof(_ah), &_ah);
 	if (ah == NULL) {

net/ipv6/netfilter/ip6t_frag.c

@@ -52,9 +52,14 @@ match(const struct sk_buff *skb,
 	struct frag_hdr _frag, *fh;
 	const struct ip6t_frag *fraginfo = matchinfo;
 	unsigned int ptr;
+	int err;
 
-	if (ipv6_find_hdr(skb, &ptr, NEXTHDR_FRAGMENT, NULL) < 0)
+	err = ipv6_find_hdr(skb, &ptr, NEXTHDR_FRAGMENT, NULL);
+	if (err < 0) {
+		if (err != -ENOENT)
+			*hotdrop = 1;
 		return 0;
+	}
 
 	fh = skb_header_pointer(skb, ptr, sizeof(_frag), &_frag);
 	if (fh == NULL) {

net/ipv6/netfilter/ip6t_hbh.c

@@ -65,9 +65,14 @@ match(const struct sk_buff *skb,
 	u8 _opttype, *tp = NULL;
 	u8 _optlen, *lp = NULL;
 	unsigned int optlen;
+	int err;
 
-	if (ipv6_find_hdr(skb, &ptr, match->data, NULL) < 0)
+	err = ipv6_find_hdr(skb, &ptr, match->data, NULL);
+	if (err < 0) {
+		if (err != -ENOENT)
+			*hotdrop = 1;
 		return 0;
+	}
 
 	oh = skb_header_pointer(skb, ptr, sizeof(_optsh), &_optsh);
 	if (oh == NULL) {

net/ipv6/netfilter/ip6t_rt.c

@@ -58,9 +58,14 @@ match(const struct sk_buff *skb,
 	unsigned int hdrlen = 0;
 	unsigned int ret = 0;
 	struct in6_addr *ap, _addr;
+	int err;
 
-	if (ipv6_find_hdr(skb, &ptr, NEXTHDR_ROUTING, NULL) < 0)
+	err = ipv6_find_hdr(skb, &ptr, NEXTHDR_ROUTING, NULL);
+	if (err < 0) {
+		if (err != -ENOENT)
+			*hotdrop = 1;
 		return 0;
+	}
 
 	rh = skb_header_pointer(skb, ptr, sizeof(_route), &_route);
 	if (rh == NULL) {