[PATCH net v3] ipv6: Cap TLV scan in ip6_tnl_parse_tlv_enc_lim

[Daniel Borkmann <daniel@iogearbox.net>]

Commit 47d3d7ac656a ("ipv6: Implement limits on Hop-by-Hop and
Destination options") added net.ipv6.max_{hbh,dst}_opts_{cnt,len}
and applied them in ip6_parse_tlv(), the generic TLV walker
invoked from ipv6_destopt_rcv() and ipv6_parse_hopopts().

ip6_tnl_parse_tlv_enc_lim() does not go through ip6_parse_tlv();
it has its own hand-rolled TLV scanner inside its NEXTHDR_DEST
branch which looks for IPV6_TLV_TNL_ENCAP_LIMIT. That inner
loop is bounded only by optlen, which can be up to 2048 bytes.
Stuffing the Destination Options header with 2046 Pad1 (type=0)
entries advances the scanner a single byte at a time, yielding
~2000 TLV iterations per extension header.

Reusing max_dst_opts_cnt to bound the TLV iterations, matching
the semantics from 47d3d7ac656a, would require duplicating
ip6_parse_tlv() to also validate Pad1/PadN payload. It would
also mandate enforcing max_dst_opts_len, since otherwise an
attacker shifts the axis to few options with a giant PadN and
recovers the original DoS. Allowing up to 8 options before the
tunnel encapsulation limit TLV is liberal enough; in practice
encap limit is the first TLV. Thus, go with a hard-coded limit
IP6_TUNNEL_MAX_DEST_TLVS (8).

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
  v2->v3:
   - Hard code limit of 8 instead of max_dst_opts_cnt (Ido)
  v1->v2:
   - Use abs() given max_dst_opts_cnt's negative meaning (Justin)
   - Remove unlikely (Justin)

 net/ipv6/ip6_tunnel.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index 907c6a2af331..4546a60942ab 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -62,6 +62,8 @@ MODULE_LICENSE("GPL");
 MODULE_ALIAS_RTNL_LINK("ip6tnl");
 MODULE_ALIAS_NETDEV("ip6tnl0");
 
+#define IP6_TUNNEL_MAX_DEST_TLVS    8
+
 #define IP6_TUNNEL_HASH_SIZE_SHIFT  5
 #define IP6_TUNNEL_HASH_SIZE (1 << IP6_TUNNEL_HASH_SIZE_SHIFT)
 
@@ -430,11 +432,15 @@ __u16 ip6_tnl_parse_tlv_enc_lim(struct sk_buff *skb, __u8 *raw)
 				break;
 		}
 		if (nexthdr == NEXTHDR_DEST) {
+			int tlv_cnt = 0;
 			u16 i = 2;
 
 			while (1) {
 				struct ipv6_tlv_tnl_enc_lim *tel;
 
+				if (unlikely(tlv_cnt++ >= IP6_TUNNEL_MAX_DEST_TLVS))
+					break;
+
 				/* No more room for encapsulation limit */
 				if (i + sizeof(*tel) > optlen)
 					break;
-- 
2.43.0