[PATCH net v2] ipv6: Apply max_dst_opts_cnt to 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.

Reuse max_dst_opts_cnt to bound the TLV iterations, matching
the semantics from 47d3d7ac656a.

Fixes: 47d3d7ac656a ("ipv6: Implement limits on Hop-by-Hop and Destination options")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 v1->v2:
   - Remove unlikely (Justin)
   - Use abs() given max_dst_opts_cnt's negative meaning (Justin)

 net/ipv6/ip6_tunnel.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index 907c6a2af331..0f50b7fcb24e 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -430,11 +430,16 @@ __u16 ip6_tnl_parse_tlv_enc_lim(struct sk_buff *skb, __u8 *raw)
 				break;
 		}
 		if (nexthdr == NEXTHDR_DEST) {
+			int tlv_max = abs(READ_ONCE(init_net.ipv6.sysctl.max_dst_opts_cnt));
+			int tlv_cnt = 0;
 			u16 i = 2;
 
 			while (1) {
 				struct ipv6_tlv_tnl_enc_lim *tel;
 
+				if (tlv_cnt++ >= tlv_max)
+					break;
+
 				/* No more room for encapsulation limit */
 				if (i + sizeof(*tel) > optlen)
 					break;
-- 
2.43.0