Re: [PATCH net] ipv6: Apply max_dst_opts_cnt to ip6_tnl_parse_tlv_enc_lim

[Justin Iurman <justin.iurman@gmail.com>]

On 4/18/26 00:03, Daniel Borkmann wrote:
> 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>
> ---
>   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..0ab76f93c136 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 = 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 (unlikely(tlv_cnt++ >= tlv_max))
> +					break;
> +
>   				/* No more room for encapsulation limit */
>   				if (i + sizeof(*tel) > optlen)
>   					break;

Good point on reusing max_dst_opts_cnt in ip6_tnl_parse_tlv_enc_lim(), 
but this patch is not ready yet.

We need to be careful: max_dst_opts_cnt can be negative. If this is the 
case, ip6_tnl_parse_tlv_enc_lim() would probably return 0, which is not 
what we want here. From the doc:

max_dst_opts_number - INTEGER
         Maximum number of non-padding TLVs allowed in a Destination
         options extension header. If this value is less than zero
         then unknown options are disallowed and the number of known
         TLVs allowed is the absolute value of this number.

         Default: 8

Since ip6_tnl_parse_tlv_enc_lim() does not check for specific option 
types (e.g., Pad1, PadN, you-name-it) and does not differentiate known 
from unknown options during parsing, I would simply use the absolute 
value of max_dst_opts_cnt by default.

Also, I wouldn't use unlikely() because it could harm us more than it 
helps in this specific context (consistent with ip6_parse_tlv()).