Re: [PATCH] ipv6: Prevent overrun when parsing v6 header options

[Eric Dumazet]

On 5/30/19 8:28 AM, Young Xiao wrote:
> The fragmentation code tries to parse the header options in order
> to figure out where to insert the fragment option.  Since nexthdr points
> to an invalid option, the calculation of the size of the network header
> can made to be much larger than the linear section of the skb and data
> is read outside of it.
> 
> This vulnerability is similar to CVE-2017-9074.
> 
> Signed-off-by: Young Xiao <92siuyang@xxxxxxxxx>
> ---
>  net/ipv6/mip6.c | 24 ++++++++++++++----------
>  1 file changed, 14 insertions(+), 10 deletions(-)
> 
> diff --git a/net/ipv6/mip6.c b/net/ipv6/mip6.c
> index 64f0f7b..30ed1c5 100644
> --- a/net/ipv6/mip6.c
> +++ b/net/ipv6/mip6.c
> @@ -263,8 +263,6 @@ static int mip6_destopt_offset(struct xfrm_state *x, struct sk_buff *skb,
>  			       u8 **nexthdr)
>  {
>  	u16 offset = sizeof(struct ipv6hdr);
> -	struct ipv6_opt_hdr *exthdr =
> -				   (struct ipv6_opt_hdr *)(ipv6_hdr(skb) + 1);
>  	const unsigned char *nh = skb_network_header(skb);
>  	unsigned int packet_len = skb_tail_pointer(skb) -
>  		skb_network_header(skb);
> @@ -272,7 +270,8 @@ static int mip6_destopt_offset(struct xfrm_state *x, struct sk_buff *skb,
>  
>  	*nexthdr = &ipv6_hdr(skb)->nexthdr;
>  
> -	while (offset + 1 <= packet_len) {
> +	while (offset <= packet_len) {
> +		struct ipv6_opt_hdr *exthdr;
>  
>  		switch (**nexthdr) {
>  		case NEXTHDR_HOP:
> @@ -299,12 +298,15 @@ static int mip6_destopt_offset(struct xfrm_state *x, struct sk_buff *skb,
>  			return offset;
>  		}
>  
> +		if (offset + sizeof(struct ipv6_opt_hdr) > packet_len)
> +			return -EINVAL;
> +
> +		exthdr = (struct ipv6_opt_hdr *)(nh + offset);
>  		offset += ipv6_optlen(exthdr);
>  		*nexthdr = &exthdr->nexthdr;
> -		exthdr = (struct ipv6_opt_hdr *)(nh + offset);
>  	}
>  
> -	return offset;
> +	return -EINVAL;
>  }
>


Ok, but have you checked that callers have been fixed ?

xfrm6_transport_output() seems buggy as well,
unless the skbs are linearized before entering these functions ?

Thanks.