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

[Eric Dumazet]

On 5/30/19 8:04 PM, Yang Xiao wrote:
> On Fri, May 31, 2019 at 1:17 AM Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
>>
>>
>>
>> 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 ?
> 
> I've checked the callers. There are two callers:
> xfrm6_transport_output() and xfrm6_ro_output(). There are checks in
> both function.
> 
> ------------------------------------------------------------------------------
>         hdr_len = x->type->hdr_offset(x, skb, &prevhdr);
>         if (hdr_len < 0)
>                 return hdr_len;
> ------------------------------------------------------------------------------
>>
>> xfrm6_transport_output() seems buggy as well,
>> unless the skbs are linearized before entering these functions ?
> I can not understand what you mean about this comment.
> Could you explain it in more detail.


If we had a problem, then the memmove(ipv6_hdr(skb), iph, hdr_len);
 in xfrm6_transport_output() would be buggy, since iph could also point to freed memory.