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

[Yang Xiao <92siuyang@gmail.com>]

On Fri, May 31, 2019 at 1:17 AM Eric Dumazet <eric.dumazet@gmail.com> 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@gmail.com>
> > ---
> >  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.

>
> Thanks.
>
>
>