Re: [PATCH bpf] bpf: check for data_len before upgrading mss when 6 to 4

[Willem de Bruijn]

On Wed, May 5, 2021 at 8:45 PM Dongseok Yi <dseok.yi@xxxxxxxxxxx> wrote:
>
> On Wed, May 05, 2021 at 10:55:10PM +0200, Daniel Borkmann wrote:
> > On 4/29/21 12:08 PM, Dongseok Yi wrote:
> > > tcp_gso_segment check for the size of GROed payload if it is bigger
> > > than the mss. bpf_skb_proto_6_to_4 increases mss, but the mss can be
> > > bigger than the size of GROed payload unexpectedly if data_len is not
> > > big enough.
> > >
> > > Assume that skb gso_size = 1372 and data_len = 8. bpf_skb_proto_6_to_4

Is this a typo and is this intended to read skb->data_len = 1380?

The issue is that payload length (1380) is greater than mss with ipv6
(1372), but less than mss with ipv4 (1392).

I don't understand data_len = 8 or why the patch compares
skb->data_len to len_diff (20).

One simple solution if this packet no longer needs to be segmented
might be to reset the gso_type completely.

In general, I would advocate using BPF_F_ADJ_ROOM_FIXED_GSO. When
converting from IPv6 to IPv4, fixed gso will end up building packets
that are slightly below the MTU. That opportunity cost is negligible
(especially with TSO). Unfortunately, I see that that flag is
available for bpf_skb_adjust_room but not for bpf_skb_proto_6_to_4.


> > > would increse the gso_size to 1392. tcp_gso_segment will get an error
> > > with 1380 <= 1392.
> > >
> > > Check for the size of GROed payload if it is really bigger than target
> > > mss when increase mss.
> > >
> > > Fixes: 6578171a7ff0 (bpf: add bpf_skb_change_proto helper)
> > > Signed-off-by: Dongseok Yi <dseok.yi@xxxxxxxxxxx>
> > > ---
> > >   net/core/filter.c | 4 +++-
> > >   1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/net/core/filter.c b/net/core/filter.c
> > > index 9323d34..3f79e3c 100644
> > > --- a/net/core/filter.c
> > > +++ b/net/core/filter.c
> > > @@ -3308,7 +3308,9 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
> > >             }
> > >
> > >             /* Due to IPv4 header, MSS can be upgraded. */
> > > -           skb_increase_gso_size(shinfo, len_diff);
> > > +           if (skb->data_len > len_diff)
> >
> > Could you elaborate some more on what this has to do with data_len specifically
> > here? I'm not sure I follow exactly your above commit description. Are you saying
> > that you're hitting in tcp_gso_segment():
> >
> >          [...]
> >          mss = skb_shinfo(skb)->gso_size;
> >          if (unlikely(skb->len <= mss))
> >                  goto out;
> >          [...]
>
> Yes, right
>
> >
> > Please provide more context on the bug, thanks!
>
> tcp_gso_segment():
>         [...]
>         __skb_pull(skb, thlen);
>
>         mss = skb_shinfo(skb)->gso_size;
>         if (unlikely(skb->len <= mss))
>         [...]
>
> skb->len will have total GROed TCP payload size after __skb_pull.
> skb->len <= mss will not be happened in a normal GROed situation. But
> bpf_skb_proto_6_to_4 would upgrade MSS by increasing gso_size, it can
> hit an error condition.
>
> We should ensure the following condition.
> total GROed TCP payload > the original mss + (IPv6 size - IPv4 size)
>
> Due to
> total GROed TCP payload = the original mss + skb->data_len
> IPv6 size - IPv4 size = len_diff
>
> Finally, we can get the condition.
> skb->data_len > len_diff
>
> >
> > > +                   skb_increase_gso_size(shinfo, len_diff);
> > > +
> > >             /* Header must be checked, and gso_segs recomputed. */
> > >             shinfo->gso_type |= SKB_GSO_DODGY;
> > >             shinfo->gso_segs = 0;
> > >
>
>