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

From: "Dongseok Yi" <[email-masked]>
Date: Thu, 6 May 2021 09:45:07 +0900

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
> > 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 <[email-masked]>
> > ---
> >   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;
> >