RE: [PATCH bpf] bpf: check for data_len before upgrading mss when 6 to 4
From: Dongseok Yi
Date: Wed May 05 2021 - 20:45:17 EST
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 <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;
> Please provide more context on the bug, thanks!
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)
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;