From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To: Dongseok Yi <dseok.yi@samsung.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>, bpf <bpf@vger.kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
Andrii Nakryiko <andrii@kernel.org>,
Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
Yonghong Song <yhs@fb.com>,
John Fastabend <john.fastabend@gmail.com>,
KP Singh <kpsingh@kernel.org>,
"David S. Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>,
Network Development <netdev@vger.kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Subject: Re: [PATCH bpf] bpf: check for data_len before upgrading mss when 6 to 4
Date: Wed, 5 May 2021 21:45:37 -0400 [thread overview]
Message-ID: <CA+FuTScC96R5o24c-sbY-CEV4EYOVFepFR85O4uGtCLwOjnzEw@mail.gmail.com> (raw)
In-Reply-To: <02bf01d74211$0ff4aed0$2fde0c70$@samsung.com>
On Wed, May 5, 2021 at 8:45 PM Dongseok Yi <dseok.yi@samsung.com> 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@samsung.com>
> > > ---
> > > 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;
> > >
>
>
next prev parent reply other threads:[~2021-05-06 1:46 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CGME20210429102143epcas2p4c8747c09a9de28f003c20389c050394a@epcas2p4.samsung.com>
2021-04-29 10:08 ` [PATCH bpf] bpf: check for data_len before upgrading mss when 6 to 4 Dongseok Yi
2021-05-05 20:55 ` Daniel Borkmann
2021-05-06 0:45 ` Dongseok Yi
2021-05-06 1:45 ` Willem de Bruijn [this message]
2021-05-06 2:27 ` Dongseok Yi
2021-05-06 18:21 ` Willem de Bruijn
2021-05-07 0:53 ` Dongseok Yi
2021-05-07 1:25 ` Willem de Bruijn
2021-05-07 1:45 ` Yunsheng Lin
2021-05-07 1:53 ` Willem de Bruijn
2021-05-07 8:25 ` Dongseok Yi
2021-05-07 9:11 ` Yunsheng Lin
2021-05-07 10:36 ` Dongseok Yi
2021-05-07 13:50 ` Willem de Bruijn
2021-05-10 2:22 ` Dongseok Yi
2021-05-10 13:19 ` Willem de Bruijn
2021-05-10 13:46 ` Willem de Bruijn
2021-05-11 1:11 ` Dongseok Yi
2021-05-11 17:38 ` Willem de Bruijn
2021-05-12 0:45 ` Dongseok Yi
[not found] ` <CGME20210511065056epcas2p1788505019deb274f5c57650a2f5d7ef0@epcas2p1.samsung.com>
2021-05-11 6:36 ` [PATCH bpf v2] bpf: check BPF_F_ADJ_ROOM_FIXED_GSO when upgrading mss in " Dongseok Yi
2021-05-11 17:42 ` Willem de Bruijn
2021-05-12 6:56 ` Dongseok Yi
[not found] ` <CGME20210512074058epcas2p35536c27bdfafaa6431e164c142007f96@epcas2p3.samsung.com>
2021-05-12 7:27 ` [PATCH bpf-next v3] bpf: check for BPF_F_ADJ_ROOM_FIXED_GSO when bpf_skb_change_proto Dongseok Yi
2021-05-12 14:13 ` Willem de Bruijn
2021-05-18 20:10 ` patchwork-bot+netdevbpf
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