Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   "Dongseok Yi" <dseok.yi@samsung.com>
To:     "'Daniel Borkmann'" <daniel@iogearbox.net>, <bpf@vger.kernel.org>
Cc:     "'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>, <netdev@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <willemdebruijn.kernel@gmail.com>
In-Reply-To: <8c2ea41a-3fc5-d560-16e5-bf706949d857@iogearbox.net>
Subject: RE: [PATCH bpf] bpf: check for data_len before upgrading mss when 6
 to 4
Date:   Thu, 6 May 2021 09:45:07 +0900
Message-ID: <02bf01d74211$0ff4aed0$2fde0c70$@samsung.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Language: ko
Thread-Index: AQKypHYW3xad5/j2XvChPebQmKG2owG+YoocAqFpyMuo+9TVEA==
Content-Type: text/plain; charset="utf-8"
CMS-TYPE: 102P
DLP-Filter: Pass
References: <CGME20210429102143epcas2p4c8747c09a9de28f003c20389c050394a@epcas2p4.samsung.com>
        <1619690903-1138-1-git-send-email-dseok.yi@samsung.com>
        <8c2ea41a-3fc5-d560-16e5-bf706949d857@iogearbox.net>
Precedence: bulk

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@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;
> >