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[209.85.221.43]) by smtp.gmail.com with ESMTPSA id da25sm481125edb.38.2021.05.05.18.46.16 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 05 May 2021 18:46:16 -0700 (PDT) Received: by mail-wr1-f43.google.com with SMTP id d11so3813593wrw.8 for ; Wed, 05 May 2021 18:46:16 -0700 (PDT) X-Received: by 2002:adf:aa9a:: with SMTP id h26mr1848982wrc.419.1620265575663; Wed, 05 May 2021 18:46:15 -0700 (PDT) MIME-Version: 1.0 References: <1619690903-1138-1-git-send-email-dseok.yi@samsung.com> <8c2ea41a-3fc5-d560-16e5-bf706949d857@iogearbox.net> <02bf01d74211$0ff4aed0$2fde0c70$@samsung.com> In-Reply-To: <02bf01d74211$0ff4aed0$2fde0c70$@samsung.com> From: Willem de Bruijn Date: Wed, 5 May 2021 21:45:37 -0400 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH bpf] bpf: check for data_len before upgrading mss when 6 to 4 To: Dongseok Yi Cc: Daniel Borkmann , bpf , Alexei Starovoitov , Andrii Nakryiko , Martin KaFai Lau , Song Liu , Yonghong Song , John Fastabend , KP Singh , "David S. Miller" , Jakub Kicinski , Network Development , linux-kernel , Willem de Bruijn Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Wed, May 5, 2021 at 8:45 PM Dongseok Yi 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 > > > --- > > > 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; > > > > >