From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-12.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3FE91C43461 for ; Fri, 7 May 2021 01:54:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E8DC361027 for ; Fri, 7 May 2021 01:54:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231288AbhEGBzd (ORCPT ); Thu, 6 May 2021 21:55:33 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58816 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232025AbhEGBzb (ORCPT ); Thu, 6 May 2021 21:55:31 -0400 Received: from mail-qv1-xf2c.google.com (mail-qv1-xf2c.google.com [IPv6:2607:f8b0:4864:20::f2c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 71EDEC061574; Thu, 6 May 2021 18:54:30 -0700 (PDT) Received: by mail-qv1-xf2c.google.com with SMTP id u1so4127819qvg.11; Thu, 06 May 2021 18:54:30 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=a+4dNNiI1+2u64fASl7JHolzckwlzH1hpOrNavtuif4=; b=Nmtb6cpQ+yEbnxRHw4KiuEyASAmEbgclDkhdLpPM336+li0ZY9IvwYj01QV+JXSUEs EkzBPZaNpLlJJqrqx8wnvWtMeeFyFH9WVYBZALnuJpFq3zlWjNTl9VzMbUgo/qBqQ4lS frm+vZe9q5Fi7TPPpKmhQbgO9ezpg99/v0Pv0+xNHQNkdvrLz3gmy0Iwx9II/n9K5zcU mipPduphRRMUpRhVAB53GheXfBb5JUOOK+3R1ZWX43vTNK9IvukBT3bgWH5bmoRuwJ2F ABojNx5xBC7omeS1QqCVAIwFO/tYpqUVuHeWpVEiVVjB4IaHyV8xXlc1NIIXAwT5KsJn czRA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=a+4dNNiI1+2u64fASl7JHolzckwlzH1hpOrNavtuif4=; b=q4BHhqAteYIn44ANq+dh0exifEy9aGUZvFfCIfwH1NUDFsN6ND69NC5PRRoqGWmuEH Br6w43mZFYQXwHlO5RtjSfZifpaTgsskrazEIqmAfPTwv3r0G26xCyjnHm+lk7MfP8gl 8NHygw7IUa07gpzX0bcyyLNoLD1/esMLAyIGj8Sy/TlvU+2o34d3ImfpFxzhnWNn4K3u ms7+NtZxcVJllrIPTWIrjt7mO5+KTFRNfA/Rf60zmnbMP2Zj15DNqZdo4UlPL1uoe3dT C4KUq9QlXZfQeVOt+KwdAxzMaKqJPkWag5LGV5auhzUhrUyX9+E8iScwMTkpuC4wwS+O 5o/A== X-Gm-Message-State: AOAM532v1eyCPSifdna4CEpIJmblCVTrECa3wB+zLWAsZ2C9Q54ousbA XhvK2RLMkWTahOoZQLRWshc984BzVTyW4Vgt2kE= X-Google-Smtp-Source: ABdhPJxpCh4QaB+Ozysrm7kXuFm2/+7FkDJ/uoybNP9JrAet8644s0jFRYQOA6BMTQ44A3eHdp/3Wbh/+O3KWLIqTvo= X-Received: by 2002:ad4:542c:: with SMTP id g12mr7385295qvt.38.1620352469567; Thu, 06 May 2021 18:54:29 -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> <02c801d7421f$65287a90$2f796fb0$@samsung.com> <001801d742db$68ab8060$3a028120$@samsung.com> <436dbc62-451b-9b29-178d-9da28f47ef24@huawei.com> In-Reply-To: <436dbc62-451b-9b29-178d-9da28f47ef24@huawei.com> From: Willem de Bruijn Date: Thu, 6 May 2021 21:53:45 -0400 Message-ID: Subject: Re: [PATCH bpf] bpf: check for data_len before upgrading mss when 6 to 4 To: Yunsheng Lin Cc: Dongseok Yi , 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 Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 6, 2021 at 9:45 PM Yunsheng Lin wrote: > > On 2021/5/7 9:25, Willem de Bruijn wrote: > >>>> head_skb's data_len is the sum of skb_gro_len for each skb of the frags. > >>>> data_len could be 8 if server sent a small size packet and it is GROed > >>>> to head_skb. > >>>> > >>>> Please let me know if I am missing something. > >>> > >>> This is my understanding of the data path. This is a forwarding path > >>> for TCP traffic. > >>> > >>> GRO is enabled and will coalesce multiple segments into a single large > >>> packet. In bad cases, the coalesced packet payload is > MSS, but < MSS > >>> + 20. > >>> > >>> Somewhere between GRO and GSO you have a BPF program that converts the > >>> IPv6 address to IPv4. > >> > >> Your understanding is right. The data path is GRO -> BPF 6 to 4 -> > >> GSO. > >> > >>> > >>> There is no concept of head_skb at the time of this BPF program. It is > >>> a single SKB, with an skb linear part and multiple data items in the > >>> frags (no frag_list). > >> > >> Sorry for the confusion. head_skb what I mentioned was a skb linear > >> part. I'm considering a single SKB with frags too. > >> > >>> > >>> When entering the GSO stack, this single skb now has a payload length > >>> < MSS. So it would just make a valid TCP packet on its own? > >>> > >>> skb_gro_len is only relevant inside the GRO stack. It internally casts > >>> the skb->cb[] to NAPI_GRO_CB. This field is a scratch area that may be > >>> reused for other purposes later by other layers of the datapath. It is > >>> not safe to read this inside bpf_skb_proto_6_to_4. > >> > >> The condition what I made uses skb->data_len not skb_gro_len. Does > >> skb->data_len have a different meaning on each layer? As I know, > >> data_len indicates the amount of frags or frag_list. skb->data_len > >> should be > 20 in the sample case because the payload size of the skb > >> linear part is the same with mss. > > > > Ah, got it. > > > > data_len is the length of the skb minus the length in the skb linear > > section (as seen in skb_headlen). > > > > So this gso skb consists of two segments, the first one entirely > > linear, the payload of the second is in skb_shinfo(skb)->frags[0]. > > > > It is not guaranteed that gso skbs built from two individual skbs end > > up looking like that. Only protocol headers in the linear segment and > > the payload of both in frags is common. > > > >> We can modify netif_needs_gso as another option to hit > >> skb_needs_linearize in validate_xmit_skb. But I think we should compare > >> skb->gso_size and skb->data_len too to check if mss exceed a payload > >> size. > > > > The rest of the stack does not build such gso packets with payload len > > < mss, so we should not have to add workarounds in the gso hot path > > for this. > > > > Also no need to linearize this skb. I think that if the bpf program > > would just clear the gso type, the packet would be sent correctly. > > Unless I'm missing something. > > Does the checksum/len field in ip and tcp/udp header need adjusting > before clearing gso type as the packet has became bigger? gro takes care of this. see for instance inet_gro_complete for updates to the ip header. > Also, instead of testing skb->data_len, may test the skb->len? > > skb->len - (mac header + ip/ipv6 header + udp/tcp header) > mss + len_diff Yes. Essentially doing the same calculation as the gso code that is causing the packet to be dropped. > > > > But I don't mean to argue that it should do that in production. > > Instead, not playing mss games would solve this and stay close to the > > original datapath if no bpf program had been present. Including > > maintaining the GSO invariant of sending out the same chain of packets > > as received (bar the IPv6 to IPv4 change). > > > > This could be achieved by adding support for the flag > > BPF_F_ADJ_ROOM_FIXED_GSO in the flags field of bpf_skb_change_proto. > > And similar to bpf_skb_net_shrink: > > > > /* Due to header shrink, MSS can be upgraded. */ > > if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO)) > > skb_increase_gso_size(shinfo, len_diff); > > > > The other case, from IPv4 to IPv6 is more difficult to address, as not > > reducing the MSS will result in packets exceeding MTU. That calls for > > workarounds like MSS clamping. Anyway, that is out of scope here. > > > > > > > >>> > >>> > >>>>> > >>>>> One simple solution if this packet no longer needs to be segmented > >>>>> might be to reset the gso_type completely. > >>>> > >>>> I am not sure gso_type can be cleared even when GSO is needed. > >>>> > >>>>> > >>>>> 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; > >>>>>>>> > >>>>>> > >>>>>> > >>>> > >> > > > > . > > >