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=-15.4 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 42594C433E0 for ; Fri, 5 Feb 2021 00:07:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 198FD64FA9 for ; Fri, 5 Feb 2021 00:07:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231448AbhBEAHW (ORCPT ); Thu, 4 Feb 2021 19:07:22 -0500 Received: from www62.your-server.de ([213.133.104.62]:60740 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231366AbhBEAHV (ORCPT ); Thu, 4 Feb 2021 19:07:21 -0500 Received: from sslproxy01.your-server.de ([78.46.139.224]) by www62.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92.3) (envelope-from ) id 1l7odw-000208-RC; Fri, 05 Feb 2021 01:06:36 +0100 Received: from [85.7.101.30] (helo=pc-9.home) by sslproxy01.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1l7odw-000LFw-IN; Fri, 05 Feb 2021 01:06:36 +0100 Subject: Re: [PATCH bpf-next V15 2/7] bpf: fix bpf_fib_lookup helper MTU check for SKB ctx To: Jesper Dangaard Brouer , bpf@vger.kernel.org Cc: netdev@vger.kernel.org, Daniel Borkmann , Alexei Starovoitov , maze@google.com, lmb@cloudflare.com, shaun@tigera.io, Lorenzo Bianconi , marek@cloudflare.com, John Fastabend , Jakub Kicinski , eyal.birger@gmail.com, colrack@gmail.com References: <161228314075.576669.15427172810948915572.stgit@firesoul> <161228321177.576669.11521750082473556168.stgit@firesoul> From: Daniel Borkmann Message-ID: Date: Fri, 5 Feb 2021 01:06:35 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <161228321177.576669.11521750082473556168.stgit@firesoul> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Authenticated-Sender: daniel@iogearbox.net X-Virus-Scanned: Clear (ClamAV 0.102.4/26070/Thu Feb 4 13:22:39 2021) Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On 2/2/21 5:26 PM, Jesper Dangaard Brouer wrote: > BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use > bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size, > by adjusting fib_params 'tot_len' with the packet length plus the expected > encap size. (Just like the bpf_check_mtu helper supports). He discovered > that for SKB ctx the param->tot_len was not used, instead skb->len was used > (via MTU check in is_skb_forwardable() that checks against netdev MTU). > > Fix this by using fib_params 'tot_len' for MTU check. If not provided (e.g. > zero) then keep existing TC behaviour intact. Notice that 'tot_len' for MTU > check is done like XDP code-path, which checks against FIB-dst MTU. > > V13: > - Only do ifindex lookup one time, calling dev_get_by_index_rcu(). > > V10: > - Use same method as XDP for 'tot_len' MTU check > > Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status") > Reported-by: Carlo Carraro > Signed-off-by: Jesper Dangaard Brouer > Acked-by: John Fastabend [...] I was about to apply the series just now, but on a last double check there is a subtle logic bug in here that still needs fixing unfortunately. :/ See below: > @@ -5568,7 +5565,9 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb, > struct bpf_fib_lookup *, params, int, plen, u32, flags) > { > struct net *net = dev_net(skb->dev); > + struct net_device *dev; > int rc = -EAFNOSUPPORT; > + bool check_mtu = false; > > if (plen < sizeof(*params)) > return -EINVAL; > @@ -5576,23 +5575,30 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb, > if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT)) > return -EINVAL; > > + dev = dev_get_by_index_rcu(net, params->ifindex); > + if (unlikely(!dev)) > + return -ENODEV; Based on your earlier idea, you try to avoid refetching the dev this way, so here it's being looked up via params->ifindex provided from the BPF prog ... > + if (params->tot_len) > + check_mtu = true; > + > switch (params->family) { > #if IS_ENABLED(CONFIG_INET) > case AF_INET: > - rc = bpf_ipv4_fib_lookup(net, params, flags, false); > + rc = bpf_ipv4_fib_lookup(net, dev, params, flags, check_mtu); ... however, bpf_ipv{4,6}_fib_lookup() might change params->ifindex here to indicate nexthop output dev: [...] dev = nhc->nhc_dev; params->rt_metric = res.fi->fib_priority; params->ifindex = dev->ifindex; [...] > break; > #endif > #if IS_ENABLED(CONFIG_IPV6) > case AF_INET6: > - rc = bpf_ipv6_fib_lookup(net, params, flags, false); > + rc = bpf_ipv6_fib_lookup(net, dev, params, flags, check_mtu); > break; > #endif > } > > - if (!rc) { > - struct net_device *dev; > - > - dev = dev_get_by_index_rcu(net, params->ifindex); > + if (rc == BPF_FIB_LKUP_RET_SUCCESS && !check_mtu) { > + /* When tot_len isn't provided by user, > + * check skb against net_device MTU > + */ > if (!is_skb_forwardable(dev, skb)) > rc = BPF_FIB_LKUP_RET_FRAG_NEEDED; ... so using old cached dev from above will result in wrong MTU check & subsequent passing of wrong params->mtu_result = dev->mtu this way. So one way to fix is that we would need to pass &dev to bpf_ipv{4,6}_fib_lookup(). > } > > Thanks, Daniel