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=-11.4 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=no 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 D0C5EECE58D for ; Fri, 11 Oct 2019 17:54:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9FF69206A1 for ; Fri, 11 Oct 2019 17:54:29 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="dQSWOWZ7" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728701AbfJKRy2 (ORCPT ); Fri, 11 Oct 2019 13:54:28 -0400 Received: from mail-io1-f65.google.com ([209.85.166.65]:43949 "EHLO mail-io1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728470AbfJKRy2 (ORCPT ); Fri, 11 Oct 2019 13:54:28 -0400 Received: by mail-io1-f65.google.com with SMTP id v2so23293465iob.10 for ; Fri, 11 Oct 2019 10:54:26 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=klWwlYRs7+LiJ2Y5+2gsV3kwDdcfDNsLegAGXEtU4i0=; b=dQSWOWZ7M6wFJaU+LTCyc3CiXtYMDCw+raEAw7k0q1FD2xbJfCMGCbomG//iL/TJtZ ND4UhmTS+ka/BWsmb5CPRZ0Luq/vxxkRWbIAVMSe751LRfyhZbHU0rvCKO56z7I8UyKZ +USixZkh/okkxHBjsX7W/uIdwqp3t/pMR6lRER89qLeQQGvoUTPwsmCXYXC6hP4RIbLu s0QBTDsxqNt05+l9X7tlJXdG0pj95lOg0tzeINmhr+5ySREQmtLdU56MXdfj8cnTpKeK C5gzLUrP8tU73eBhStGabW1jrI+ROedkLxwQAkjCLF0wBxWoxNUiGI7PjXtgYH3t6YBp GNGg== 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=klWwlYRs7+LiJ2Y5+2gsV3kwDdcfDNsLegAGXEtU4i0=; b=GuocBBhBqA0+1rp0jcCCpPfK7c9YVmMbljSGO29QOhoWi/iSCVeTALTtqzSWAGUJik vJFSY2fdQjqjVZeYncZ6bnYD1g4jPnXrwVXTBUHbcewqR1Mh7Coq1mqQbkM1uKyGxcXP 2ln2Vad5TEF/Sp7LGJ1/C0QoQ6PtOeATv1F1hQU/8BPKFaHpGu7pxLcdaNBqWkXifqDl HLpgDj6zwEhnrwjlfqNnG3mXftFlAzZ88FjosQlzQTVNU2AyiseunQbyW4RWY4MAQ42v Y2P/u15+ptWMF9j98g090MfGmel+z03gFymPoeT4RtZOzC34ug4g7eJKTxyXgYN83lI0 kmyA== X-Gm-Message-State: APjAAAXwRkPURVbOR8xkx2aaDPAF3HOkGgIt53z/mJoiRjGBY3fsYFIq hsAN8qI1YoOl5GNQ0tLxaicZ8s1jEdu8qa3IAp+33w== X-Google-Smtp-Source: APXvYqwDqPDWzie+Xw3ejHxcjAEyzyrjgblPVTJWk830kS+dlZb/52/9NviV5zzjTGxQFVAPU4yRGOyKRdFVeMdWhDs= X-Received: by 2002:a6b:5a09:: with SMTP id o9mr2376387iob.241.1570816465203; Fri, 11 Oct 2019 10:54:25 -0700 (PDT) MIME-Version: 1.0 References: <20191010083102.GA1336@splinter> <20191011154224.GA23486@splinter> In-Reply-To: <20191011154224.GA23486@splinter> From: Wei Wang Date: Fri, 11 Oct 2019 10:54:13 -0700 Message-ID: Subject: Re: Race condition in route lookup To: Ido Schimmel , Martin KaFai Lau Cc: Jesse Hathaway , Linux Kernel Network Developers Content-Type: text/plain; charset="UTF-8" Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Fri, Oct 11, 2019 at 8:42 AM Ido Schimmel wrote: > > On Fri, Oct 11, 2019 at 09:36:51AM -0500, Jesse Hathaway wrote: > > On Thu, Oct 10, 2019 at 3:31 AM Ido Schimmel wrote: > > > I think it's working as expected. Here is my theory: > > > > > > If CPU0 is executing both the route get request and forwarding packets > > > through the directly connected interface, then the following can happen: > > > > > > - In process context, per-CPU dst entry cached in the nexthop > > > is found. Not yet dumped to user space > > > > > > - Routes are added / removed, therefore invalidating the > > > cache by bumping 'net->ipv4.rt_genid' > > > > > > - In softirq, packet is forwarded through the nexthop. The > > > cached dst entry is found to be invalid. Therefore, it is replaced by a > > > newer dst entry. dst_dev_put() is called on old entry which assigns the > > > blackhole netdev to 'dst->dev'. This netdev has an ifindex of 0 because > > > it is not registered. > > > > > > - After softirq finished executing, your route get request > > > from t0 is resumed and the old dst entry is dumped to user space with > > > ifindex of 0. > > > > > > I tested this on my system using your script to generate the route get > > > requests. I pinned it to the same CPU forwarding packets through the > > > nexthop. To constantly invalidate the cache I created another script > > > that simply adds and removes IP addresses from an interface. > > > > > > If I stop the packet forwarding or the script that invalidates the > > > cache, then I don't see any '*' answers to my route get requests. > > > > Thanks for the reply and analysis Ido, I tested with an additional script which > > adds and deletes a route in a loop, as you also saw this increased the > > frequency of blackhole route replies from the first script. > > > > Questions: > > > > 1. We saw this behavior occurring with TCP connections traversing our routers, > > though I was able to reproduce it with only local route requests on our router. > > Would you expect this same behavior for TCP traffic only in the kernel which > > does not go to userspace? > > Yes, the problem is in the input path where received packets need to be > forwarded. > > > > > 2. These blackhole routes occur even though our main routing table is not > > changing, however a separate route table managed by bird on the Linux router is > > changing. Is this still expected behavior given that the ip-rules and main > > route table used by these route requests are not changing? > > Yes, there is a per-netns counter that is incremented whenever cached > dst entries need to be invalidated. Since it is per-netns it is > incremented regardless of the routing table to which your insert the > route. > > > > > 3. We were previously rejecting these packets with an iptables rule which sent > > an ICMP prohibited message to the sender, this caused TCP connections to break > > with a EHOSTUNREACH, should we be silently dropping these packets instead? > > > > 4. If we should just be dropping these packets, why does the kernel not drop > > them instead of letting them traverse the iptables rules? > > I actually believe the current behavior is a bug that needs to be fixed. > See below. > > > > > > BTW, the blackhole netdev was added in 5.3. I assume (didn't test) that > > > with older kernel versions you'll see 'lo' instead of '*'. > > > > Yes indeed! Thanks for solving that mystery as well, our routers are running > > 5.1, but we upgraded to 5.4-rc2 to determine whether the issue was still > > present in the latest kernel. > > Do you remember when you started seeing this behavior? I think it > started in 4.13 with commit ffe95ecf3a2e ("Merge branch > 'net-remove-dst-garbage-collector-logic'"). > > Let me add Wei to see if/how this can be fixed. > > Wei, in case you don't have the original mail with the description of > the problem, it can be found here [1]. > > I believe that the issue Jesse is experiencing is the following: > > - Received packet A is forwarded and cached dst entry is > taken from the nexthop ('nhc->nhc_rth_input'). Calls skb_dst_set() > > - Given Jesse has busy routers ("ingesting full BGP routing tables > from multiple ISPs"), route is added / deleted and rt_cache_flush() is > called > > - Received packet B tries to use the same cached dst entry > from t0, but rt_cache_valid() is no longer true and it is replaced in > rt_cache_route() by the newer one. This calls dst_dev_put() on the > original dst entry which assigns the blackhole netdev to 'dst->dev' > > - dst_input(skb) is called on packet A and it is dropped due > to 'dst->dev' being the blackhole netdev > > The following patch "fixes" the problem for me: > > diff --git a/net/ipv4/route.c b/net/ipv4/route.c > index 42221a12bdda..1c67bdb80fd5 100644 > --- a/net/ipv4/route.c > +++ b/net/ipv4/route.c > @@ -1482,7 +1482,6 @@ static bool rt_cache_route(struct fib_nh_common *nhc, struct rtable *rt) > prev = cmpxchg(p, orig, rt); > if (prev == orig) { > if (orig) { > - dst_dev_put(&orig->dst); > dst_release(&orig->dst); > } > } else { > > But if this dst entry is cached in some inactive socket and the netdev > on which it took a reference needs to be unregistered, then we can > potentially wait forever. No? > Yes. That's exactly the reason we need to free the dev here. Otherwise as you described, we will see "unregister_netdevice: waiting for xxx to become free. Usage count = x" flushing the screen... Not fun... > I'm thinking that it can be fixed by making 'nhc_rth_input' per-CPU, in > a similar fashion to what Eric did in commit d26b3a7c4b3b ("ipv4: percpu > nh_rth_output cache"). > Hmm... Yes... I would think a per-CPU input cache should work for the case above. Another idea is: instead of calling dst_dev_put() in rt_cache_route() to switch out the dev, we call, rt_add_uncached_list() to add this obsolete dst cache to the uncached list. And if the device gets unregistered, rt_flush_dev() takes care of all dst entries in the uncached list. I think that would work too. diff --git a/net/ipv4/route.c b/net/ipv4/route.c index dc1f510a7c81..ee618d4234ce 100644 --- a/net/ipv4/route.c +++ b/net/ipv4/route.c @@ -1482,7 +1482,7 @@ static bool rt_cache_route(struct fib_nh_common *nhc, struct rtable *rt) prev = cmpxchg(p, orig, rt); if (prev == orig) { if (orig) { - dst_dev_put(&orig->dst); + rt_add_uncached_list(orig); dst_release(&orig->dst); } } else { + Martin for his idea and input. > Two questions: > > 1. Do you agree with the above analysis? > 2. Do you have a simpler/better solution in mind? > > Thanks > > [1] https://lore.kernel.org/netdev/CANSNSoVM1Uo106xfJtGpTyXNed8kOL4JiXqf3A1eZHBa7z3=yg@mail.gmail.com/T/#medece9445d617372b4842d44525ef0d3ba1ea083