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=-5.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 30401C47404 for ; Fri, 11 Oct 2019 15:42:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F1B19206A1 for ; Fri, 11 Oct 2019 15:42:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=messagingengine.com header.i=@messagingengine.com header.b="na9Zif5U" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728142AbfJKPma (ORCPT ); Fri, 11 Oct 2019 11:42:30 -0400 Received: from out4-smtp.messagingengine.com ([66.111.4.28]:45527 "EHLO out4-smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726521AbfJKPm3 (ORCPT ); Fri, 11 Oct 2019 11:42:29 -0400 Received: from compute3.internal (compute3.nyi.internal [10.202.2.43]) by mailout.nyi.internal (Postfix) with ESMTP id B841721391; Fri, 11 Oct 2019 11:42:28 -0400 (EDT) Received: from mailfrontend2 ([10.202.2.163]) by compute3.internal (MEProxy); Fri, 11 Oct 2019 11:42:28 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d= messagingengine.com; h=cc:content-type:date:from:in-reply-to :message-id:mime-version:references:subject:to:x-me-proxy :x-me-proxy:x-me-sender:x-me-sender:x-sasl-enc; s=fm1; bh=JmBCnY ZbCaZgiYppXViAaVqhV8l/6mNGGak957ySjZo=; b=na9Zif5U0MHh4vp23OCYBz 2Fyq8yt0OmH7CF92d3FfejV4CNkHRghAwrNuyfjLhyVJU7JGynfgWvWRdYDdsNW7 vtAA3UcgIj2IHoaV6TlGLSu4QbFgyIKTNa1D97TdHIScF+EYVbyxkh2jVmzFBsQn XFZ+iO7anW8z//bpwAF8V+anUkxyiQqQaJMbtTzCyZVdWrIkKobN8XhzqhhsS6wZ bjAw21jZQoa/M2ZMOE8upFxwfG5P4PaTAYTCbbzJ3WXECbWRWh0XrGbI7tPwYZLx jQtEsHuouS7p68sMJiZnf8h5L4qlLzX+XG3wu3GmaA/NaHsnox83AGRDy8U55c+w == X-ME-Sender: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgedufedrieehgdelgecutefuodetggdotefrodftvf curfhrohhfihhlvgemucfhrghsthforghilhdpqfgfvfdpuffrtefokffrpgfnqfghnecu uegrihhlohhuthemuceftddtnecusecvtfgvtghiphhivghnthhsucdlqddutddtmdenuc fjughrpeffhffvuffkfhggtggujggfsehttdertddtredvnecuhfhrohhmpefkughoucfu tghhihhmmhgvlhcuoehiughoshgthhesihguohhstghhrdhorhhgqeenucffohhmrghinh epkhgvrhhnvghlrdhorhhgpdhfohhrvghvvghrrdhnohenucfkphepjeelrddujeejrdef kedrvddtheenucfrrghrrghmpehmrghilhhfrhhomhepihguohhstghhsehiughoshgthh drohhrghenucevlhhushhtvghrufhiiigvpedt X-ME-Proxy: Received: from localhost (bzq-79-177-38-205.red.bezeqint.net [79.177.38.205]) by mail.messagingengine.com (Postfix) with ESMTPA id EED9DD6005F; Fri, 11 Oct 2019 11:42:27 -0400 (EDT) Date: Fri, 11 Oct 2019 18:42:24 +0300 From: Ido Schimmel To: Jesse Hathaway , weiwan@google.com Cc: netdev@vger.kernel.org Subject: Re: Race condition in route lookup Message-ID: <20191011154224.GA23486@splinter> References: <20191010083102.GA1336@splinter> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.12.1 (2019-06-15) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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? 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"). 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