From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753180AbaHDWYO (ORCPT ); Mon, 4 Aug 2014 18:24:14 -0400 Received: from shards.monkeyblade.net ([149.20.54.216]:46696 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751939AbaHDWYM (ORCPT ); Mon, 4 Aug 2014 18:24:12 -0400 Date: Mon, 04 Aug 2014 15:24:11 -0700 (PDT) Message-Id: <20140804.152411.1486639478907964423.davem@davemloft.net> To: wei.liu2@citrix.com Cc: zoltan.kiss@citrix.com, konrad.wilk@oracle.com, boris.ostrovsky@oracle.com, david.vrabel@citrix.com, Ian.Campbell@citrix.com, paul.durrant@citrix.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org Subject: Re: [PATCH] xen-netfront: Fix handling packets on compound pages with skb_segment From: David Miller In-Reply-To: <20140803091110.GA7093@zion.uk.xensource.com> References: <20140801110246.GB17947@zion.uk.xensource.com> <20140802.153337.1399638752235662112.davem@davemloft.net> <20140803091110.GA7093@zion.uk.xensource.com> X-Mailer: Mew version 6.5 on Emacs 24.1 / Mule 6.0 (HANACHIRUSATO) Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.5.7 (shards.monkeyblade.net [149.20.54.216]); Mon, 04 Aug 2014 15:24:12 -0700 (PDT) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Wei Liu Date: Sun, 3 Aug 2014 10:11:10 +0100 > On Sat, Aug 02, 2014 at 03:33:37PM -0700, David Miller wrote: >> From: Wei Liu >> Date: Fri, 1 Aug 2014 12:02:46 +0100 >> >> > On Thu, Jul 31, 2014 at 01:25:20PM -0700, David Miller wrote: >> >> If you were to have a 64-slot TX queue, you ought to be able to handle >> >> this theoretical 51 slot SKB. >> > >> > There's two problems: >> > 1. IIRC a single page ring has 256 slots, allowing 64 slots packet >> > yields 4 in-flight packets in worst case. >> > 2. Older netback could not handle this large number of slots and it's >> > likely to deem the frontend malicious. >> > >> > For #1, we don't actually care that much if guest screws itself by >> > generating 64 slot packets. #2 is more concerning. >> >> How many slots can the older netback handle? > > I listed those two problems in the context "if we were to lift this > limit in the latest net-next tree", so "older netback" actually refers > to netback from 3.10 to 3.16. > > The current implementation allows the number of slots X: > 1. X <= 18, valid packet > 2. 18 < X < fatal_slot_count, dropped > 3. X >= fatal_slot_count, malicious frontend > > fatal_slot_count has default value of 20. Given what I've seen so far, I think the only option is to linearize the packet. BTW, we do have a netdev->gso_max_segs tunable drivers can set, but it might not cover all of the cases you need to handle. Maybe we can create a similar tunable which triggers skb_needs_linearize() in the transmit path. The advantage of such a tunable is that this can be worked with inside of TCP to avoid creating such packets in the first place. For example, all of the MAX_SKB_FRAGS checks you see in net/ipv4/tcp.c could be replaced with tests against this new tunable in struct netdevice.