From mboxrd@z Thu Jan 1 00:00:00 1970 From: Zoltan Kiss Subject: Re: [PATCH] xen-netfront: Fix handling packets on compound pages with skb_segment Date: Tue, 5 Aug 2014 14:00:30 +0100 Message-ID: <53E0D56E.7020205@citrix.com> References: <20140801110246.GB17947@zion.uk.xensource.com> <20140802.153337.1399638752235662112.davem@davemloft.net> <20140803091110.GA7093@zion.uk.xensource.com> <20140804.152411.1486639478907964423.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit Cc: , , , , , , , To: David Miller , Return-path: In-Reply-To: <20140804.152411.1486639478907964423.davem@davemloft.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 04/08/14 23:24, David Miller wrote: > 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. I think that would have more performance penalty than calling skb_gso_segment, but maybe I'm wrong. > > 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. Indeed. Even a packet with one frag can be too scattered for us. > > 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. You would need to implement xennet_count_skb_frag_slots and count the slots for every skb heading to a device with this tunable set. And not just for TCP, but for any packet source. I think it would be better to check for that tunable in dev_hard_start_xmit, and mask out the GSO bits in 'features' to force segmentation there. That would do essentially the same as this patch, but not in the netfront's start_xmit. One minor flaw is that it does one round of segmentation only, which doesn't handle the theoretical worst case. Zoli