From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wei Liu Subject: Re: [PATCH] xen-netfront: Fix handling packets on compound pages with skb_segment Date: Tue, 5 Aug 2014 11:53:32 +0100 Message-ID: <20140805105332.GC11230@zion.uk.xensource.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="us-ascii" Cc: , , , , , , , , , To: David Miller Return-path: Content-Disposition: inline In-Reply-To: <20140804.152411.1486639478907964423.davem@davemloft.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Mon, Aug 04, 2014 at 03:24:11PM -0700, 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. > > 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. +1 for this. Avoiding generating such packets in transmit path in the first place is even better. Wei. > -- > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html