Re: [PATCH] xen-netfront: Fix handling packets on compound pages with skb_segment

[Zoltan Kiss]

On 04/08/14 23:24, David Miller wrote:
> From: Wei Liu <wei.liu2@xxxxxxxxxx>
> 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 <wei.liu2@xxxxxxxxxx>
> > > 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
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