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

[David Miller]

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.

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.
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