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

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.


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