On Mon, Aug 04, 2014 at 03:24:11PM -0700, David Miller wrote:

From: Wei Liu <redacted>
Date: Sun, 3 Aug 2014 10:11:10 +0100

quoted

On Sat, Aug 02, 2014 at 03:33:37PM -0700, David Miller wrote:

quoted

From: Wei Liu <redacted>
Date: Fri, 1 Aug 2014 12:02:46 +0100

quoted

On Thu, Jul 31, 2014 at 01:25:20PM -0700, David Miller wrote:

quoted

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.

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