Re: [Qemu-devel] How to lock-up your tap-based VM network

[Paul Brook <paul@codesourcery.com>]

> Paul Brook wrote:
> >> But anyway, this flow control mechanism is buggy - what if instead of
> >> an interface down, you just have a *slow* guest?  That should not push
> >> back so much that it makes other guests networking with each other
> >> slow down.
> >
> > The OP described multiple guests connected via a host bridge. In this
> > case it is entirely the host's responsibility to arbitrate between
> > multiple guests. If one interface can block the bridge simply by failing
> > to respond in a timely manner then this is a serious bug or
> > misconfiguration of your host bridge.
> 
> It's not the bridge, I think it's limited to the tun driver as bridge
> participant and how it is configured/used by QEMU.

If one tap interface can prevent correct operation of another by failing to 
read data, then this is clearly a host kernel bug. 
I've no idea whether it's a bug in the TAP implementation (e.g. shared queue 
between independent devices), a bug in the bridging implementation (drops 
unrelated packets when one port is slow), or some interaction between the two, 
but it's definitely not a qemu bug.    

I'm sure there are plenty of ways to DoS a qemu instance, and prevent it 
reading data from its tap interface.  How/whether this effects other unrelated 
processes on the host machine is not something qemu can or should know about.

Paul