Re: [PATCH 3/5] vhost: support upto 509 memory regions

[Michael S. Tsirkin]

On Wed, Jun 17, 2015 at 05:12:57PM +0200, Igor Mammedov wrote:
> On Wed, 17 Jun 2015 16:32:02 +0200
> "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> 
> > On Wed, Jun 17, 2015 at 03:20:44PM +0200, Paolo Bonzini wrote:
> > > 
> > > 
> > > On 17/06/2015 15:13, Michael S. Tsirkin wrote:
> > > > > > Considering userspace can be malicious, I guess yes.
> > > > > I don't think it's a valid concern in this case,
> > > > > setting limit back from 509 to 64 will not help here in any way,
> > > > > userspace still can create as many vhost instances as it needs
> > > > > to consume memory it desires.
> > > > 
> > > > Not really since vhost char device isn't world-accessible.
> > > > It's typically opened by a priveledged tool, the fd is
> > > > then passed to an unpriveledged userspace, or permissions dropped.
> > > 
> > > Then what's the concern anyway?
> > > 
> > > Paolo
> > 
> > Each fd now ties up 16K of kernel memory.  It didn't use to, so
> > priveledged tool could safely give the unpriveledged userspace
> > a ton of these fds.
> if privileged tool gives out unlimited amount of fds then it
> doesn't matter whether fd ties 4K or 16K, host still could be DoSed.
> 

Of course it does not give out unlimited fds, there's a way
for the sysadmin to specify the number of fds. Look at how libvirt
uses vhost, it should become clear I think.

-- 
MST
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