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

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Date:	Wed, 17 Jun 2015 15:13:22 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Igor Mammedov <imammedo@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	pbonzini@...hat.com
Subject: Re: [PATCH 3/5] vhost: support upto 509 memory regions

On Wed, Jun 17, 2015 at 02:23:39PM +0200, Igor Mammedov wrote:
> On Wed, 17 Jun 2015 13:51:56 +0200
> "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
> > On Wed, Jun 17, 2015 at 01:48:03PM +0200, Igor Mammedov wrote:
> > > > > So far it's kernel limitation and this patch fixes crashes
> > > > > that users see now, with the rest of patches enabling performance
> > > > > not to regress.
> > > > 
> > > > When I say regression I refer to an option to limit the array
> > > > size again after userspace started using the larger size.
> > > Is there a need to do so?
> > 
> > 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.

> > 
> > > Userspace that cares about memory footprint won't use many slots
> > > keeping it low and user space that can't do without many slots
> > > or doesn't care will have bigger memory footprint.
> > 
> > We really can't trust userspace to do the right thing though.
> > 
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