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

[Michael S. Tsirkin]

On Thu, Jun 18, 2015 at 11:12:24AM +0200, Igor Mammedov wrote:
> On Wed, 17 Jun 2015 18:30:02 +0200
> "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> 
> > On Wed, Jun 17, 2015 at 06:09:21PM +0200, Igor Mammedov wrote:
> > > On Wed, 17 Jun 2015 17:38:40 +0200
> > > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> > > 
> > > > 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.
> > > then it just means that tool has to take into account a new limits
> > > to partition host in sensible manner.
> > 
> > Meanwhile old tools are vulnerable to OOM attacks.
> I've chatted with libvirt folks, it doesn't care about how much memory
> vhost would consume nor do any host capacity planning in that regard.

Exactly, it's up to host admin.

> But lets assume that there are tools that do this so
> how about instead of hardcoding limit make it a module parameter
> with default set to 64. That would allow users to set higher limit
> if they need it and nor regress old tools. it will also give tools
> interface for reading limit from vhost module.

And now you need to choose between security and functionality :(

> > 
> > > Exposing limit as module parameter might be of help to tool for
> > > getting/setting it in a way it needs.
> > 
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