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    SubjectRe: [PATCH 3/5] vhost: support upto 509 memory regions
    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@redhat.com> 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@redhat.com> 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.

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

    --
    MST


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