Re: [PATCH RFC v2 00/13] IOMMUFD Generic interface

Daniel P. Berrangé
2022-09-23T08:55:08Z

On Thu, Sep 22, 2022 at 12:31:20PM -0300, Jason Gunthorpe wrote:
> On Thu, Sep 22, 2022 at 04:00:00PM +0100, Daniel P. Berrangé wrote:
> > On Thu, Sep 22, 2022 at 11:51:54AM -0300, Jason Gunthorpe wrote:
> > > On Thu, Sep 22, 2022 at 03:49:02PM +0100, Daniel P. Berrangé wrote:
> > > > On Thu, Sep 22, 2022 at 11:08:23AM -0300, Jason Gunthorpe wrote:
> > > > > On Thu, Sep 22, 2022 at 12:20:50PM +0100, Daniel P. Berrangé wrote:
> > > > > > On Wed, Sep 21, 2022 at 03:44:24PM -0300, Jason Gunthorpe wrote:
> > > > > > > On Wed, Sep 21, 2022 at 12:06:49PM -0600, Alex Williamson wrote:
> > > > > > > > The issue is where we account these pinned pages, where accounting is
> > > > > > > > necessary such that a user cannot lock an arbitrary number of pages
> > > > > > > > into RAM to generate a DoS attack.
> > > > > > >
> > > > > > > It is worth pointing out that preventing a DOS attack doesn't actually
> > > > > > > work because a *task* limit is trivially bypassed by just spawning
> > > > > > > more tasks. So, as a security feature, this is already very
> > > > > > > questionable.
> > > > > >
> > > > > > The malicious party on host VM hosts is generally the QEMU process.
> > > > > > QEMU is normally prevented from spawning more tasks, both by SELinux
> > > > > > controls and be the seccomp sandbox blocking clone() (except for
> > > > > > thread creation). We need to constrain what any individual QEMU can
> > > > > > do to the host, and the per-task mem locking limits can do that.
> > > > >
> > > > > Even with syscall limits simple things like execve (enabled eg for
> > > > > qemu self-upgrade) can corrupt the kernel task-based accounting to the
> > > > > point that the limits don't work.
> > > >
> > > > Note, execve is currently blocked by default too by the default
> > > > seccomp sandbox used with libvirt, as well as by the SELinux
> > > > policy again. self-upgrade isn't a feature that exists (yet).
> > >
> > > That userspace has disabled half the kernel isn't an excuse for the
> > > kernel to be insecure by design :( This needs to be fixed to enable
> > > features we know are coming so..
> > >
> > > What would libvirt land like to see given task based tracking cannot
> > > be fixed in the kernel?
> >
> > There needs to be a mechanism to control individual VMs, whether by
> > task or by cgroup. User based limits are not suited to what we need
> > to achieve.
>
> The kernel has already standardized on user based limits here for
> other subsystems - libvirt and qemu cannot ignore that it exists. It
> is only a matter of time before qemu starts using these other
> subsystem features (eg io_uring) and has problems.
>
> So, IMHO, the future must be that libvirt/etc sets an unlimited
> rlimit, because the user approach is not going away in the kernel and
> it sounds like libvirt cannot accommodate it at all.
>
> This means we need to provide a new mechanism for future libvirt to
> use. Are you happy with cgroups?

Yes, we use cgroups extensively already.

With regards,
Daniel
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