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

Daniel P. Berrangé
2022-09-22T14:49:25Z

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).

With regards,
Daniel
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