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

Jason Gunthorpe
2022-09-22T14:08:49Z

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.

Also, you are skipping the fact that since every subsystem does this
differently and wrong a qemu can still go at least 3x over the
allocation using just normal allowed functionalities.

Again, as a security feature this fundamentally does not work. We
cannot account for a FD owned resource inside the task based
mm_struct. There are always going to be exploitable holes.

What you really want is a cgroup based limit that is consistently
applied in the kernel.

Regardless, since this seems pretty well entrenched I continue to
suggest my simpler alternative of making it fd based instead of user
based. At least that doesn't have the unsolvable bugs related to task
accounting.

Jason