linux-kernel - Re: [RFC] /dev/ioasid uAPI proposal

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Date:   Fri, 4 Jun 2021 13:03:36 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Robin Murphy <robin.murphy@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Kirti Wankhede <kwankhede@...dia.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

On Fri, Jun 04, 2021 at 05:57:19PM +0200, Paolo Bonzini wrote:
> On 04/06/21 17:50, Jason Gunthorpe wrote:
> > > Extending the scenarios where WBINVD is not a nop is not a problem for me.
> > > If possible I wouldn't mind keeping the existing kvm-vfio connection via the
> > > device, if only because then the decision remains in the VFIO camp (whose
> > > judgment I trust more than mine on this kind of issue).
> > Really the question to answer is what "security proof" do you want
> > before the wbinvd can be enabled
> 
> I don't want a security proof myself; I want to trust VFIO to make the right
> judgment and I'm happy to defer to it (via the KVM-VFIO device).
> 
> Given how KVM is just a device driver inside Linux, VMs should be a slightly
> more roundabout way to do stuff that is accessible to bare metal; not a way
> to gain extra privilege.

Okay, fine, lets turn the question on its head then.

VFIO should provide a IOCTL VFIO_EXECUTE_WBINVD so that userspace VFIO
application can make use of no-snoop optimizations. The ability of KVM
to execute wbinvd should be tied to the ability of that IOCTL to run
in a normal process context.

So, under what conditions do we want to allow VFIO to giave a process
elevated access to the CPU:

> >   1) User has access to a device that can issue no-snoop TLPS
> >   2) User has access to an IOMMU that can not block no-snoop (today)
> >   3) Require CAP_SYS_RAW_IO
> >   4) Anyone

Jason