linux-kernel - Re: Plan for /dev/ioasid RFC v2

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Date:   Thu, 17 Jun 2021 21:52:39 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Jason Wang <jasowang@...hat.com>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "Enrico Weigelt, metux IT consult" <lkml@...ux.net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Shenming Lu <lushenming@...wei.com>,
        Eric Auger <eric.auger@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Robin Murphy <robin.murphy@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        David Woodhouse <dwmw2@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>
Subject: Re: Plan for /dev/ioasid RFC v2

On Thu, Jun 17, 2021 at 07:31:03AM +0000, Tian, Kevin wrote:
> > > Yes. function 1 is block-DMA while function 0 still attached to IOASID.
> > > Actually unbind from IOMMU fd doesn't change the security context.
> > > the change is conducted when attaching/detaching device to/from an
> > > IOASID.
> > 
> > But I think you're suggesting that the IOMMU context is simply the
> > device's default domain, so vfio is left in the position where the user
> > gained access to the device by binding it to an iommu_fd, but now the
> > device exists outside of the iommu_fd.

I don't think unbind should be allowed. Close the fd and re-open it if
you want to attach to a different iommu_fd.

> > to gate device access on binding the device to the iommu_fd?  The user
> > can get an accessible device_fd unbound from an iommu_fd on the reverse
> > path.
> 
> yes, binding to iommu_fd is not the appropriate point of gating
> device access.

Binding is the only point we have enough information to make a
full security decision. Device FDs that are not bound must be
inoperable until bound.

The complexities with revoking mmap/etc are what lead me to conclude
that unbind is not worth doing - we can't go back to an inoperable
state very easially.

> Yes, that was the original impression. But after figuring out the new
> block-DMA behavior, I'm not sure whether /dev/iommu must maintain
> its own group integrity check. If it trusts vfio, I feel it's fine to avoid 
> such check which even allows a group of devices bound to different
> IOMMU fd's if user likes. Also if we want to sustain the current vfio
> semantics which doesn't require all devices in the group bound to
> vfio driver, seems it's pointless to enforce such integrity check in
> /dev/iommu.
> 
> Jason, what's your opinion?

I think the iommu code should do all of this, I don't see why vfio
should be dealing with *iommu* isolation.

The rest of this email got a bit long for me to catch up on, sorry :\

Jason