linux-kernel - RE: [PATCH v3 06/14] vfio/type1: Add VFIO_IOMMU_PASID

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Date:   Thu, 9 Jul 2020 00:32:27 +0000
From:   "Liu, Yi L" <yi.l.liu@...el.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>
CC:     "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>,
        "Sun, Yi Y" <yi.y.sun@...el.com>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "peterx@...hat.com" <peterx@...hat.com>,
        "Wu, Hao" <hao.wu@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v3 06/14] vfio/type1: Add VFIO_IOMMU_PASID_REQUEST
 (alloc/free)

Hi Alex,

> Alex Williamson <alex.williamson@...hat.com>
> Sent: Thursday, July 9, 2020 3:55 AM
> 
> On Wed, 8 Jul 2020 08:16:16 +0000
> "Liu, Yi L" <yi.l.liu@...el.com> wrote:
> 
> > Hi Alex,
> >
> > > From: Liu, Yi L < yi.l.liu@...el.com>
> > > Sent: Friday, July 3, 2020 2:28 PM
> > >
> > > Hi Alex,
> > >
> > > > From: Alex Williamson <alex.williamson@...hat.com>
> > > > Sent: Friday, July 3, 2020 5:19 AM
> > > >
> > > > On Wed, 24 Jun 2020 01:55:19 -0700
> > > > Liu Yi L <yi.l.liu@...el.com> wrote:
> > > >
> > > > > This patch allows user space to request PASID allocation/free, e.g.
> > > > > when serving the request from the guest.
> > > > >
> > > > > PASIDs that are not freed by userspace are automatically freed when
> > > > > the IOASID set is destroyed when process exits.
> > [...]
> > > > > +static int vfio_iommu_type1_pasid_request(struct vfio_iommu *iommu,
> > > > > +					  unsigned long arg)
> > > > > +{
> > > > > +	struct vfio_iommu_type1_pasid_request req;
> > > > > +	unsigned long minsz;
> > > > > +
> > > > > +	minsz = offsetofend(struct vfio_iommu_type1_pasid_request,
> range);
> > > > > +
> > > > > +	if (copy_from_user(&req, (void __user *)arg, minsz))
> > > > > +		return -EFAULT;
> > > > > +
> > > > > +	if (req.argsz < minsz || (req.flags &
> ~VFIO_PASID_REQUEST_MASK))
> > > > > +		return -EINVAL;
> > > > > +
> > > > > +	if (req.range.min > req.range.max)
> > > >
> > > > Is it exploitable that a user can spin the kernel for a long time in
> > > > the case of a free by calling this with [0, MAX_UINT] regardless of their
> actual
> > > allocations?
> > >
> > > IOASID can ensure that user can only free the PASIDs allocated to the user.
> but
> > > it's true, kernel needs to loop all the PASIDs within the range provided
> > > by user.
> it
> > > may take a long time. is there anything we can do? one thing may limit the
> range
> > > provided by user?
> >
> > thought about it more, we have per-VM pasid quota (say 1000), so even if
> > user passed down [0, MAX_UNIT], kernel will only loop the 1000 pasids at
> > most. do you think we still need to do something on it?
> 
> How do you figure that?  vfio_iommu_type1_pasid_request() accepts the
> user's min/max so long as (max > min) and passes that to
> vfio_iommu_type1_pasid_free(), then to vfio_pasid_free_range()  which
> loops as:
> 
> 	ioasid_t pasid = min;
> 	for (; pasid <= max; pasid++)
> 		ioasid_free(pasid);
> 
> A user might only be able to allocate 1000 pasids, but apparently they
> can ask to free all they want.
> 
> It's also not obvious to me that calling ioasid_free() is only allowing
> the user to free their own passid.  Does it?  It would be a pretty
> gaping hole if a user could free arbitrary pasids.  A r-b tree of
> passids might help both for security and to bound spinning in a loop.

oh, yes. BTW. instead of r-b tree in VFIO, maybe we can add an ioasid_set
parameter for ioasid_free(), thus to prevent the user from freeing PASIDs
that doesn't belong to it. I remember Jacob mentioned it before.

@Jacob, is it still in your plan?

Regards,
Yi Liu

> Thanks,
> 
> Alex