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

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Date:   Wed, 8 Jul 2020 08:16:16 +0000
From:   "Liu, Yi L" <yi.l.liu@...el.com>
To:     "Liu, Yi L" <yi.l.liu@...el.com>,
        Alex Williamson <alex.williamson@...hat.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>,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.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,

> 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?

Regards,
Yi Liu