Re: [PATCH v3 06/14] vfio/type1: Add VFIO_IOMMU_PASID_REQUEST (alloc/free)

[Alex Williamson]

On Wed, 8 Jul 2020 08:16:16 +0000
"Liu, Yi L" <yi.l.liu@xxxxxxxxx> wrote:

> Hi Alex,
> 
> > From: Liu, Yi L < yi.l.liu@xxxxxxxxx>
> > Sent: Friday, July 3, 2020 2:28 PM
> > 
> > Hi Alex,
> >   
> > > From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > > Sent: Friday, July 3, 2020 5:19 AM
> > >
> > > On Wed, 24 Jun 2020 01:55:19 -0700
> > > Liu Yi L <yi.l.liu@xxxxxxxxx> 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.
Thanks,

Alex