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

[Tian, Kevin]

> From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> Sent: Thursday, July 9, 2020 8:32 AM
> 
> Hi Alex,
> 
> > Alex Williamson <alex.williamson@xxxxxxxxxx>
> > Sent: Thursday, July 9, 2020 3:55 AM
> >
> > 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

Agree. I thought ioasid_free should at least carry a token since the
user space is only allowed to manage PASIDs in its own set...

> > 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.
> 

check current ioasid_free:

        spin_lock(&ioasid_allocator_lock);
        ioasid_data = xa_load(&active_allocator->xa, ioasid);
        if (!ioasid_data) {
                pr_err("Trying to free unknown IOASID %u\n", ioasid);
                goto exit_unlock;
        }

Allow an user to trigger above lock paths with MAX_UINT times might still
be bad. 

Thanks
Kevin