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

["Liu, Yi L" <yi.l.liu@intel.com>]

Hi Kevin,

> From: Tian, Kevin <kevin.tian@intel.com>
> Sent: Thursday, July 9, 2020 9:57 AM
> 
> > From: Liu, Yi L <yi.l.liu@intel.com>
> > Sent: Thursday, July 9, 2020 8:32 AM
> >
> > Hi Alex,
> >
> > > Alex Williamson <alex.williamson@redhat.com>
> > > Sent: Thursday, July 9, 2020 3:55 AM
> > >
> > > On Wed, 8 Jul 2020 08:16:16 +0000
> > > "Liu, Yi L" <yi.l.liu@intel.com> wrote:
> > >
> > > > Hi Alex,
> > > >
> > > > > From: Liu, Yi L < yi.l.liu@intel.com>
> > > > > Sent: Friday, July 3, 2020 2:28 PM
> > > > >
> > > > > Hi Alex,
> > > > >
> > > > > > From: Alex Williamson <alex.williamson@redhat.com>
> > > > > > Sent: Friday, July 3, 2020 5:19 AM
> > > > > >
> > > > > > On Wed, 24 Jun 2020 01:55:19 -0700 Liu Yi L
> > > > > > <yi.l.liu@intel.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
> 
> 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.

yeah, how about the below two options:

- comparing the max - min with the quota before calling ioasid_free().
  If max - min > current quota of the user, then should fail it. If
  max - min < quota, then call ioasid_free() one by one. still trigger
  the above lock path with quota times.

- pass the max and min to ioasid_free(), let ioasid_free() decide. should
  be able to avoid trigger the lock multiple times, and ioasid has have a
  track on how may PASIDs have been allocated, if max - min is larger than
  the allocated number, should fail anyway.
 
thoughts on the above reply?

Regards,
Yi Liu

> Thanks
> Kevin