Re: [PATCH v2 3/3] iommu/vt-d: Sanity check uapi argsz filled by users

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 11 Jun 2020 13:02:24 -0700
Jacob Pan <jacob.jun.pan@linux.intel.com> wrote:

> On Thu, 11 Jun 2020 11:08:16 -0600
> Alex Williamson <alex.williamson@redhat.com> wrote:
> 
> > On Wed, 10 Jun 2020 21:12:15 -0700
> > Jacob Pan <jacob.jun.pan@linux.intel.com> wrote:
> >   
> > > IOMMU UAPI data has an argsz field which is filled by user. As the
> > > data structures expands, argsz may change. As the UAPI data are
> > > shared among different architectures, extensions of UAPI data could
> > > be a result of one architecture which has no impact on another.
> > > Therefore, these argsz santity checks are performed in the model
> > > specific IOMMU drivers. This patch adds sanity checks in the VT-d
> > > to ensure argsz passed by userspace matches feature flags and other
> > > contents.
> > > 
> > > Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > > ---
> > >  drivers/iommu/intel-iommu.c | 16 ++++++++++++++++
> > >  drivers/iommu/intel-svm.c   | 12 ++++++++++++
> > >  2 files changed, 28 insertions(+)
> > > 
> > > diff --git a/drivers/iommu/intel-iommu.c
> > > b/drivers/iommu/intel-iommu.c index 27ebf4b9faef..c98b5109684b
> > > 100644 --- a/drivers/iommu/intel-iommu.c
> > > +++ b/drivers/iommu/intel-iommu.c
> > > @@ -5365,6 +5365,7 @@ intel_iommu_sva_invalidate(struct
> > > iommu_domain *domain, struct device *dev, struct device_domain_info
> > > *info; struct intel_iommu *iommu;
> > >  	unsigned long flags;
> > > +	unsigned long minsz;
> > >  	int cache_type;
> > >  	u8 bus, devfn;
> > >  	u16 did, sid;
> > > @@ -5385,6 +5386,21 @@ intel_iommu_sva_invalidate(struct
> > > iommu_domain *domain, struct device *dev, if (!(dmar_domain->flags
> > > & DOMAIN_FLAG_NESTING_MODE)) return -EINVAL;
> > >  
> > > +	minsz = offsetofend(struct iommu_cache_invalidate_info,
> > > padding);    
> > 
> > Would it still be better to look for the end of the last field that's
> > actually used to avoid the code churn and oversights if/when the
> > padding field does get used and renamed?
> >   
> My thought was that if the padding gets partially re-purposed, the
> remaining padding would still be valid for minsz check. The extension
> rule ensures that there is no size change other the variable size union
> at the end. So use padding would avoid the churn, or i am totally wrong?

No, it's trying to predict the future either way.  I figured checking
minsz against the fields we actually consume allows complete use of the
padding fields and provides a little leniency to the user.  We'd need
to be careful though that if those fields are later used by this
driver, the code would still need to accept the smaller size.  If the
union was named rather than anonymous we could just use offsetof() to
avoid directly referencing the padding fields.
 
> > Per my comment on patch 1/, this also seems like where the device
> > specific IOMMU driver should also have the responsibility of receiving
> > a __user pointer to do the copy_from_user() here.  vfio can't know
> > which flags require which fields to make a UAPI with acceptable
> > compatibility guarantees otherwise.
> >   
> Right, VFIO cannot do compatibility guarantees, it is just seem to be
> that VFIO has enough information to copy_from_user sanely & safely and
> handle over to IOMMU. Please help define the roles/responsibilities in
> my other email. Then I will follow the guideline.

We can keep that part of the discussion in the other thread.  Thanks,

Alex

> > > +	if (inv_info->argsz < minsz)
> > > +		return -EINVAL;
> > > +
> > > +	/* Sanity check user filled invalidation dat sizes */
> > > +	if (inv_info->granularity == IOMMU_INV_GRANU_ADDR &&
> > > +		inv_info->argsz != offsetofend(struct
> > > iommu_cache_invalidate_info,
> > > +					addr_info))
> > > +		return -EINVAL;
> > > +
> > > +	if (inv_info->granularity == IOMMU_INV_GRANU_PASID &&
> > > +		inv_info->argsz != offsetofend(struct
> > > iommu_cache_invalidate_info,
> > > +					pasid_info))
> > > +		return -EINVAL;
> > > +
> > >  	spin_lock_irqsave(&device_domain_lock, flags);
> > >  	spin_lock(&iommu->lock);
> > >  	info = get_domain_info(dev);
> > > diff --git a/drivers/iommu/intel-svm.c b/drivers/iommu/intel-svm.c
> > > index 35b43fe819ed..64dc2c66dfff 100644
> > > --- a/drivers/iommu/intel-svm.c
> > > +++ b/drivers/iommu/intel-svm.c
> > > @@ -235,15 +235,27 @@ int intel_svm_bind_gpasid(struct iommu_domain
> > > *domain, struct device *dev, struct dmar_domain *dmar_domain;
> > >  	struct intel_svm_dev *sdev;
> > >  	struct intel_svm *svm;
> > > +	unsigned long minsz;
> > >  	int ret = 0;
> > >  
> > >  	if (WARN_ON(!iommu) || !data)
> > >  		return -EINVAL;
> > >  
> > > +	/*
> > > +	 * We mandate that no size change in IOMMU UAPI data
> > > before the
> > > +	 * variable size union at the end.
> > > +	 */
> > > +	minsz = offsetofend(struct iommu_gpasid_bind_data,
> > > padding);    
> > 
> > Same.  Thanks,
> > 
> > Alex
> >   
> > > +	if (data->argsz < minsz)
> > > +		return -EINVAL;
> > > +
> > >  	if (data->version != IOMMU_GPASID_BIND_VERSION_1 ||
> > >  	    data->format != IOMMU_PASID_FORMAT_INTEL_VTD)
> > >  		return -EINVAL;
> > >  
> > > +	if (data->argsz != offsetofend(struct
> > > iommu_gpasid_bind_data, vtd))
> > > +		return -EINVAL;
> > > +
> > >  	if (!dev_is_pci(dev))
> > >  		return -ENOTSUPP;
> > >      
> >   
> 
> [Jacob Pan]
>