Re: [PATCH v2 1/3] iommu/uapi: Define uapi version and capabilities

From: Jacob Pan <jacob.jun.pan@linux.intel.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: "Tian, Kevin" <kevin.tian@intel.com>,
	"Raj, Ashok" <ashok.raj@intel.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.com>,
	LKML <linux-kernel@vger.kernel.org>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	David Woodhouse <dwmw2@infradead.org>
Subject: Re: [PATCH v2 1/3] iommu/uapi: Define uapi version and capabilities
Date: Tue, 14 Apr 2020 15:32:26 -0700	[thread overview]
Message-ID: <20200414153226.30d8cab3@jacob-builder> (raw)
In-Reply-To: <20200414101304.27e587eb@jacob-builder>

On Tue, 14 Apr 2020 10:13:04 -0700
Jacob Pan <jacob.jun.pan@linux.intel.com> wrote:

> > > >  In any of the proposed solutions, the
> > > > IOMMU driver is ultimately responsible for validating the user
> > > > data, so do we want vfio performing the copy_from_user() to an
> > > > object that could later be assumed to be sanitized, or should
> > > > vfio just pass a user pointer to make it obvious that the
> > > > consumer is responsible for all the user protections?  Seems
> > > > like the latter.      
> > > I like the latter as well.
> > >     
On a second thought, I think the former is better. Two reasons:

1. IOMMU API such as page_response is also used in baremetal. So it is
not suitable to pass a __user *.
https://www.spinics.net/lists/arm-kernel/msg798677.html

2. Some data are in the mandatory (fixed offset, never removed or
extended) portion of the uAPI structure. It is simpler for VFIO to
extract that and pass it to IOMMU API. For example, the PASID value used
for unbind_gpasid(). VFIO also need to sanitize the PASID value to make
sure it belongs to the same VM that did the allocation.

> > > >  That still really
> > > > doesn't address what's in that user data blob yet, but the vfio
> > > > interface could be:
> > > > 
> > > > struct {
> > > > 	__u32 argsz;
> > > > 	__u32 flags;
> > > > 	__u8  data[];
> > > > }
> > > > 
> > > > Where flags might be partitioned like we do for DEVICE_FEATURE
> > > > to indicate the format of data and what vfio should do with it,
> > > > and data might simply be defined as a (__u64 __user *).
> > > >       
> > > So, __user * will be passed to IOMMU driver if VFIO checks minsz
> > > include flags and they are valid.
> > > IOMMU driver can copy the rest based on the mandatory
> > > version/minsz and flags in the IOMMU uAPI structs.
> > > Does it sound right? This is really choice #2.    
> > 
> > Sounds like each IOMMU UAPI struct just needs to have an embedded
> > size and flags field, but yes.
> >   
> Yes, an argsz field can be added to each UAPI. There are already flags
> or the equivalent. IOMMU driver can process the __user * based on the
> argsz, flags, check argsz against offsetofend(iommu_uapi_struct,
> last_element), etc.;
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