RE: [Xen-devel] [PATCH] xen: grant-table: Check truncation when giving access to a frame

[Paul Durrant]

> -----Original Message-----
> From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxx] On Behalf Of
> Julien Grall
> Sent: 13 June 2016 12:10
> To: David Vrabel; boris.ostrovsky@xxxxxxxxxx; jgross@xxxxxxxx;
> sstabellini@xxxxxxxxxx; konrad.wilk@xxxxxxxxxx
> Cc: Andrew Cooper; steve.capper@xxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> JBeulich@xxxxxxxx; xen-devel@xxxxxxxxxxxxx
> Subject: Re: [Xen-devel] [PATCH] xen: grant-table: Check truncation when
> giving access to a frame
> 
> Hi David,
> 
> On 13/06/16 11:57, David Vrabel wrote:
> > On 13/06/16 11:50, Julien Grall wrote:
> >> The version 1 of the grant-table protocol only supports frame encoded on
> >> 32-bit.
> >>
> >> When the platform is supporting 48-bit physical address, the frame will
> >> be encoded on 36-bit which will lead a truncation and give access to
> >> the wrong frame.
> >>
> >> On ARM Xen will always allow the guest to use all the physical address,
> >> although today the RAM is always located under 40-bits (see
> >> xen/include/public/arch-arm.h).
> >>
> >> Add a truncation check in gnttab_update_entry_v1 to prevent the guest
> to
> >> give access to the wrong frame.
> >
> > In hindsight, we shouldn't have dropped the V2 support from Linux.
> > Should we reinstate it?
> 
> What were the reasons to drop the v2 support from Linux? More
> importantly why people did choose to stay on v1?
> 

One of the main reasons for v2's existence was to support a version of the netif protocol that pushed guest receive-side copy into the guest itself. This was done by granting pages from dom0, or from other guests, to the guest performing the copy. To do this securely a couple of things were needed:

- The ability to have (copy only) sub-page grants.
- The ability to transitively grant a ref from one domain to another.

Unfortunately the idea did not scale as it became bottle-necked on dom0's grant table size, and there were some nasty corner cases to work around (which is why we also have a swap-grant-ref hypercall). In the end, guest copy was dropped and then there was really no need to use grant table v2. Using version 1 is simpler, and gives you more grant entries per page of table, so everyone stuck with that.

  Paul

> Cheers,
> 
> --
> Julien Grall
> 
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