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

[David Vrabel]

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?

David

> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> 
> ---
>     This is limiting us to a 44-bit address space whilst ARM can support
>     up to 48-bit today. This number of bit will increase to 52-bit in
>     upcoming processors [1].
> 
>     It might be good to start thinking to extend the version 1 of the
>     protocol to use 64-bit frame number.
> 
>     [1] https://community.arm.com/groups/processors/blog/2016/01/05/armv8-a-architecture-evolution
> ---
>  drivers/xen/grant-table.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index bb36b1e..f47c2e99 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -224,6 +224,13 @@ static void gnttab_update_entry_v1(grant_ref_t ref, domid_t domid,
>  {
>  	gnttab_shared.v1[ref].domid = domid;
>  	gnttab_shared.v1[ref].frame = frame;
> +
> +	/*
> +	 * V1 only supports 32-bit frame, check the truncation
> +	 * to avoid giving access to the wrong frame.
> +	 */
> +	BUG_ON(gnttab_shared.v1[ref].frame != frame);
> +
>  	wmb();
>  	gnttab_shared.v1[ref].flags = flags;
>  }
>