Re: [Xen-devel] [PATCH v2 2/4] x86: allow 64-bit PV guest kernels to suppress user mode exposure of M2P

[Tim Deegan <tim@xxxxxxx>]

At 15:22 +0000 on 20 Mar (1426864937), Jan Beulich wrote:
> Xen L4 entries being uniformly installed into any L4 table and 64-bit
> PV kernels running in ring 3 means that user mode was able to see the
> read-only M2P presented by Xen to the guests. While apparently not
> really representing an exploitable information leak, this still very
> certainly was never meant to be that way.
> 
> Building on the fact that these guests already have separate kernel and
> user mode page tables we can allow guest kernels to tell Xen that they
> don't want user mode to see this table. We can't, however, do this by
> default: There is no ABI requirement that kernel and user mode page
> tables be separate. Therefore introduce a new VM-assist flag allowing
> the guest to control respective hypervisor behavior:
> - when not set, L4 tables get created with the respective slot blank,
>   and whenever the L4 table gets used as a kernel one the missing
>   mapping gets inserted,
> - when set, L4 tables get created with the respective slot initialized
>   as before, and whenever the L4 table gets used as a user one the
>   mapping gets zapped.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Much nicer, thanks.  Reviewed-by: Tim Deegan <tim@xxxxxxx>, for this
and the two patches that follow it.

Cheers,

Tim.

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