Re: [PATCH][4.15] x86: mirror compat argument translation area for 32-bit PV

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.02.2021 12:35, Roger Pau Monné wrote:
> On Mon, Feb 22, 2021 at 11:27:07AM +0100, Jan Beulich wrote:
>> Now that we guard the entire Xen VA space against speculative abuse
>> through hypervisor accesses to guest memory, the argument translation
>> area's VA also needs to live outside this range, at least for 32-bit PV
>> guests. To avoid extra is_hvm_*() conditionals, use the alternative VA
>> uniformly.
> 
> Since you are double mapping the per-domain virtual area, won't it
> make more sense to map it just once outside of the Xen virtual space
> area? (so it's always using PML4_ADDR(511))

This would then require conditionals in paths using other parts of
the per-domain mappings for 64-bit PV, as the same range is under
guest control there.

> Is there anything concerning in the per-domain area that should be
> protected against speculative accesses?

First of all this is an unrelated question - I'm not changing what
gets accessed there, but only through which addresses these accesses
happen. What lives there are GDT/LDT mappings, map cache, and the
argument translation area. The guest has no control (or very limited
when considering GDT/LDT one) over the accesses made to this space.

Jan