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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, Feb 22, 2021 at 04:47:38PM +0000, Andrew Cooper wrote:
> On 22/02/2021 14:22, Jan Beulich wrote:
> > On 22.02.2021 15:14, Andrew Cooper wrote:
> >> On 22/02/2021 10:27, 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.
> >>>
> >>> While this could be conditionalized upon CONFIG_PV32 &&
> >>> CONFIG_SPECULATIVE_HARDEN_GUEST_ACCESS, omitting such extra conditionals
> >>> keeps the code more legible imo.
> >>>
> >>> Fixes: 4dc181599142 ("x86/PV: harden guest memory accesses against speculative abuse")
> >>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> >>>
> >>> --- a/xen/arch/x86/mm.c
> >>> +++ b/xen/arch/x86/mm.c
> >>> @@ -1727,6 +1727,11 @@ void init_xen_l4_slots(l4_pgentry_t *l4t
> >>>                 (ROOT_PAGETABLE_FIRST_XEN_SLOT + slots -
> >>>                  l4_table_offset(XEN_VIRT_START)) * sizeof(*l4t));
> >>>      }
> >>> +
> >>> +    /* Slot 511: Per-domain mappings mirror. */
> >>> +    if ( !is_pv_64bit_domain(d) )
> >>> +        l4t[l4_table_offset(PERDOMAIN2_VIRT_START)] =
> >>> +            l4e_from_page(d->arch.perdomain_l3_pg, __PAGE_HYPERVISOR_RW);
> >> This virtual address is inside the extended directmap.
> > No. That one covers only the range excluding the last L4 slot.
> >
> >>   You're going to
> >> need to rearrange more things than just this, to make it safe.
> > I specifically picked that entry because I don't think further
> > arrangements are needed.
> 
> map_domain_page() will blindly hand this virtual address if an
> appropriate mfn is passed, because there are no suitability checks.
> 
> The error handling isn't great, but at least any attempt to use that
> pointer would fault, which is now no longer the case.

AFAICT map_domain_page will never populate the error page virtual
address, as the slot end (MAPCACHE_VIRT_END) is way lower than
-MAX_ERRNO?

We could add:

BUILD_BUG_ON((PERDOMAIN_VIRT_SLOT(PERDOMAIN_SLOTS) - 1) >= (unsigned long)-MAX_ERRNO);

For safety somewhere.

Roger.