x86/mm: also flush TLB when putting writable foreign page reference

Jann's explanation of the problem:

"start situation:
 - domain A and domain B are PV domains
 - domain A and B both have currently scheduled vCPUs, and the vCPUs
   are not scheduled away
 - domain A has XSM_TARGET access to domain B
 - page X is owned by domain B and has no mappings
 - page X is zeroed

 steps:
 - domain A uses do_mmu_update() to map page X in domain A as writable
 - domain A accesses page X through the new PTE, creating a TLB entry
 - domain A removes its mapping of page X
   - type count of page X goes to 0
   - tlbflush_timestamp of page X is bumped
 - domain B maps page X as L1 pagetable
   - type of page X changes to PGT_l1_page_table
   - TLB flush is forced using domain_dirty_cpumask of domain B
   - page X is mapped as L1 pagetable in domain B

 At this point, domain B's vCPUs are guaranteed to have no
 incorrectly-typed stale TLB entries for page X, but AFAICS domain A's
 vCPUs can still have stale TLB entries that map page X as writable,
 permitting domain A to control a live pagetable of domain B."

Domain A necessarily is Dom0 (DomU-s with XSM_TARGET permission are
being created only for HVM domains, but domain B needs to be PV here),
so this is not a security issue, but nevertheless seems desirable to
correct.

Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Don't consider page's time stamp (relevant only for the owning
    domain).

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -1266,6 +1266,18 @@ void put_page_from_l1e(l1_pgentry_t l1e,
     if ( (l1e_get_flags(l1e) & _PAGE_RW) && 
          ((l1e_owner == pg_owner) || !paging_mode_external(pg_owner)) )
     {
+        /*
+         * Don't leave stale writable TLB entries in the unmapping domain's
+         * page tables, to prevent them allowing access to pages required to
+         * be read-only (e.g. after pg_owner changed them to page table or
+         * segment descriptor pages).
+         */
+        if ( unlikely(l1e_owner != pg_owner) )
+        {
+            perfc_incr(need_flush_tlb_flush);
+            flush_tlb_mask(l1e_owner->domain_dirty_cpumask);
+        }
+
         put_page_and_type(page);
     }
     else