x86/mm: Have alloc_l[23]_table clear partial_flags when preempting

In order to allow recursive pagetable promotions and demotions to be
interrupted, Xen must keep track of the state of the sub-pages
promoted or demoted.  This is stored in two elements in the page
struct: nr_entries_validated and partial_flags.

The rule is that entries [0, nr_entries_validated) should always be
validated and hold a general reference count.  If partial_flags is
zero, then [nr_entries_validated] is not validated and no reference
count is held.  If PTF_partial_set is set, then [nr_entries_validated]
is partially validated.

At the moment, a distinction is made between promotion and demotion
with regard to whether the entry itself "holds" a general reference
count: when entry promotion is interrupted (i.e., returns -ERESTART),
the entry is not considered to hold a reference; when entry demotion
is interrupted, the entry is still considered to hold a general
reference.

PTF_partial_general_ref is used to distinguish between these cases.
If clear, it's a partial promotion => no general reference count held
by the entry; if set, it's partial demotion, so a general reference
count held.  Because promotions and demotions can be interleaved, this
value is passed to get_page_and_type_from_mfn and put_page_from_l*e,
to be able to properly handle reference counts.

Unfortunately, when alloc_l[23]_table check hypercall_preempt_check()
and return -ERESTART, they set nr_entries_validated, but don't clear
partial_flags.

If we were picking up from a previously-interrupted promotion, that
means that PTF_partial_set would be set even though
[nr_entries_validated] was not partially validated.  This means that
if the page in this state were de-validated, put_page_type() would
erroneously be called on that entry.

Perhaps worse, if we were racing with a de-validation, then we might
leave both PTF_partial_set and PTF_partial_general_ref; and when
de-validation picked up again, both the type and the general ref would
be erroneously dropped from [nr_entries_validated].

In a sense, the real issue here is code duplication.  Rather than
duplicate the interruption code, set rc to -EINTR and fall through to
the code which already handles that case correctly.

Given the logic at this point, it should be impossible for
partial_flags to be non-zero; add an ASSERT() to catch any changes.

This is part of XSA-299.

Reported-by: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/mm.c

@@ -1525,21 +1525,13 @@ static int alloc_l2_table(struct page_info *page, unsigned long type)
     for ( i = page->nr_validated_ptes; i < L2_PAGETABLE_ENTRIES;
           i++, partial_flags = 0 )
     {
-        l2_pgentry_t l2e;
+        l2_pgentry_t l2e = pl2e[i];
 
         if ( i > page->nr_validated_ptes && hypercall_preempt_check() )
-        {
-            page->nr_validated_ptes = i;
-            rc = -ERESTART;
-            break;
-        }
-
-        if ( !is_guest_l2_slot(d, type, i) )
+            rc = -EINTR;
+        else if ( !is_guest_l2_slot(d, type, i) )
             continue;
-
-        l2e = pl2e[i];
-
-        if ( !(l2e_get_flags(l2e) & _PAGE_PRESENT) )
+        else if ( !(l2e_get_flags(l2e) & _PAGE_PRESENT) )
         {
             if ( !pv_l1tf_check_l2e(d, l2e) )
                 continue;
@@ -1612,13 +1604,8 @@ static int alloc_l3_table(struct page_info *page)
         l3_pgentry_t l3e = pl3e[i];
 
         if ( i > page->nr_validated_ptes && hypercall_preempt_check() )
-        {
-            page->nr_validated_ptes = i;
-            rc = -ERESTART;
-            break;
-        }
-
-        if ( is_pv_32bit_domain(d) && (i == 3) )
+            rc = -EINTR;
+        else if ( is_pv_32bit_domain(d) && (i == 3) )
         {
             if ( !(l3e_get_flags(l3e) & _PAGE_PRESENT) ||
                  (l3e_get_flags(l3e) & l3_disallow_mask(d)) )