x86/mm: alloc/free_lN_table: Retain partial_flags on -EINTR

When validating or de-validating pages (in alloc_lN_table and
free_lN_table respectively), the `partial_flags` local variable is
used to keep track of whether the "current" PTE started the entire
operation in a "may be partial" state.

One of the patches in XSA-299 addressed the fact that it is possible
for a previously-partially-validated entry to subsequently be found to
have invalid entries (indicated by returning -EINVAL); in which case
page->partial_flags needs to be set to indicate that the current PTE
may have the partial bit set (and thus _put_page_type() should be
called with PTF_partial_set).

Unfortunately, the patches in XSA-299 assumed that once
put_page_from_lNe() returned -ERESTART on a page, it was not possible
for it to return -EINTR.  This turns out to be true for
alloc_lN_table() and free_lN_table, but not for _get_page_type() and
_put_page_type(): both can return -EINTR when called on pages with
PGT_partial set.  In these cases, the pages PGT_partial will still be
set; failing to set partial_flags appropriately may allow an attacker
to do a privilege escalation similar to those described in XSA-299.

Fix this by always copying the local partial_flags variable into
page->partial_flags when exiting early.

NB that on the "get" side, no adjustment to nr_validated_entries is
needed: whether pte[i] is partially validated or entirely
un-validated, we want nr_validated_entries = i.  On the "put" side,
however, we need to adjust nr_validated_entries appropriately: if
pte[i] is entirely validated, we want nr_validated_entries = i + 1; if
pte[i] is partially validated, we want nr_validated_entries = i.

This is part of XSA-310.

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

xen/arch/x86/mm.c

@@ -1557,7 +1557,7 @@ static int alloc_l2_table(struct page_info *page, unsigned long type)
         if ( rc == -EINTR && i )
         {
             page->nr_validated_ptes = i;
-            page->partial_flags = 0;
+            page->partial_flags = partial_flags;;
             rc = -ERESTART;
         }
         else if ( rc < 0 && rc != -EINTR )
@@ -1660,7 +1660,7 @@ static int alloc_l3_table(struct page_info *page)
         else if ( rc == -EINTR && i )
         {
             page->nr_validated_ptes = i;
-            page->partial_flags = 0;
+            page->partial_flags = partial_flags;
             rc = -ERESTART;
         }
         if ( rc < 0 )
@@ -1982,8 +1982,8 @@ static int free_l2_table(struct page_info *page)
     }
     else if ( rc == -EINTR && i < L2_PAGETABLE_ENTRIES - 1 )
     {
-        page->nr_validated_ptes = i + 1;
-        page->partial_flags = 0;
+        page->nr_validated_ptes = i + !(partial_flags & PTF_partial_set);
+        page->partial_flags = partial_flags;
         rc = -ERESTART;
     }
 
@@ -2030,8 +2030,8 @@ static int free_l3_table(struct page_info *page)
     }
     else if ( rc == -EINTR && i < L3_PAGETABLE_ENTRIES - 1 )
     {
-        page->nr_validated_ptes = i + 1;
-        page->partial_flags = 0;
+        page->nr_validated_ptes = i + !(partial_flags & PTF_partial_set);
+        page->partial_flags = partial_flags;
         rc = -ERESTART;
     }
     return rc > 0 ? 0 : rc;
@@ -2061,8 +2061,8 @@ static int free_l4_table(struct page_info *page)
     }
     else if ( rc == -EINTR && i < L4_PAGETABLE_ENTRIES - 1 )
     {
-        page->nr_validated_ptes = i + 1;
-        page->partial_flags = 0;
+        page->nr_validated_ptes = i + !(partial_flags & PTF_partial_set);
+        page->partial_flags = partial_flags;
         rc = -ERESTART;
     }