x86/mm: Don't check for invalid bits in non-present PTEs.

If _PAGE_PRESENT is clean in a pagetable entry, any pattern of bits
is valid in the rest of the entry.  OSes that special-case
PFEC_invalid_bits (since it should never happen) will be confused
by our setting it in this way.

Signed-off-by: Tim Deegan <tim@xen.org>

1 files changed, 16 insertions, 5 deletions

diff --git a/xen/arch/x86/mm/guest_walk.c b/xen/arch/x86/mm/guest_walk.c
index bc4a82f878..88fc4f8cf4 100644
--- a/xen/arch/x86/mm/guest_walk.c
+++ b/xen/arch/x86/mm/guest_walk.c
@@ -179,8 +179,11 @@ guest_walk_tables(struct vcpu *v, struct p2m_domain *p2m,
     l4p = (guest_l4e_t *) top_map;
     gw->l4e = l4p[guest_l4_table_offset(va)];
     gflags = guest_l4e_get_flags(gw->l4e) ^ iflags;
+    if ( !(gflags & _PAGE_PRESENT) ) {
+        rc |= _PAGE_PRESENT;
+        goto out;
+    }
     rc |= ((gflags & mflags) ^ mflags);
-    if ( rc & _PAGE_PRESENT ) goto out;
 
     /* Map the l3 table */
     l3p = map_domain_gfn(p2m, 
@@ -193,9 +196,11 @@ guest_walk_tables(struct vcpu *v, struct p2m_domain *p2m,
     /* Get the l3e and check its flags*/
     gw->l3e = l3p[guest_l3_table_offset(va)];
     gflags = guest_l3e_get_flags(gw->l3e) ^ iflags;
-    rc |= ((gflags & mflags) ^ mflags);
-    if ( rc & _PAGE_PRESENT )
+    if ( !(gflags & _PAGE_PRESENT) ) {
+        rc |= _PAGE_PRESENT;
         goto out;
+    }
+    rc |= ((gflags & mflags) ^ mflags);
     
     pse1G = (gflags & _PAGE_PSE) && guest_supports_1G_superpages(v); 
 
@@ -261,9 +266,11 @@ guest_walk_tables(struct vcpu *v, struct p2m_domain *p2m,
 #endif /* All levels... */
 
     gflags = guest_l2e_get_flags(gw->l2e) ^ iflags;
-    rc |= ((gflags & mflags) ^ mflags);
-    if ( rc & _PAGE_PRESENT )
+    if ( !(gflags & _PAGE_PRESENT) ) {
+        rc |= _PAGE_PRESENT;
         goto out;
+    }
+    rc |= ((gflags & mflags) ^ mflags);
 
     pse2M = (gflags & _PAGE_PSE) && guest_supports_superpages(v); 
 
@@ -321,6 +328,10 @@ guest_walk_tables(struct vcpu *v, struct p2m_domain *p2m,
             goto out;
         gw->l1e = l1p[guest_l1_table_offset(va)];
         gflags = guest_l1e_get_flags(gw->l1e) ^ iflags;
+        if ( !(gflags & _PAGE_PRESENT) ) {
+            rc |= _PAGE_PRESENT;
+            goto out;
+        }
         rc |= ((gflags & mflags) ^ mflags);
     }