x86/pv: Remove unsafe bits from the mod_l?_entry() fastpath

All changes in writeability and cacheability must go through full
re-validation.

Rework the logic as a whitelist, to make it clearer to follow.

This is XSA-182

Reported-by: Jérémie Boutoille <jboutoille@ext.quarkslab.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Tim Deegan <tim@xen.org>

xen/arch/x86/mm.c

@@ -1695,6 +1695,14 @@ static inline int update_intpte(intpte_t *p,
                   _t ## e_get_intpte(_o), _t ## e_get_intpte(_n),   \
                   (_m), (_v), (_ad))
 
+/*
+ * PTE flags that a guest may change without re-validating the PTE.
+ * All other bits affect translation, caching, or Xen's safety.
+ */
+#define FASTPATH_FLAG_WHITELIST                                     \
+    (_PAGE_NX_BIT | _PAGE_AVAIL_HIGH | _PAGE_AVAIL | _PAGE_GLOBAL | \
+     _PAGE_DIRTY | _PAGE_ACCESSED | _PAGE_USER)
+
 /* Update the L1 entry at pl1e to new value nl1e. */
 static int mod_l1_entry(l1_pgentry_t *pl1e, l1_pgentry_t nl1e,
                         unsigned long gl1mfn, int preserve_ad,
@@ -1735,9 +1743,8 @@ static int mod_l1_entry(l1_pgentry_t *pl1e, l1_pgentry_t nl1e,
             return -EINVAL;
         }
 
-        /* Fast path for identical mapping, r/w, presence, and cachability. */
-        if ( !l1e_has_changed(ol1e, nl1e,
-                              PAGE_CACHE_ATTRS | _PAGE_RW | _PAGE_PRESENT) )
+        /* Fast path for sufficiently-similar mappings. */
+        if ( !l1e_has_changed(ol1e, nl1e, ~FASTPATH_FLAG_WHITELIST) )
         {
             adjust_guest_l1e(nl1e, pt_dom);
             if ( UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, pt_vcpu,
@@ -1819,11 +1826,8 @@ static int mod_l2_entry(l2_pgentry_t *pl2e,
             return -EINVAL;
         }
 
-        /* Fast path for identical mapping and presence. */
-        if ( !l2e_has_changed(ol2e, nl2e,
-                              unlikely(opt_allow_superpage)
-                              ? _PAGE_PSE | _PAGE_RW | _PAGE_PRESENT
-                              : _PAGE_PRESENT) )
+        /* Fast path for sufficiently-similar mappings. */
+        if ( !l2e_has_changed(ol2e, nl2e, ~FASTPATH_FLAG_WHITELIST) )
         {
             adjust_guest_l2e(nl2e, d);
             if ( UPDATE_ENTRY(l2, pl2e, ol2e, nl2e, pfn, vcpu, preserve_ad) )
@@ -1888,8 +1892,8 @@ static int mod_l3_entry(l3_pgentry_t *pl3e,
             return -EINVAL;
         }
 
-        /* Fast path for identical mapping and presence. */
-        if ( !l3e_has_changed(ol3e, nl3e, _PAGE_PRESENT) )
+        /* Fast path for sufficiently-similar mappings. */
+        if ( !l3e_has_changed(ol3e, nl3e, ~FASTPATH_FLAG_WHITELIST) )
         {
             adjust_guest_l3e(nl3e, d);
             rc = UPDATE_ENTRY(l3, pl3e, ol3e, nl3e, pfn, vcpu, preserve_ad);
@@ -1952,8 +1956,8 @@ static int mod_l4_entry(l4_pgentry_t *pl4e,
             return -EINVAL;
         }
 
-        /* Fast path for identical mapping and presence. */
-        if ( !l4e_has_changed(ol4e, nl4e, _PAGE_PRESENT) )
+        /* Fast path for sufficiently-similar mappings. */
+        if ( !l4e_has_changed(ol4e, nl4e, ~FASTPATH_FLAG_WHITELIST) )
         {
             adjust_guest_l4e(nl4e, d);
             rc = UPDATE_ENTRY(l4, pl4e, ol4e, nl4e, pfn, vcpu, preserve_ad);

xen/include/asm-x86/page.h

@@ -308,6 +308,7 @@ void efi_update_l4_pgtable(unsigned int l4idx, l4_pgentry_t);
 #define _PAGE_AVAIL2   _AC(0x800,U)
 #define _PAGE_AVAIL    _AC(0xE00,U)
 #define _PAGE_PSE_PAT _AC(0x1000,U)
+#define _PAGE_AVAIL_HIGH (_AC(0x7ff, U) << 12)
 /* non-architectural flags */
 #define _PAGE_PAGED   0x2000U
 #define _PAGE_SHARED  0x4000U