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From: Jan Beulich <jbeulich@suse.com>
Subject: x86: discard type information when stealing pages

While a page having just a single general reference left necessarily
has a zero type reference count too, its type may still be valid (and
in validated state; at present this is only possible and relevant for
PGT_seg_desc_page, as page tables have their type forcibly zapped when
their type reference count drops to zero, and
PGT_{writable,shared}_page pages don't require any validation). In
such a case when the page is being re-used with the same type again,
validation is being skipped. As validation criteria differ between
32- and 64-bit guests, pages to be transferred between guests need to
have their validation indicator zapped (and with it we zap all other
type information at once).

This is XSA-214.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4466,6 +4466,17 @@ int steal_page(
         y = cmpxchg(&page->count_info, x, x & ~PGC_count_mask);
     } while ( y != x );
 
+    /*
+     * With the sole reference dropped temporarily, no-one can update type
+     * information. Type count also needs to be zero in this case, but e.g.
+     * PGT_seg_desc_page may still have PGT_validated set, which we need to
+     * clear before transferring ownership (as validation criteria vary
+     * depending on domain type).
+     */
+    BUG_ON(page->u.inuse.type_info & (PGT_count_mask | PGT_locked |
+                                      PGT_pinned));
+    page->u.inuse.type_info = 0;
+
     /* Swizzle the owner then reinstate the PGC_allocated reference. */
     page_set_owner(page, NULL);
     y = page->count_info;