Subject: x86/mm: don't bypass preemption checks
From: Jan Beulich <[email-masked]>

While unlikely, it is not impossible for a multi-vCPU guest to leverage
bypasses of preemption checks to drive Xen into an unbounded loop.

This is XSA-264.

Signed-off-by: Jan Beulich <[email-masked]>
Reviewed-by: Andrew Cooper <[email-masked]>

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -2526,7 +2526,7 @@ static int _put_page_type(struct page_in
                 nx = x & ~(PGT_validated|PGT_partial);
                 if ( unlikely((y = cmpxchg(&page->u.inuse.type_info,
                                            x, nx)) != x) )
-                    continue;
+                    goto maybe_preempt;
                 /* We cleared the 'valid bit' so we do the clean up. */
                 rc = _put_final_page_type(page, x, preemptible, ptpg);
                 ptpg = NULL;
@@ -2558,12 +2558,13 @@ static int _put_page_type(struct page_in
              */
             cpu_relax();
             y = page->u.inuse.type_info;
-            continue;
+            goto maybe_preempt;
         }
 
         if ( likely((y = cmpxchg(&page->u.inuse.type_info, x, nx)) == x) )
             break;
 
+    maybe_preempt:
         if ( preemptible && hypercall_preempt_check() )
             return -EINTR;
     }
@@ -2676,12 +2677,11 @@ static int __get_page_type(struct page_i
             if ( !(x & PGT_partial) )
             {
                 /* Someone else is updating validation of this page. Wait... */
-                while ( (y = page->u.inuse.type_info) == x )
-                {
+                do {
                     if ( preemptible && hypercall_preempt_check() )
                         return -EINTR;
                     cpu_relax();
-                }
+                } while ( (y = page->u.inuse.type_info) == x );
                 continue;
             }
             /* Type ref count was left at 1 when PGT_partial got set. */