Subject: x86/shadow: fix refcount overflow check
From: Jan Beulich <[email-masked]>

Commit c385d27079 ("x86 shadow: for multi-page shadows, explicitly track
the first page") reduced the refcount width to 25, without adjusting the
overflow check. Eliminate the disconnect by using a manifest constant.

Interestingly, up to commit 047782fa01 ("Out-of-sync L1 shadows: OOS
snapshot") the refcount was 27 bits wide, yet the check was already
using 26.

This is XSA-249.

Signed-off-by: Jan Beulich <[email-masked]>
Reviewed-by: George Dunlap <[email-masked]>
Reviewed-by: Tim Deegan <[email-masked]>
---
v2: Simplify expression back to the style it was.

--- a/xen/arch/x86/mm/shadow/private.h
+++ b/xen/arch/x86/mm/shadow/private.h
@@ -529,7 +529,7 @@ static inline int sh_get_ref(struct doma
     x = sp->u.sh.count;
     nx = x + 1;
 
-    if ( unlikely(nx >= 1U<<26) )
+    if ( unlikely(nx >= (1U << PAGE_SH_REFCOUNT_WIDTH)) )
     {
         SHADOW_PRINTK("shadow ref overflow, gmfn=%lx smfn=%lx\n",
                        __backpointer(sp), mfn_x(smfn));
--- a/xen/include/asm-x86/mm.h
+++ b/xen/include/asm-x86/mm.h
@@ -82,7 +82,8 @@ struct page_info
             unsigned long type:5;   /* What kind of shadow is this? */
             unsigned long pinned:1; /* Is the shadow pinned? */
             unsigned long head:1;   /* Is this the first page of the shadow? */
-            unsigned long count:25; /* Reference count */
+#define PAGE_SH_REFCOUNT_WIDTH 25
+            unsigned long count:PAGE_SH_REFCOUNT_WIDTH; /* Reference count */
         } sh;
 
         /* Page is on a free list: ((count_info & PGC_count_mask) == 0). */