Subject: x86/paging: don't unconditionally BUG() on finding SHARED_M2P_ENTRY
From: Jan Beulich <[email-masked]>

PV guests can fully control the values written into the P2M.

This is XSA-251.

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

--- a/xen/arch/x86/mm/paging.c
+++ b/xen/arch/x86/mm/paging.c
@@ -276,7 +276,7 @@ void paging_mark_pfn_dirty(struct domain
         return;
 
     /* Shared MFNs should NEVER be marked dirty */
-    BUG_ON(SHARED_M2P(pfn));
+    BUG_ON(paging_mode_translate(d) && SHARED_M2P(pfn));
 
     /*
      * Values with the MSB set denote MFNs that aren't really part of the