xen/arm: mm: Access a PT entry before the table is unmapped

xen_pt_next_level() will retrieve the MFN from the entry right after the
page-table has been unmapped.

After calling xen_unmap_table(), there is no guarantee the mapping will
still be valid. Depending on the implementation, this may result to a
data abort in Xen.

Re-order the code to retrieve the MFN before the table is unmapped.

Fixes: 53abb9a1dc ("xen/arm: mm: Rework Xen page-tables walk during update")
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Paul Durrant <paul@xen.org>

xen/arch/arm/mm.c

@@ -1036,6 +1036,7 @@ static int xen_pt_next_level(bool read_only, unsigned int level,
 {
     lpae_t *entry;
     int ret;
+    mfn_t mfn;
 
     entry = *table + offset;
 
@@ -1053,8 +1054,10 @@ static int xen_pt_next_level(bool read_only, unsigned int level,
     if ( lpae_is_mapping(*entry, level) )
         return XEN_TABLE_SUPER_PAGE;
 
+    mfn = lpae_get_mfn(*entry);
+
     xen_unmap_table(*table);
-    *table = xen_map_table(lpae_get_mfn(*entry));
+    *table = xen_map_table(mfn);
 
     return XEN_TABLE_NORMAL_PAGE;
 }