xen/arm: p2m: Don't check the return of p2m_get_root_pointer() with BUG_ON()

It turns out that the BUG_ON() was actually reachable with well-crafted
hypercalls. The BUG_ON() is here to prevent catch logical error, so
crashing Xen is a bit over the top.

While all the holes should now be fixed, it would be better to downgrade
the BUG_ON() to something less fatal to prevent any more DoS.

The BUG_ON() in p2m_get_entry() is now replaced by ASSERT_UNREACHABLE()
to catch mistake in debug build and return INVALID_MFN for production
build. The interface also requires to set page_order to give an idea of
the size of "hole". So 'level' is now set so we report a hole of size of
the an entry of the root page-table. This stays inline with what happen
when the GFN is higher than p2m->max_mapped_gfn.

The BUG_ON() in p2m_resolve_translation_fault() is now replaced by
ASSERT_UNREACHABLE() to catch mistake in debug build and just report a
fault for producion build.

This is part of XSA-301.

Reported-by: Julien Grall <Julien.Grall@arm.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
master commit: 31b4f4ab66
master date: 2019-10-31 16:19:14 +0100

xen/arch/arm/p2m.c

@@ -359,7 +359,12 @@ mfn_t p2m_get_entry(struct p2m_domain *p2m, gfn_t gfn,
      * the table should always be non-NULL because the gfn is below
      * p2m->max_mapped_gfn and the root table pages are always present.
      */
-    BUG_ON(table == NULL);
+    if ( !table )
+    {
+        ASSERT_UNREACHABLE();
+        level = P2M_ROOT_LEVEL;
+        goto out;
+    }
 
     for ( level = P2M_ROOT_LEVEL; level < 3; level++ )
     {