From: Jan Beulich <jbeulich@suse.com>
Subject: VT-d: fix reduced page table levels support when sharing tables

domain_pgd_maddr() contains logic to adjust the root address to be put
in the context entry in case 4-level page tables aren't supported by an
IOMMU. This logic may not be bypassed when sharing page tables.

This is CVE-2021-28710 / XSA-390.

Fixes: 25ccd093425c ("iommu: remove the share_p2m operation")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>

--- a/xen/drivers/passthrough/vtd/iommu.c
+++ b/xen/drivers/passthrough/vtd/iommu.c
@@ -340,19 +340,21 @@ static uint64_t domain_pgd_maddr(struct
     {
         pagetable_t pgt = p2m_get_pagetable(p2m_get_hostp2m(d));
 
-        return pagetable_get_paddr(pgt);
+        pgd_maddr = pagetable_get_paddr(pgt);
     }
-
-    if ( !hd->arch.vtd.pgd_maddr )
+    else
     {
-        /* Ensure we have pagetables allocated down to leaf PTE. */
-        addr_to_dma_page_maddr(d, 0, 1);
-
         if ( !hd->arch.vtd.pgd_maddr )
-            return 0;
-    }
+        {
+            /* Ensure we have pagetables allocated down to leaf PTE. */
+            addr_to_dma_page_maddr(d, 0, 1);
 
-    pgd_maddr = hd->arch.vtd.pgd_maddr;
+            if ( !hd->arch.vtd.pgd_maddr )
+                return 0;
+        }
+
+        pgd_maddr = hd->arch.vtd.pgd_maddr;
+    }
 
     /* Skip top levels of page tables for 2- and 3-level DRHDs. */
     for ( agaw = level_to_agaw(4);