[RFC PATCH 12/14] OvmfPkg/AmdSev: Don't overwrite mailbox or pagetables

["Tobin Feldman-Fitzthum" <tobin@linux.ibm.com>]

While restoring pages, the MH should avoid overwriting its
pagetables or the mailbox it uses to communicate with the HV.

Signed-off-by: Tobin Feldman-Fitzthum <tobin@linux.ibm.com>
---
 .../ConfidentialMigrationDxe.c                | 22 +++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/OvmfPkg/AmdSev/ConfidentialMigration/ConfidentialMigrationDxe.c b/OvmfPkg/AmdSev/ConfidentialMigration/ConfidentialMigrationDxe.c
index 3df3b09732..f609e16f8d 100644
--- a/OvmfPkg/AmdSev/ConfidentialMigration/ConfidentialMigrationDxe.c
+++ b/OvmfPkg/AmdSev/ConfidentialMigration/ConfidentialMigrationDxe.c
@@ -128,6 +128,10 @@ MigrationHandlerMain (
   IN OUT VOID *Buffer
   )
 {
+  UINT64                       mailbox_start;
+  UINT64                       mailbox_end;
+  UINT64                       pagetable_start;
+  UINT64                       pagetable_end;
   UINT64                       params_base;
   MH_COMMAND_PARAMETERS        *params;
   VOID                         *page_va;
@@ -139,10 +143,16 @@ MigrationHandlerMain (
   //
   // Shared pages must be offset by UNENC_VIRT_ADDR_BASE.
   //
-  params_base = PcdGet32 (PcdConfidentialMigrationMailboxBase) + UNENC_VIRT_ADDR_BASE;
+  mailbox_start = PcdGet32 (PcdConfidentialMigrationMailboxBase);
+  params_base = mailbox_start + UNENC_VIRT_ADDR_BASE;
   params = (VOID *)params_base;
   page_va = (VOID *)params_base + 0x1000;
 
+  mailbox_end = mailbox_start + 2 * EFI_PAGE_SIZE;
+
+  pagetable_start = mMigrationHelperPageTables;
+  pagetable_end = pagetable_start + 11 * EFI_PAGE_SIZE;
+
   DisableInterrupts();
   params->go = 0;
 
@@ -163,7 +173,15 @@ MigrationHandlerMain (
       break;
 
     case MH_FUNC_RESTORE_PAGE:
-      CopyMem((VOID *)params->gpa, page_va, 4096);
+      //
+      // Don't import a page that covers the mailbox or pagetables.
+      //
+      if ((params->gpa >= mailbox_start && params->gpa < mailbox_end) ||
+          (params->gpa >= pagetable_start && params->gpa < pagetable_end)) {
+      }
+      else {
+        CopyMem((VOID *)params->gpa, page_va, 4096);
+      }
       params->ret = MH_SUCCESS;
       break;
 
-- 
2.20.1