MdeModulePkg/Variable: Init var policy after SMM variable is ready (#1269)

## Description

On a MM system, the main UEFI variable logic resides in MMRAM. In that
case, the variable policy logic in `VarCheckPolicyLib`, such as
`VarCheckPolicyLibStandaloneMm` is linked against the MM driver also in
that case `VariableStandaloneMm`.

The MM variable driver indicates its presence to the RT DXE driver via
`gEfiSmmVariableProtocolGuid` to indicate variable read support is
available from MM. This triggers installation of the variable
architectural protocol in DXE.

Today, variable policy is initialized by calling
`VariablePolicySmmDxeMain()` in `VariableSmmRuntimeInitialize()`. In
turn, this installs `gEdkiiVariablePolicyProtocolGuid`. Functions in
`gEdkiiVariablePolicyProtocolGuid` may trigger MMIs. However, it is
possible that the MM variable driver which is linked against the code
with the variable policy MMI handlers (i.e. `VarCheckPolicyLib`) is not
loaded yet.

Therefore, this change moves invocation of `VariablePolicySmmDxeMain()`
to `SmmVariableReady()` which is called on installation of
`gEfiSmmVariableProtocolGuid` indicating variable MM services are ready.
`gEdkiiVariablePolicyProtocolGuid` is still installed prior to the
variable architectural protocol being installed.

- [x] Impacts functionality?
- [ ] Impacts security?
- [ ] Breaking change?
- [ ] Includes tests?
- [ ] Includes documentation?
- [ ] Backport to release branch?

## How This Was Tested

- Verify variable policy is initialized as expected on QEMU Q35/SBSA and
a physical Intel system.
- Check that the variable image handle passed to
`VariablePolicySmmDxeMain()` is correct.

## Integration Instructions

N/A - Some drivers may dispatch differently if they depend on
`gEdkiiVariablePolicyProtocolGuid`. However, this is not considered
breaking as that is an inherent expectation in dispatch based on
dependency expressions.

Signed-off-by: Michael Kubacki <[email protected]>

Author: makubacki

MdeModulePkg/Universal/Variable/RuntimeDxe/VariablePolicySmmDxe.c

@@ -813,7 +813,7 @@ VariablePolicyVirtualAddressCallback (
   The driver's entry point.
 
   @param[in] ImageHandle  The firmware allocated handle for the EFI image.
-  @param[in] SystemTable  A pointer to the EFI System Table.
+  // @param[in] SystemTable  A pointer to the EFI System Table. // MU_CHANGE - Initialize var policy after SMM Variable is ready
 
   @retval EFI_SUCCESS     The entry point executed successfully.
   @retval other           Some error occured when executing this entry point.
@@ -822,8 +822,8 @@ VariablePolicyVirtualAddressCallback (
 EFI_STATUS
 EFIAPI
 VariablePolicySmmDxeMain (
-  IN    EFI_HANDLE        ImageHandle,
-  IN    EFI_SYSTEM_TABLE  *SystemTable
+  IN    EFI_HANDLE  ImageHandle             // MU_CHANGE - Initialize var policy after SMM Variable is ready
+  // IN    EFI_SYSTEM_TABLE  *SystemTable   // MU_CHANGE - Initialize var policy after SMM Variable is ready
   )
 {
   EFI_STATUS  Status;

MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmmRuntimeDxe.c

@@ -73,8 +73,8 @@ EDKII_VAR_CHECK_PROTOCOL        mVarCheck;
 EFI_STATUS
 EFIAPI
 VariablePolicySmmDxeMain (
-  IN    EFI_HANDLE        ImageHandle,
-  IN    EFI_SYSTEM_TABLE  *SystemTable
+  IN    EFI_HANDLE  ImageHandle             // MU_CHANGE - Initialize var policy after SMM Variable is ready
+  // IN    EFI_SYSTEM_TABLE  *SystemTable   // MU_CHANGE - Initialize var policy after SMM Variable is ready
   );
 
 /**
@@ -1692,6 +1692,7 @@ SmmVariableReady (
   )
 {
   EFI_STATUS  Status;
+  EFI_HANDLE  VariablePolicyHandle;   // MU_CHANGE - Initialize var policy after SMM Variable is ready
 
   Status = gBS->LocateProtocol (&gEfiSmmVariableProtocolGuid, NULL, (VOID **)&mSmmVariable);
   if (EFI_ERROR (Status)) {
@@ -1758,6 +1759,16 @@ SmmVariableReady (
   gRT->SetVariable         = RuntimeServiceSetVariable;
   gRT->QueryVariableInfo   = RuntimeServiceQueryVariableInfo;
 
+  // MU_CHANGE [BEGIN] - Initialize var policy after SMM Variable is ready
+  VariablePolicyHandle = (Context != NULL) ? Context : mHandle;
+  if (Context == NULL) {
+    DEBUG ((DEBUG_ERROR, "Variable policy was installed on a handle other than the variable image handle.\n"));
+  }
+
+  // Initialize the VariablePolicy protocol and engine.
+  VariablePolicySmmDxeMain (VariablePolicyHandle);
+  // MU_CHANGE [END] - Initialize var policy after SMM Variable is ready
+
   //
   // Install the Variable Architectural Protocol on a new handle.
   //
@@ -1868,7 +1879,7 @@ VariableSmmRuntimeInitialize (
     &gEfiSmmVariableProtocolGuid,
     TPL_CALLBACK,
     SmmVariableReady,
-    NULL,
+    ImageHandle,                    // MU_CHANGE - Initialize var policy after SMM Variable is ready
     &SmmVariableRegistration
     );
 
@@ -1930,8 +1941,10 @@ VariableSmmRuntimeInitialize (
          &mVirtualAddressChangeEvent
          );
 
-  // Initialize the VariablePolicy protocol and engine.
-  VariablePolicySmmDxeMain (ImageHandle, SystemTable);
+  // MU_CHANGE [BEGIN] - Initialize var policy after SMM Variable is ready
+  // // Initialize the VariablePolicy protocol and engine.
+  // VariablePolicySmmDxeMain (ImageHandle, SystemTable);
+  // MU_CHANGE [END] - Initialize var policy after SMM Variable is ready
 
   return EFI_SUCCESS;
 }