path: root/MdeModulePkg/Universal/Variable/RuntimeDxe/VariablePolicySmmDxe.c

/** @file -- VariablePolicySmmDxe.c
This protocol allows communication with Variable Policy Engine.

Copyright (c) Microsoft Corporation.
SPDX-License-Identifier: BSD-2-Clause-Patent

**/

#include <Library/BaseLib.h>
#include <Library/UefiLib.h>
#include <Library/DebugLib.h>
#include <Library/SafeIntLib.h>
#include <Library/UefiBootServicesTableLib.h>
#include <Library/BaseMemoryLib.h>
#include <Library/MemoryAllocationLib.h>

#include <Protocol/VariablePolicy.h>
#include <Protocol/MmCommunication2.h>

#include <Guid/VarCheckPolicyMmi.h>

#include "Variable.h"

EDKII_VARIABLE_POLICY_PROTOCOL  mVariablePolicyProtocol;
EFI_MM_COMMUNICATION2_PROTOCOL  *mMmCommunication;

VOID      *mMmCommunicationBuffer;
UINTN     mMmCommunicationBufferSize;
EFI_LOCK  mMmCommunicationLock;

/**
  Internal helper function to consolidate communication method.

  @param[in,out]  CommBuffer
  @param[in,out]  CommSize    Size of the CommBuffer.

  @retval   EFI_STATUS    Result from communication method.

**/
STATIC
EFI_STATUS
InternalMmCommunicate (
  IN OUT VOID   *CommBuffer,
  IN OUT UINTN  *CommSize
  )
{
  EFI_STATUS  Status;

  if ((CommBuffer == NULL) || (CommSize == NULL)) {
    return EFI_INVALID_PARAMETER;
  }

  Status = mMmCommunication->Communicate (mMmCommunication, CommBuffer, CommBuffer, CommSize);
  return Status;
}

/**
  This API function disables the variable policy enforcement. If it's
  already been called once, will return EFI_ALREADY_STARTED.

  @retval     EFI_SUCCESS
  @retval     EFI_ALREADY_STARTED   Has already been called once this boot.
  @retval     EFI_WRITE_PROTECTED   Interface has been locked until reboot.
  @retval     EFI_WRITE_PROTECTED   Interface option is disabled by platform PCD.

**/
STATIC
EFI_STATUS
EFIAPI
ProtocolDisableVariablePolicy (
  VOID
  )
{
  EFI_STATUS                    Status;
  EFI_MM_COMMUNICATE_HEADER     *CommHeader;
  VAR_CHECK_POLICY_COMM_HEADER  *PolicyHeader;
  UINTN                         BufferSize;

  // Check the PCD for convenience.
  // This would also be rejected by the lib, but why go to MM if we don't have to?
  if (!PcdGetBool (PcdAllowVariablePolicyEnforcementDisable)) {
    return EFI_WRITE_PROTECTED;
  }

  AcquireLockOnlyAtBootTime (&mMmCommunicationLock);

  // Set up the MM communication.
  BufferSize   = mMmCommunicationBufferSize;
  CommHeader   = mMmCommunicationBuffer;
  PolicyHeader = (VAR_CHECK_POLICY_COMM_HEADER *)&CommHeader->Data;
  CopyGuid (&CommHeader->HeaderGuid, &gVarCheckPolicyLibMmiHandlerGuid);
  CommHeader->MessageLength = BufferSize - OFFSET_OF (EFI_MM_COMMUNICATE_HEADER, Data);
  PolicyHeader->Signature   = VAR_CHECK_POLICY_COMM_SIG;
  PolicyHeader->Revision    = VAR_CHECK_POLICY_COMM_REVISION;
  PolicyHeader->Command     = VAR_CHECK_POLICY_COMMAND_DISABLE;

  Status = InternalMmCommunicate (CommHeader, &BufferSize);
  DEBUG ((DEBUG_VERBOSE, "%a - MmCommunication returned %r.\n", __func__, Status));

  ReleaseLockOnlyAtBootTime (&mMmCommunicationLock);

  return (EFI_ERROR (Status)) ? Status : PolicyHeader->Result;
}

/**
  This API function returns whether or not the policy engine is
  currently being enforced.

  @param[out]   State       Pointer to a return value for whether the policy enforcement
                            is currently enabled.

  @retval     EFI_SUCCESS
  @retval     Others        An error has prevented this command from completing.

**/
STATIC
EFI_STATUS
EFIAPI
ProtocolIsVariablePolicyEnabled (
  OUT BOOLEAN  *State
  )
{
  EFI_STATUS                               Status;
  EFI_MM_COMMUNICATE_HEADER                *CommHeader;
  VAR_CHECK_POLICY_COMM_HEADER             *PolicyHeader;
  VAR_CHECK_POLICY_COMM_IS_ENABLED_PARAMS  *CommandParams;
  UINTN                                    BufferSize;

  if (State == NULL) {
    return EFI_INVALID_PARAMETER;
  }

  AcquireLockOnlyAtBootTime (&mMmCommunicationLock);

  // Set up the MM communication.
  BufferSize    = mMmCommunicationBufferSize;
  CommHeader    = mMmCommunicationBuffer;
  PolicyHeader  = (VAR_CHECK_POLICY_COMM_HEADER *)&CommHeader->Data;
  CommandParams = (VAR_CHECK_POLICY_COMM_IS_ENABLED_PARAMS *)(PolicyHeader + 1);
  CopyGuid (&CommHeader->HeaderGuid, &gVarCheckPolicyLibMmiHandlerGuid);
  CommHeader->MessageLength = BufferSize - OFFSET_OF (EFI_MM_COMMUNICATE_HEADER, Data);
  PolicyHeader->Signature   = VAR_CHECK_POLICY_COMM_SIG;
  PolicyHeader->Revision    = VAR_CHECK_POLICY_COMM_REVISION;
  PolicyHeader->Command     = VAR_CHECK_POLICY_COMMAND_IS_ENABLED;

  Status = InternalMmCommunicate (CommHeader, &BufferSize);
  DEBUG ((DEBUG_VERBOSE, "%a - MmCommunication returned %r.\n", __func__, Status));

  if (!EFI_ERROR (Status)) {
    Status = PolicyHeader->Result;
    *State = CommandParams->State;
  }

  ReleaseLockOnlyAtBootTime (&mMmCommunicationLock);

  return Status;
}

/**
  This API function validates and registers a new policy with
  the policy enforcement engine.

  @param[in]  NewPolicy     Pointer to the incoming policy structure.

  @retval     EFI_SUCCESS
  @retval     EFI_INVALID_PARAMETER   NewPolicy is NULL or is internally inconsistent.
  @retval     EFI_ALREADY_STARTED     An identical matching policy already exists.
  @retval     EFI_WRITE_PROTECTED     The interface has been locked until the next reboot.
  @retval     EFI_UNSUPPORTED         Policy enforcement has been disabled. No reason to add more policies.
  @retval     EFI_ABORTED             A calculation error has prevented this function from completing.
  @retval     EFI_OUT_OF_RESOURCES    Cannot grow the table to hold any more policies.

**/
STATIC
EFI_STATUS
EFIAPI
ProtocolRegisterVariablePolicy (
  IN CONST VARIABLE_POLICY_ENTRY  *NewPolicy
  )
{
  EFI_STATUS                    Status;
  EFI_MM_COMMUNICATE_HEADER     *CommHeader;
  VAR_CHECK_POLICY_COMM_HEADER  *PolicyHeader;
  VOID                          *PolicyBuffer;
  UINTN                         BufferSize;
  UINTN                         RequiredSize;

  if (NewPolicy == NULL) {
    return EFI_INVALID_PARAMETER;
  }

  // First, make sure that the required size does not exceed the capabilities
  // of the MmCommunication buffer.
  RequiredSize = OFFSET_OF (EFI_MM_COMMUNICATE_HEADER, Data) + sizeof (VAR_CHECK_POLICY_COMM_HEADER);
  Status       = SafeUintnAdd (RequiredSize, NewPolicy->Size, &RequiredSize);
  if (EFI_ERROR (Status) || (RequiredSize > mMmCommunicationBufferSize)) {
    DEBUG ((
      DEBUG_ERROR,
      "%a - Policy too large for buffer! %r, %d > %d \n",
      __func__,
      Status,
      RequiredSize,
      mMmCommunicationBufferSize
      ));
    return EFI_OUT_OF_RESOURCES;
  }

  AcquireLockOnlyAtBootTime (&mMmCommunicationLock);

  // Set up the MM communication.
  BufferSize   = mMmCommunicationBufferSize;
  CommHeader   = mMmCommunicationBuffer;
  PolicyHeader = (VAR_CHECK_POLICY_COMM_HEADER *)&CommHeader->Data;
  PolicyBuffer = (VOID *)(PolicyHeader + 1);
  CopyGuid (&CommHeader->HeaderGuid, &gVarCheckPolicyLibMmiHandlerGuid);
  CommHeader->MessageLength = BufferSize - OFFSET_OF (EFI_MM_COMMUNICATE_HEADER, Data);
  PolicyHeader->Signature   = VAR_CHECK_POLICY_COMM_SIG;
  PolicyHeader->Revision    = VAR_CHECK_POLICY_COMM_REVISION;
  PolicyHeader->Command     = VAR_CHECK_POLICY_COMMAND_REGISTER;

  // Copy the policy into place. This copy is safe because we've already tested above.
  CopyMem (PolicyBuffer, NewPolicy, NewPolicy->Size);

  Status = InternalMmCommunicate (CommHeader, &BufferSize);
  DEBUG ((DEBUG_VERBOSE, "%a - MmCommunication returned %r.\n", __func__, Status));

  ReleaseLockOnlyAtBootTime (&mMmCommunicationLock);

  return (EFI_ERROR (Status)) ? Status : PolicyHeader->Result;
}

/**
  This helper function takes care of the overhead of formatting, sending, and interpreting
  the results for a single DumpVariablePolicy request.

  @param[in]      PageRequested   The page of the paginated results from MM. 0 for metadata.
  @param[out]     TotalSize       The total size of the entire buffer. Returned as part of metadata.
  @param[out]     PageSize        The size of the current page being returned. Not valid as part of metadata.
  @param[out]     HasMore         A flag indicating whether there are more pages after this one.
  @param[out]     Buffer          The start of the current page from MM.

  @retval     EFI_SUCCESS             Output params have been updated (either metadata or dump page).
  @retval     EFI_INVALID_PARAMETER   One of the output params is NULL.
  @retval     Others                  Response from MM handler.

**/
STATIC
EFI_STATUS
DumpVariablePolicyHelper (
  IN  UINT32   PageRequested,
  OUT UINT32   *TotalSize,
  OUT UINT32   *PageSize,
  OUT BOOLEAN  *HasMore,
  OUT UINT8    **Buffer
  )
{
  EFI_STATUS                         Status;
  EFI_MM_COMMUNICATE_HEADER          *CommHeader;
  VAR_CHECK_POLICY_COMM_HEADER       *PolicyHeader;
  VAR_CHECK_POLICY_COMM_DUMP_PARAMS  *CommandParams;
  UINTN                              BufferSize;

  if ((TotalSize == NULL) || (PageSize == NULL) || (HasMore == NULL) || (Buffer == NULL)) {
    return EFI_INVALID_PARAMETER;
  }

  // Set up the MM communication.
  BufferSize    = mMmCommunicationBufferSize;
  CommHeader    = mMmCommunicationBuffer;
  PolicyHeader  = (VAR_CHECK_POLICY_COMM_HEADER *)&CommHeader->Data;
  CommandParams = (VAR_CHECK_POLICY_COMM_DUMP_PARAMS *)(PolicyHeader + 1);
  CopyGuid (&CommHeader->HeaderGuid, &gVarCheckPolicyLibMmiHandlerGuid);
  CommHeader->MessageLength = BufferSize - OFFSET_OF (EFI_MM_COMMUNICATE_HEADER, Data);
  PolicyHeader->Signature   = VAR_CHECK_POLICY_COMM_SIG;
  PolicyHeader->Revision    = VAR_CHECK_POLICY_COMM_REVISION;
  PolicyHeader->Command     = VAR_CHECK_POLICY_COMMAND_DUMP;

  CommandParams->PageRequested = PageRequested;

  Status = InternalMmCommunicate (CommHeader, &BufferSize);
  DEBUG ((DEBUG_VERBOSE, "%a - MmCommunication returned %r.\n", __func__, Status));

  if (!EFI_ERROR (Status)) {
    Status     = PolicyHeader->Result;
    *TotalSize = CommandParams->TotalSize;
    *PageSize  = CommandParams->PageSize;
    *HasMore   = CommandParams->HasMore;
    *Buffer    = (UINT8 *)(CommandParams + 1);
  }

  return Status;
}

/**
  This API function will dump the entire contents of the variable policy table.

  Similar to GetVariable, the first call can be made with a 0 size and it will return
  the size of the buffer required to hold the entire table.

  @param[out]     Policy  Pointer to the policy buffer. Can be NULL if Size is 0.
  @param[in,out]  Size    On input, the size of the output buffer. On output, the size
                          of the data returned.

  @retval     EFI_SUCCESS             Policy data is in the output buffer and Size has been updated.
  @retval     EFI_INVALID_PARAMETER   Size is NULL, or Size is non-zero and Policy is NULL.
  @retval     EFI_BUFFER_TOO_SMALL    Size is insufficient to hold policy. Size updated with required size.

**/
STATIC
EFI_STATUS
EFIAPI
ProtocolDumpVariablePolicy (
  OUT UINT8      *Policy OPTIONAL,
  IN OUT UINT32  *Size
  )
{
  EFI_STATUS  Status;
  UINT8       *Source;
  UINT8       *Destination;
  UINT32      PolicySize;
  UINT32      PageSize;
  BOOLEAN     HasMore;
  UINT32      PageIndex;

  if ((Size == NULL) || ((*Size > 0) && (Policy == NULL))) {
    return EFI_INVALID_PARAMETER;
  }

  AcquireLockOnlyAtBootTime (&mMmCommunicationLock);

  // Repeat this whole process until we either have a failure case or get the entire buffer.
  do {
    // First, we must check the zero page to determine the buffer size and
    // reset the internal state.
    PolicySize = 0;
    PageSize   = 0;
    HasMore    = FALSE;
    Status     = DumpVariablePolicyHelper (0, &PolicySize, &PageSize, &HasMore, &Source);
    if (EFI_ERROR (Status)) {
      break;
    }

    // If we're good, we can at least check the required size now.
    if (*Size < PolicySize) {
      *Size  = PolicySize;
      Status = EFI_BUFFER_TOO_SMALL;
      break;
    }

    // On further thought, let's update the size either way.
    *Size = PolicySize;
    // And get ready to ROCK.
    Destination = Policy;

    // Keep looping and copying until we're either done or freak out.
    for (PageIndex = 1; !EFI_ERROR (Status) && HasMore && PageIndex < MAX_UINT32; PageIndex++) {
      Status = DumpVariablePolicyHelper (PageIndex, &PolicySize, &PageSize, &HasMore, &Source);
      if (!EFI_ERROR (Status)) {
        CopyMem (Destination, Source, PageSize);
        Destination += PageSize;
      }
    }

    // Next, we check to see whether
  } while (Status == EFI_TIMEOUT);

  ReleaseLockOnlyAtBootTime (&mMmCommunicationLock);

  // There's currently no use for this, but it shouldn't be hard to implement.
  return Status;
}

/**
  This API function locks the interface so that no more policy updates
  can be performed or changes made to the enforcement until the next boot.

  @retval     EFI_SUCCESS
  @retval     Others        An error has prevented this command from completing.

**/
STATIC
EFI_STATUS
EFIAPI
ProtocolLockVariablePolicy (
  VOID
  )
{
  EFI_STATUS                    Status;
  EFI_MM_COMMUNICATE_HEADER     *CommHeader;
  VAR_CHECK_POLICY_COMM_HEADER  *PolicyHeader;
  UINTN                         BufferSize;

  AcquireLockOnlyAtBootTime (&mMmCommunicationLock);

  // Set up the MM communication.
  BufferSize   = mMmCommunicationBufferSize;
  CommHeader   = mMmCommunicationBuffer;
  PolicyHeader = (VAR_CHECK_POLICY_COMM_HEADER *)&CommHeader->Data;
  CopyGuid (&CommHeader->HeaderGuid, &gVarCheckPolicyLibMmiHandlerGuid);
  CommHeader->MessageLength = BufferSize - OFFSET_OF (EFI_MM_COMMUNICATE_HEADER, Data);
  PolicyHeader->Signature   = VAR_CHECK_POLICY_COMM_SIG;
  PolicyHeader->Revision    = VAR_CHECK_POLICY_COMM_REVISION;
  PolicyHeader->Command     = VAR_CHECK_POLICY_COMMAND_LOCK;

  Status = InternalMmCommunicate (CommHeader, &BufferSize);
  DEBUG ((DEBUG_VERBOSE, "%a - MmCommunication returned %r.\n", __func__, Status));

  ReleaseLockOnlyAtBootTime (&mMmCommunicationLock);

  return (EFI_ERROR (Status)) ? Status : PolicyHeader->Result;
}

/**
  Internal implementation to retrieve variable information for a given UEFI variable that is shared
  between different policy types.

  Currently, the two policy structure types supported (and all that is defined) are VARIABLE_POLICY_ENTRY
  and VARIABLE_LOCK_ON_VAR_STATE_POLICY.

  @param[in]      Command                               The command value to use in the communicate call.
  @param[in]      VariableName                          The name of the variable to use for the policy search.
  @param[in]      VendorGuid                            The vendor GUID of the variable to use for the policy search.
  @param[in,out]  VariablePolicyVariableNameBufferSize  On input, the size, in bytes, of the VariablePolicyVariableName
                                                        buffer.

                                                        On output, the size, in bytes, needed to store the variable
                                                        policy variable name.

                                                        If testing for the VariablePolicyVariableName buffer size
                                                        needed, set this value to zero so EFI_BUFFER_TOO_SMALL is
                                                        guaranteed to be returned if the variable policy variable name
                                                        is found.
  @param[out]     VariablePolicy                        Pointer to a buffer where the policy entry will be written
                                                        if found.
  @param[out]     VariablePolicyVariableName            Pointer to a buffer where the variable name used for the
                                                        variable policy will be written if a variable name is
                                                        registered.

                                                        If the variable policy is not associated with a variable name
                                                        (e.g. applied to variable vendor namespace) and this parameter
                                                        is given, this parameter will not be modified and
                                                        VariablePolicyVariableNameBufferSize will be set to zero to
                                                        indicate a name was not present.

                                                        If the pointer given is not NULL,
                                                        VariablePolicyVariableNameBufferSize must be non-NULL.

  @retval     EFI_SUCCESS             A variable policy entry was found and returned successfully.
  @retval     EFI_BAD_BUFFER_SIZE     An internal buffer size caused a calculation error.
  @retval     EFI_BUFFER_TOO_SMALL    The VariablePolicyVariableName buffer value is too small for the size needed.
                                      The buffer should now point to the size needed.
  @retval     EFI_NOT_READY           Variable policy has not yet been initialized.
  @retval     EFI_INVALID_PARAMETER   A required pointer argument passed is NULL. This will be returned if
                                      VariablePolicyVariableName is non-NULL and VariablePolicyVariableNameBufferSize
                                      is NULL. It can also be returned if the Command value provided is invalid.
  @retval     EFI_NOT_FOUND           A variable policy was not found for the given UEFI variable name and vendor GUID.


**/
STATIC
EFI_STATUS
InternalProtocolGetVariablePolicyInfo (
  IN      UINT32 Command,
  IN      CONST CHAR16 *VariableName,
  IN      CONST EFI_GUID *VendorGuid,
  IN OUT  UINTN *VariablePolicyVariableNameBufferSize, OPTIONAL
  OUT     VOID                        *VariablePolicy,
  OUT     CHAR16                      *VariablePolicyVariableName             OPTIONAL
  )
{
  EFI_STATUS                             Status;
  CHAR16                                 *OutputVariableName;
  EFI_MM_COMMUNICATE_HEADER              *CommHeader;
  VAR_CHECK_POLICY_COMM_HEADER           *PolicyHeader;
  VAR_CHECK_POLICY_COMM_GET_INFO_PARAMS  *CommandParams;
  UINTN                                  AllowedOutputVariableNameSize;
  UINTN                                  BufferSize;
  UINTN                                  VariableNameSize;

  if ((VariableName == NULL) || (VendorGuid == NULL) || (VariablePolicy == NULL)) {
    return EFI_INVALID_PARAMETER;
  }

  switch (Command) {
    case VAR_CHECK_POLICY_COMMAND_GET_INFO:
    case VAR_CHECK_POLICY_COMMAND_GET_LOCK_VAR_STATE_INFO:
      break;
    default:
      return EFI_INVALID_PARAMETER;
  }

  AcquireLockOnlyAtBootTime (&mMmCommunicationLock);

  VariableNameSize = StrnSizeS (
                       VariableName,
                       (VAR_CHECK_POLICY_MM_GET_INFO_BUFFER_SIZE - VAR_CHECK_POLICY_COMM_GET_INFO_PARAMS_END)
                       );
  if (VariableNameSize >= (VAR_CHECK_POLICY_MM_GET_INFO_BUFFER_SIZE - VAR_CHECK_POLICY_COMM_GET_INFO_PARAMS_END)) {
    Status = EFI_INVALID_PARAMETER;
    goto Done;
  }

  if ((VariablePolicyVariableName != NULL) && (VariablePolicyVariableNameBufferSize == NULL)) {
    return EFI_INVALID_PARAMETER;
    goto Done;
  }

  BufferSize = mMmCommunicationBufferSize;
  CommHeader = mMmCommunicationBuffer;

  Status =  SafeUintnSub (
              VAR_CHECK_POLICY_MM_GET_INFO_BUFFER_SIZE,
              VariableNameSize,
              &AllowedOutputVariableNameSize
              );
  if (EFI_ERROR (Status)) {
    Status = EFI_INVALID_PARAMETER;
    goto Done;
  }

  if (VariablePolicyVariableNameBufferSize != NULL) {
    AllowedOutputVariableNameSize = MIN (AllowedOutputVariableNameSize, *VariablePolicyVariableNameBufferSize);
  } else {
    AllowedOutputVariableNameSize = 0;
  }

  PolicyHeader  = (VAR_CHECK_POLICY_COMM_HEADER *)&CommHeader->Data;
  CommandParams = (VAR_CHECK_POLICY_COMM_GET_INFO_PARAMS *)(PolicyHeader + 1);

  CopyGuid (&CommHeader->HeaderGuid, &gVarCheckPolicyLibMmiHandlerGuid);
  CommHeader->MessageLength = BufferSize - OFFSET_OF (EFI_MM_COMMUNICATE_HEADER, Data);
  PolicyHeader->Signature   = VAR_CHECK_POLICY_COMM_SIG;
  PolicyHeader->Revision    = VAR_CHECK_POLICY_COMM_REVISION;
  PolicyHeader->Command     = Command;

  ZeroMem ((VOID *)&CommandParams->OutputPolicyEntry, sizeof (CommandParams->OutputPolicyEntry));
  CopyGuid (&CommandParams->InputVendorGuid, VendorGuid);
  Status = SafeUintnToUint32 (VariableNameSize, &CommandParams->InputVariableNameSize);
  if (EFI_ERROR (Status)) {
    Status = EFI_INVALID_PARAMETER;
    goto Done;
  }

  Status = SafeUintnToUint32 (AllowedOutputVariableNameSize, &CommandParams->OutputVariableNameSize);
  if (EFI_ERROR (Status)) {
    Status = EFI_INVALID_PARAMETER;
    goto Done;
  }

  if (AllowedOutputVariableNameSize > 0) {
    Status =  StrnCpyS (
                CommandParams->InputVariableName,
                AllowedOutputVariableNameSize / sizeof (CHAR16),
                VariableName,
                (UINTN)CommandParams->InputVariableNameSize / sizeof (CHAR16)
                );
    ASSERT_EFI_ERROR (Status);
  }

  Status = InternalMmCommunicate (CommHeader, &BufferSize);
  if (Status == EFI_SUCCESS) {
    CopyMem (
      VariablePolicy,
      (VOID *)&CommandParams->OutputPolicyEntry,
      (Command == VAR_CHECK_POLICY_COMMAND_GET_INFO) ?
      sizeof (CommandParams->OutputPolicyEntry.VariablePolicy) :
      sizeof (CommandParams->OutputPolicyEntry.LockOnVarStatePolicy)
      );

    if (VariablePolicyVariableNameBufferSize == NULL) {
      if (VariablePolicyVariableName != NULL) {
        Status = EFI_INVALID_PARAMETER;
      }

      goto Done;
    }

    if (PolicyHeader->Result == EFI_BUFFER_TOO_SMALL) {
      *VariablePolicyVariableNameBufferSize = (UINTN)CommandParams->OutputVariableNameSize;
      goto Done;
    }

    if (PolicyHeader->Result == EFI_SUCCESS) {
      if (CommandParams->OutputVariableNameSize > 0) {
        Status =  SafeUintnAdd (
                    ((UINTN)CommandParams + VAR_CHECK_POLICY_COMM_GET_INFO_PARAMS_END),
                    (UINTN)CommandParams->InputVariableNameSize,
                    (UINTN *)&OutputVariableName
                    );
        if (EFI_ERROR (Status)) {
          Status = EFI_BAD_BUFFER_SIZE;
          goto Done;
        }

        Status =  StrnCpyS (
                    VariablePolicyVariableName,
                    *VariablePolicyVariableNameBufferSize / sizeof (CHAR16),
                    OutputVariableName,
                    (UINTN)CommandParams->OutputVariableNameSize
                    );
        ASSERT_EFI_ERROR (Status);
        *VariablePolicyVariableNameBufferSize = (UINTN)CommandParams->OutputVariableNameSize;
      } else {
        // A variable policy variable name is not present. Return values according to interface.
        *VariablePolicyVariableNameBufferSize = 0;
      }
    }
  }

Done:
  ReleaseLockOnlyAtBootTime (&mMmCommunicationLock);

  return (EFI_ERROR (Status)) ? Status : PolicyHeader->Result;
}

/**
  This function will return variable policy information for a UEFI variable with a
  registered variable policy.

  @param[in]      VariableName                          The name of the variable to use for the policy search.
  @param[in]      VendorGuid                            The vendor GUID of the variable to use for the policy search.
  @param[in,out]  VariablePolicyVariableNameBufferSize  On input, the size, in bytes, of the VariablePolicyVariableName
                                                        buffer.

                                                        On output, the size, in bytes, needed to store the variable
                                                        policy variable name.

                                                        If testing for the VariablePolicyVariableName buffer size
                                                        needed, set this value to zero so EFI_BUFFER_TOO_SMALL is
                                                        guaranteed to be returned if the variable policy variable name
                                                        is found.
  @param[out]     VariablePolicy                        Pointer to a buffer where the policy entry will be written
                                                        if found.
  @param[out]     VariablePolicyVariableName            Pointer to a buffer where the variable name used for the
                                                        variable policy will be written if a variable name is
                                                        registered.

                                                        If the variable policy is not associated with a variable name
                                                        (e.g. applied to variable vendor namespace) and this parameter
                                                        is given, this parameter will not be modified and
                                                        VariablePolicyVariableNameBufferSize will be set to zero to
                                                        indicate a name was not present.

                                                        If the pointer given is not NULL,
                                                        VariablePolicyVariableNameBufferSize must be non-NULL.

  @retval     EFI_SUCCESS             A variable policy entry was found and returned successfully.
  @retval     EFI_BAD_BUFFER_SIZE     An internal buffer size caused a calculation error.
  @retval     EFI_BUFFER_TOO_SMALL    The VariablePolicyVariableName buffer value is too small for the size needed.
                                      The buffer should now point to the size needed.
  @retval     EFI_NOT_READY           Variable policy has not yet been initialized.
  @retval     EFI_INVALID_PARAMETER   A required pointer argument passed is NULL. This will be returned if
                                      VariablePolicyVariableName is non-NULL and VariablePolicyVariableNameBufferSize
                                      is NULL.
  @retval     EFI_NOT_FOUND           A variable policy was not found for the given UEFI variable name and vendor GUID.

**/
STATIC
EFI_STATUS
EFIAPI
ProtocolGetVariablePolicyInfo (
  IN      CONST CHAR16 *VariableName,
  IN      CONST EFI_GUID *VendorGuid,
  IN OUT  UINTN *VariablePolicyVariableNameBufferSize, OPTIONAL
  OUT     VARIABLE_POLICY_ENTRY       *VariablePolicy,
  OUT     CHAR16                      *VariablePolicyVariableName             OPTIONAL
  )
{
  return InternalProtocolGetVariablePolicyInfo (
           VAR_CHECK_POLICY_COMMAND_GET_INFO,
           VariableName,
           VendorGuid,
           VariablePolicyVariableNameBufferSize,
           VariablePolicy,
           VariablePolicyVariableName
           );
}

/**
  This function will return the Lock on Variable State policy information for the policy
  associated with the given UEFI variable.

  @param[in]      VariableName                              The name of the variable to use for the policy search.
  @param[in]      VendorGuid                                The vendor GUID of the variable to use for the policy
                                                            search.
  @param[in,out]  VariableLockPolicyVariableNameBufferSize  On input, the size, in bytes, of the
                                                            VariableLockPolicyVariableName buffer.

                                                            On output, the size, in bytes, needed to store the variable
                                                            policy variable name.

                                                            If testing for the VariableLockPolicyVariableName buffer
                           P                                 size needed, set this value to zero so EFI_BUFFER_TOO_SMALL
                                                            is guaranteed to be returned if the variable policy variable
                                                            name is found.
  @param[out]     VariablePolicy                            Pointer to a buffer where the policy entry will be written
                                                            if found.
  @param[out]     VariableLockPolicyVariableName            Pointer to a buffer where the variable name used for the
                                                            variable lock on variable state policy will be written if
                                                            a variable name is registered.

                                                            If the lock on variable policy is not associated with a
                                                            variable name (e.g. applied to variable vendor namespace)
                                                            and this parameter is given, this parameter will not be
                                                            modified and VariableLockPolicyVariableNameBufferSize will
                                                            be set to zero to indicate a name was not present.

                                                            If the pointer given is not NULL,
                                                            VariableLockPolicyVariableNameBufferSize must be non-NULL.

  @retval     EFI_SUCCESS             A Lock on Variable State variable policy entry was found and returned
                                      successfully.
  @retval     EFI_BAD_BUFFER_SIZE     An internal buffer size caused a calculation error.
  @retval     EFI_BUFFER_TOO_SMALL    The VariableLockPolicyVariableName buffer is too small for the size needed.
                                      The buffer should now point to the size needed.
  @retval     EFI_NOT_READY           Variable policy has not yet been initialized.
  @retval     EFI_INVALID_PARAMETER   A required pointer argument passed is NULL. This will be returned if
                                      VariableLockPolicyVariableName is non-NULL and
                                      VariableLockPolicyVariableNameBufferSize is NULL.
  @retval     EFI_NOT_FOUND           A Lock on Variable State variable policy was not found for the given UEFI
                                      variable name and vendor GUID.

**/
STATIC
EFI_STATUS
EFIAPI
ProtocolGetLockOnVariableStateVariablePolicyInfo (
  IN      CONST CHAR16 *VariableName,
  IN      CONST EFI_GUID *VendorGuid,
  IN OUT  UINTN *VariableLockPolicyVariableNameBufferSize, OPTIONAL
  OUT     VARIABLE_LOCK_ON_VAR_STATE_POLICY   *VariablePolicy,
  OUT     CHAR16                              *VariableLockPolicyVariableName             OPTIONAL
  )
{
  return InternalProtocolGetVariablePolicyInfo (
           VAR_CHECK_POLICY_COMMAND_GET_LOCK_VAR_STATE_INFO,
           VariableName,
           VendorGuid,
           VariableLockPolicyVariableNameBufferSize,
           VariablePolicy,
           VariableLockPolicyVariableName
           );
}

/**
  This helper function locates the shared comm buffer and assigns it to input pointers.

  @param[in,out]  BufferSize      On input, the minimum buffer size required INCLUDING the MM communicate header.
                                  On output, the size of the matching buffer found.
  @param[out]     LocatedBuffer   A pointer to the matching buffer.

  @retval     EFI_SUCCESS
  @retval     EFI_INVALID_PARAMETER   One of the output pointers was NULL.
  @retval     EFI_OUT_OF_RESOURCES    Not enough memory to allocate a comm buffer.

**/
STATIC
EFI_STATUS
InitMmCommonCommBuffer (
  IN OUT  UINTN  *BufferSize,
  OUT     VOID   **LocatedBuffer
  )
{
  EFI_STATUS  Status;

  Status = EFI_SUCCESS;

  // Make sure that we're working with good pointers.
  if ((BufferSize == NULL) || (LocatedBuffer == NULL)) {
    return EFI_INVALID_PARAMETER;
  }

  // Allocate the runtime memory for the comm buffer.
  *LocatedBuffer = AllocateRuntimePool (*BufferSize);
  if (*LocatedBuffer == NULL) {
    Status      = EFI_OUT_OF_RESOURCES;
    *BufferSize = 0;
  }

  EfiInitializeLock (&mMmCommunicationLock, TPL_NOTIFY);

  return Status;
}

/**
  Convert internal pointer addresses to virtual addresses.

  @param[in] Event      Event whose notification function is being invoked.
  @param[in] Context    The pointer to the notification function's context, which
                        is implementation-dependent.
**/
STATIC
VOID
EFIAPI
VariablePolicyVirtualAddressCallback (
  IN  EFI_EVENT  Event,
  IN  VOID       *Context
  )
{
  EfiConvertPointer (0, (VOID **)&mMmCommunication);
  EfiConvertPointer (0, (VOID **)&mMmCommunicationBuffer);
}

/**
  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.

  @retval EFI_SUCCESS     The entry point executed successfully.
  @retval other           Some error occured when executing this entry point.

**/
EFI_STATUS
EFIAPI
VariablePolicySmmDxeMain (
  IN    EFI_HANDLE        ImageHandle,
  IN    EFI_SYSTEM_TABLE  *SystemTable
  )
{
  EFI_STATUS  Status;
  BOOLEAN     ProtocolInstalled;
  BOOLEAN     VirtualAddressChangeRegistered;
  EFI_EVENT   VirtualAddressChangeEvent;

  Status                         = EFI_SUCCESS;
  ProtocolInstalled              = FALSE;
  VirtualAddressChangeRegistered = FALSE;

  // Update the minimum buffer size.
  mMmCommunicationBufferSize = VAR_CHECK_POLICY_MM_COMM_BUFFER_SIZE;
  // Locate the shared comm buffer to use for sending MM commands.
  Status = InitMmCommonCommBuffer (&mMmCommunicationBufferSize, &mMmCommunicationBuffer);
  if (EFI_ERROR (Status)) {
    DEBUG ((DEBUG_ERROR, "%a - Failed to locate a viable MM comm buffer! %r\n", __func__, Status));
    ASSERT_EFI_ERROR (Status);
    return Status;
  }

  // Locate the MmCommunication protocol.
  Status = gBS->LocateProtocol (&gEfiMmCommunication2ProtocolGuid, NULL, (VOID **)&mMmCommunication);
  if (EFI_ERROR (Status)) {
    DEBUG ((DEBUG_ERROR, "%a - Failed to locate MmCommunication protocol! %r\n", __func__, Status));
    ASSERT_EFI_ERROR (Status);
    return Status;
  }

  // Configure the VariablePolicy protocol structure.
  mVariablePolicyProtocol.Revision                                 = EDKII_VARIABLE_POLICY_PROTOCOL_REVISION;
  mVariablePolicyProtocol.DisableVariablePolicy                    = ProtocolDisableVariablePolicy;
  mVariablePolicyProtocol.IsVariablePolicyEnabled                  = ProtocolIsVariablePolicyEnabled;
  mVariablePolicyProtocol.RegisterVariablePolicy                   = ProtocolRegisterVariablePolicy;
  mVariablePolicyProtocol.DumpVariablePolicy                       = ProtocolDumpVariablePolicy;
  mVariablePolicyProtocol.LockVariablePolicy                       = ProtocolLockVariablePolicy;
  mVariablePolicyProtocol.GetVariablePolicyInfo                    = ProtocolGetVariablePolicyInfo;
  mVariablePolicyProtocol.GetLockOnVariableStateVariablePolicyInfo = ProtocolGetLockOnVariableStateVariablePolicyInfo;

  // Register all the protocols and return the status.
  Status = gBS->InstallMultipleProtocolInterfaces (
                  &ImageHandle,
                  &gEdkiiVariablePolicyProtocolGuid,
                  &mVariablePolicyProtocol,
                  NULL
                  );
  if (EFI_ERROR (Status)) {
    DEBUG ((DEBUG_ERROR, "%a - Failed to install protocol! %r\n", __func__, Status));
    goto Exit;
  } else {
    ProtocolInstalled = TRUE;
  }

  // Normally, we might want to register a callback
  // to lock the interface, but this is integrated
  // into the existing callbacks in VaraiableSmm.c
  // and VariableDxe.c.

  //
  // Register a VirtualAddressChange callback for the MmComm protocol and Comm buffer.
  Status = gBS->CreateEventEx (
                  EVT_NOTIFY_SIGNAL,
                  TPL_NOTIFY,
                  VariablePolicyVirtualAddressCallback,
                  NULL,
                  &gEfiEventVirtualAddressChangeGuid,
                  &VirtualAddressChangeEvent
                  );
  if (EFI_ERROR (Status)) {
    DEBUG ((DEBUG_ERROR, "%a - Failed to create VirtualAddressChange event! %r\n", __func__, Status));
    goto Exit;
  } else {
    VirtualAddressChangeRegistered = TRUE;
  }

Exit:
  //
  // If we're about to return a failed status (and unload this driver), we must first undo anything that
  // has been successfully done.
  if (EFI_ERROR (Status)) {
    if (ProtocolInstalled) {
      gBS->UninstallProtocolInterface (&ImageHandle, &gEdkiiVariablePolicyProtocolGuid, &mVariablePolicyProtocol);
    }

    if (VirtualAddressChangeRegistered) {
      gBS->CloseEvent (VirtualAddressChangeEvent);
    }
  }

  return Status;
}