UefiCpuPkg/PiSmmCpuDxeSmm: [CVE-2017-5753] Fix bounds check bypass

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1194

Speculative execution is used by processor to avoid having to wait for
data to arrive from memory, or for previous operations to finish, the
processor may speculate as to what will be executed.

If the speculation is incorrect, the speculatively executed instructions
might leave hints such as which memory locations have been brought into
cache. Malicious actors can use the bounds check bypass method (code
gadgets with controlled external inputs) to infer data values that have
been used in speculative operations to reveal secrets which should not
otherwise be accessed.

It is possible for SMI handler(s) to call EFI_SMM_CPU_PROTOCOL service
ReadSaveState() and use the content in the 'CommBuffer' (controlled
external inputs) as the 'CpuIndex'. So this commit will insert AsmLfence
API to mitigate the bounds check bypass issue within SmmReadSaveState().

For SmmReadSaveState():

The 'CpuIndex' will be passed into function ReadSaveStateRegister(). And
then in to ReadSaveStateRegisterByIndex().

With the call:
ReadSaveStateRegisterByIndex (
  CpuIndex,
  SMM_SAVE_STATE_REGISTER_IOMISC_INDEX,
  sizeof(IoMisc.Uint32),
  &IoMisc.Uint32
  );

The 'IoMisc' can be a cross boundary access during speculative execution.
Later, 'IoMisc' is used as the index to access buffers 'mSmmCpuIoWidth'
and 'mSmmCpuIoType'. One can observe which part of the content within
those buffers was brought into cache to possibly reveal the value of
'IoMisc'.

Hence, this commit adds a AsmLfence() after the check of 'CpuIndex'
within function SmmReadSaveState() to prevent the speculative execution.

A more detailed explanation of the purpose of commit is under the
'Bounds check bypass mitigation' section of the below link:
https://software.intel.com/security-software-guidance/insights/host-firmware-speculative-execution-side-channel-mitigation

And the document at:
https://software.intel.com/security-software-guidance/api-app/sites/default/files/337879-analyzing-potential-bounds-Check-bypass-vulnerabilities.pdf

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>

UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.c

@@ -237,6 +237,11 @@ SmmReadSaveState (
   if ((CpuIndex >= gSmst->NumberOfCpus) || (Buffer == NULL)) {
     return EFI_INVALID_PARAMETER;
   }
+  //
+  // The AsmLfence() call here is to ensure the above check for the CpuIndex
+  // has been completed before the execution of subsequent codes.
+  //
+  AsmLfence ();
 
   //
   // Check for special EFI_SMM_SAVE_STATE_REGISTER_PROCESSOR_ID