UefiCpuPkg, OvmfPkg: Disable interrupts when using the GHCB

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3008

The QemuFlashPtrWrite() flash services runtime uses the GHCB and VmgExit()
directly to perform the flash write when running as an SEV-ES guest. If an
interrupt arrives between VmgInit() and VmgExit(), the Dr7 read in the
interrupt handler will generate a #VC, which can overwrite information in
the GHCB that QemuFlashPtrWrite() has set. This has been seen with the
timer interrupt firing and the CpuExceptionHandlerLib library code,
UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/
  Xcode5ExceptionHandlerAsm.nasm and
  ExceptionHandlerAsm.nasm
reading the Dr7 register while QemuFlashPtrWrite() is using the GHCB. In
general, it is necessary to protect the GHCB whenever it is used, not just
in QemuFlashPtrWrite().

Disable interrupts around the usage of the GHCB by modifying the VmgInit()
and VmgDone() interfaces:
- VmgInit() will take an extra parameter that is a pointer to a BOOLEAN
  that will hold the interrupt state at the time of invocation. VmgInit()
  will get and save this interrupt state before updating the GHCB.
- VmgDone() will take an extra parameter that is used to indicate whether
  interrupts are to be (re)enabled. Before exiting, VmgDone() will enable
  interrupts if that is requested.

Fixes: 437eb3f7a8
Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Acked-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <c326a4fd78253f784b42eb317589176cf7d8592a.1604685192.git.thomas.lendacky@amd.com>

UefiCpuPkg/Library/MpInitLib/MpLib.c

@@ -884,6 +884,7 @@ ApWakeupFunction (
           GHCB                      *Ghcb;
           UINT64                    Status;
           BOOLEAN                   DoDecrement;
+          BOOLEAN                   InterruptState;
 
           DoDecrement = (BOOLEAN) (CpuMpData->InitFlag == ApInitConfig);
 
@@ -891,7 +892,7 @@ ApWakeupFunction (
             Msr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);
             Ghcb = Msr.Ghcb;
 
-            VmgInit (Ghcb);
+            VmgInit (Ghcb, &InterruptState);
 
             if (DoDecrement) {
               DoDecrement = FALSE;
@@ -905,11 +906,11 @@ ApWakeupFunction (
 
             Status = VmgExit (Ghcb, SVM_EXIT_AP_RESET_HOLD, 0, 0);
             if ((Status == 0) && (Ghcb->SaveArea.SwExitInfo2 != 0)) {
-              VmgDone (Ghcb);
+              VmgDone (Ghcb, InterruptState);
               break;
             }
 
-            VmgDone (Ghcb);
+            VmgDone (Ghcb, InterruptState);
           }
 
           //