Re: [edk2-devel] [PATCH v3 10/11] UefiCpuPkg, OvmfPkg: Disable interrupts when using the GHCB

["Dong, Eric" <eric.dong@intel.com>]

Acked-by: Eric Dong <eric.dong@intel.com>

-----Original Message-----
From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of Lendacky, Thomas
Sent: Thursday, October 29, 2020 10:18 PM
To: devel@edk2.groups.io
Cc: Brijesh Singh <brijesh.singh@amd.com>; Dong, Eric <eric.dong@intel.com>; Ni, Ray <ray.ni@intel.com>; Laszlo Ersek <lersek@redhat.com>; Kumar, Rahul1 <rahul1.kumar@intel.com>; Justen, Jordan L <jordan.l.justen@intel.com>; Ard Biesheuvel <ard.biesheuvel@arm.com>
Subject: [edk2-devel] [PATCH v3 10/11] UefiCpuPkg, OvmfPkg: Disable interrupts when using the GHCB

From: Tom Lendacky <thomas.lendacky@amd.com>

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: 437eb3f7a8db7681afe0e6064d3a8edb12abb766
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>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 UefiCpuPkg/Include/Library/VmgExitLib.h               | 14 ++++++++---
 OvmfPkg/Library/VmgExitLib/VmgExitLib.c               | 26 +++++++++++++++++---
 OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c         |  5 ++--
 OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c |  5 ++--
 UefiCpuPkg/Library/MpInitLib/DxeMpLib.c               |  5 ++--
 UefiCpuPkg/Library/MpInitLib/MpLib.c                  |  7 +++---
 UefiCpuPkg/Library/VmgExitLibNull/VmgExitLibNull.c    | 18 ++++++++------
 7 files changed, 55 insertions(+), 25 deletions(-)

diff --git a/UefiCpuPkg/Include/Library/VmgExitLib.h b/UefiCpuPkg/Include/Library/VmgExitLib.h
index 07e8af6450b9..061948cf840d 100644
--- a/UefiCpuPkg/Include/Library/VmgExitLib.h
+++ b/UefiCpuPkg/Include/Library/VmgExitLib.h
@@ -50,13 +50,16 @@ VmgExit (
   Performs the necessary steps in preparation for invoking VMGEXIT. Must be
   called before setting any fields within the GHCB.
 
-  @param[in, out]  Ghcb       A pointer to the GHCB
+  @param[in, out]  Ghcb            A pointer to the GHCB
+  @param[in, out]  InterruptState  A pointer to hold the current interrupt
+                                   state, used for restoring in VmgDone 
+ ()
 
 **/
 VOID
 EFIAPI
 VmgInit (
-  IN OUT GHCB                *Ghcb
+  IN OUT GHCB                *Ghcb,
+  IN OUT BOOLEAN             *InterruptState
   );
 
 /**
@@ -65,13 +68,16 @@ VmgInit (
   Performs the necessary steps to cleanup after invoking VMGEXIT. Must be
   called after obtaining needed fields within the GHCB.
 
-  @param[in, out]  Ghcb       A pointer to the GHCB
+  @param[in, out]  Ghcb            A pointer to the GHCB
+  @param[in]       InterruptState  An indicator to conditionally (re)enable
+                                   interrupts
 
 **/
 VOID
 EFIAPI
 VmgDone (
-  IN OUT GHCB                *Ghcb
+  IN OUT GHCB                *Ghcb,
+  IN     BOOLEAN             InterruptState
   );
 
 /**
diff --git a/OvmfPkg/Library/VmgExitLib/VmgExitLib.c b/OvmfPkg/Library/VmgExitLib/VmgExitLib.c
index 0540df8a04d4..bc5cd61d751f 100644
--- a/OvmfPkg/Library/VmgExitLib/VmgExitLib.c
+++ b/OvmfPkg/Library/VmgExitLib/VmgExitLib.c
@@ -132,15 +132,27 @@ VmgExit (
   Performs the necessary steps in preparation for invoking VMGEXIT. Must be
   called before setting any fields within the GHCB.
 
-  @param[in, out]  Ghcb       A pointer to the GHCB
+  @param[in, out]  Ghcb            A pointer to the GHCB
+  @param[in, out]  InterruptState  A pointer to hold the current interrupt
+                                   state, used for restoring in VmgDone 
+ ()
 
 **/
 VOID
 EFIAPI
 VmgInit (
-  IN OUT GHCB                *Ghcb
+  IN OUT GHCB                *Ghcb,
+  IN OUT BOOLEAN             *InterruptState
   )
 {
+  //
+  // Be sure that an interrupt can't cause a #VC while the GHCB is  // 
+ being used.
+  //
+  *InterruptState = GetInterruptState ();  if (*InterruptState) {
+    DisableInterrupts ();
+  }
+
   SetMem (&Ghcb->SaveArea, sizeof (Ghcb->SaveArea), 0);  }
 
@@ -150,15 +162,21 @@ VmgInit (
   Performs the necessary steps to cleanup after invoking VMGEXIT. Must be
   called after obtaining needed fields within the GHCB.
 
-  @param[in, out]  Ghcb       A pointer to the GHCB
+  @param[in, out]  Ghcb            A pointer to the GHCB
+  @param[in]       InterruptState  An indicator to conditionally (re)enable
+                                   interrupts
 
 **/
 VOID
 EFIAPI
 VmgDone (
-  IN OUT GHCB                *Ghcb
+  IN OUT GHCB                *Ghcb,
+  IN     BOOLEAN             InterruptState
   )
 {
+  if (InterruptState) {
+    EnableInterrupts ();
+  }
 }
 
 /**
diff --git a/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c b/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
index 9bf9d160179c..1671db3a01b1 100644
--- a/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
+++ b/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
@@ -1568,6 +1568,7 @@ VmgExitHandleVc (
   SEV_ES_INSTRUCTION_DATA   InstructionData;
   UINT64                    ExitCode, Status;
   EFI_STATUS                VcRet;
+  BOOLEAN                   InterruptState;
 
   VcRet = EFI_SUCCESS;
 
@@ -1578,7 +1579,7 @@ VmgExitHandleVc (
   Regs = SystemContext.SystemContextX64;
   Ghcb = Msr.Ghcb;
 
-  VmgInit (Ghcb);
+  VmgInit (Ghcb, &InterruptState);
 
   ExitCode = Regs->ExceptionData;
   switch (ExitCode) {
@@ -1662,7 +1663,7 @@ VmgExitHandleVc (
     VcRet = EFI_PROTOCOL_ERROR;
   }
 
-  VmgDone (Ghcb);
+  VmgDone (Ghcb, InterruptState);
 
   return VcRet;
 }
diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
index f9b21b54137d..1b0742967f71 100644
--- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
+++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
@@ -52,6 +52,7 @@ QemuFlashPtrWrite (
   if (MemEncryptSevEsIsEnabled ()) {
     MSR_SEV_ES_GHCB_REGISTER  Msr;
     GHCB                      *Ghcb;
+    BOOLEAN                   InterruptState;
 
     Msr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);
     Ghcb = Msr.Ghcb;
@@ -63,12 +64,12 @@ QemuFlashPtrWrite (
     // #VC exception. Instead, use the the VMGEXIT MMIO write support directly
     // to perform the update.
     //
-    VmgInit (Ghcb);
+    VmgInit (Ghcb, &InterruptState);
     Ghcb->SharedBuffer[0] = Value;
     Ghcb->SaveArea.SwScratch = (UINT64) (UINTN) Ghcb->SharedBuffer;
     VmgSetOffsetValid (Ghcb, GhcbSwScratch);
     VmgExit (Ghcb, SVM_EXIT_MMIO_WRITE, (UINT64) (UINTN) Ptr, 1);
-    VmgDone (Ghcb);
+    VmgDone (Ghcb, InterruptState);
   } else {
     *Ptr = Value;
   }
diff --git a/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c b/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
index 2c00d72ddefe..7839c249760e 100644
--- a/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
+++ b/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
@@ -171,6 +171,7 @@ GetSevEsAPMemory (
   EFI_PHYSICAL_ADDRESS      StartAddress;
   MSR_SEV_ES_GHCB_REGISTER  Msr;
   GHCB                      *Ghcb;
+  BOOLEAN                   InterruptState;
 
   //
   // Allocate 1 page for AP jump table page @@ -192,9 +193,9 @@ GetSevEsAPMemory (
   Msr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);
   Ghcb = Msr.Ghcb;
 
-  VmgInit (Ghcb);
+  VmgInit (Ghcb, &InterruptState);
   VmgExit (Ghcb, SVM_EXIT_AP_JUMP_TABLE, 0, (UINT64) (UINTN) StartAddress);
-  VmgDone (Ghcb);
+  VmgDone (Ghcb, InterruptState);
 
   return (UINTN) StartAddress;
 }
diff --git a/UefiCpuPkg/Library/MpInitLib/MpLib.c b/UefiCpuPkg/Library/MpInitLib/MpLib.c
index 6d977d45bcdd..1f47ff3f73b5 100644
--- a/UefiCpuPkg/Library/MpInitLib/MpLib.c
+++ b/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);
           }
 
           //
diff --git a/UefiCpuPkg/Library/VmgExitLibNull/VmgExitLibNull.c b/UefiCpuPkg/Library/VmgExitLibNull/VmgExitLibNull.c
index b47e282aff82..89b065cb3ff3 100644
--- a/UefiCpuPkg/Library/VmgExitLibNull/VmgExitLibNull.c
+++ b/UefiCpuPkg/Library/VmgExitLibNull/VmgExitLibNull.c
@@ -57,15 +57,16 @@ VmgExit (
   Performs the necessary steps in preparation for invoking VMGEXIT. Must be
   called before setting any fields within the GHCB.
 
-  The base library function does nothing.
-
-  @param[in, out]  Ghcb       A pointer to the GHCB
+  @param[in, out]  Ghcb            A pointer to the GHCB
+  @param[in, out]  InterruptState  A pointer to hold the current interrupt
+                                   state, used for restoring in VmgDone 
+ ()
 
 **/
 VOID
 EFIAPI
 VmgInit (
-  IN OUT GHCB                *Ghcb
+  IN OUT GHCB                *Ghcb,
+  IN OUT BOOLEAN             *InterruptState
   )
 {
 }
@@ -76,15 +77,16 @@ VmgInit (
   Performs the necessary steps to cleanup after invoking VMGEXIT. Must be
   called after obtaining needed fields within the GHCB.
 
-  The base library function does nothing.
-
-  @param[in, out]  Ghcb       A pointer to the GHCB
+  @param[in, out]  Ghcb            A pointer to the GHCB
+  @param[in]       InterruptState  An indicator to conditionally (re)enable
+                                   interrupts
 
 **/
 VOID
 EFIAPI
 VmgDone (
-  IN OUT GHCB                *Ghcb
+  IN OUT GHCB                *Ghcb,
+  IN     BOOLEAN             InterruptState
   )
 {
 }
--
2.28.0