OvfmPkg/VmgExitLib: Validate #VC MMIO is to un-encrypted memory

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3108

When SEV-ES is active, and MMIO operation will trigger a #VC and the
VmgExitLib exception handler will process this MMIO operation.

A malicious hypervisor could try to extract information from encrypted
memory by setting a reserved bit in the guests nested page tables for
a non-MMIO area. This can result in the encrypted data being copied into
the GHCB shared buffer area and accessed by the hypervisor.

Prevent this by ensuring that the MMIO source/destination is un-encrypted
memory. For the APIC register space, access is allowed in general.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <0cf28470ad5e694af45f7f0b35296628f819567d.1610045305.git.thomas.lendacky@amd.com>

OvmfPkg/AmdSev/AmdSevX64.dsc

@@ -237,6 +237,7 @@
   CpuExceptionHandlerLib|UefiCpuPkg/Library/CpuExceptionHandlerLib/SecPeiCpuExceptionHandlerLib.inf
 !endif
   VmgExitLib|OvmfPkg/Library/VmgExitLib/SecVmgExitLib.inf
+  MemEncryptSevLib|OvmfPkg/Library/BaseMemEncryptSevLib/SecMemEncryptSevLib.inf
 
 [LibraryClasses.common.PEI_CORE]
   HobLib|MdePkg/Library/PeiHobLib/PeiHobLib.inf

OvmfPkg/Library/BaseMemEncryptSevLib/DxeMemEncryptSevLib.inf

@@ -14,7 +14,7 @@
   FILE_GUID                      = c1594631-3888-4be4-949f-9c630dbc842b
   MODULE_TYPE                    = BASE
   VERSION_STRING                 = 1.0
-  LIBRARY_CLASS                  = MemEncryptSevLib|DXE_DRIVER DXE_RUNTIME_DRIVER DXE_SMM_DRIVER UEFI_DRIVER
+  LIBRARY_CLASS                  = MemEncryptSevLib|DXE_CORE DXE_DRIVER DXE_RUNTIME_DRIVER DXE_SMM_DRIVER UEFI_DRIVER
 
 #
 # The following information is for reference only and not required by the build

OvmfPkg/Library/VmgExitLib/SecVmgExitLib.inf

@@ -35,6 +35,8 @@
   BaseLib
   BaseMemoryLib
   DebugLib
+  LocalApicLib
+  MemEncryptSevLib
   PcdLib
 
 [FixedPcd]

OvmfPkg/Library/VmgExitLib/VmgExitLib.inf

@@ -35,4 +35,6 @@
   BaseLib
   BaseMemoryLib
   DebugLib
+  LocalApicLib
+  MemEncryptSevLib
 

OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c

@@ -9,6 +9,7 @@
 #include <Base.h>
 #include <Uefi.h>
 #include <Library/BaseMemoryLib.h>
+#include <Library/LocalApicLib.h>
 #include <Library/MemEncryptSevLib.h>
 #include <Library/VmgExitLib.h>
 #include <Register/Amd/Msr.h>
@@ -595,6 +596,61 @@ UnsupportedExit (
   return Status;
 }
 
+/**
+  Validate that the MMIO memory access is not to encrypted memory.
+
+  Examine the pagetable entry for the memory specified. MMIO should not be
+  performed against encrypted memory. MMIO to the APIC page is always allowed.
+
+  @param[in] Ghcb           Pointer to the Guest-Hypervisor Communication Block
+  @param[in] MemoryAddress  Memory address to validate
+  @param[in] MemoryLength   Memory length to validate
+
+  @retval 0          Memory is not encrypted
+  @return            New exception value to propogate
+
+**/
+STATIC
+UINT64
+ValidateMmioMemory (
+  IN GHCB   *Ghcb,
+  IN UINTN  MemoryAddress,
+  IN UINTN  MemoryLength
+  )
+{
+  MEM_ENCRYPT_SEV_ADDRESS_RANGE_STATE  State;
+  GHCB_EVENT_INJECTION                 GpEvent;
+  UINTN                                Address;
+
+  //
+  // Allow APIC accesses (which will have the encryption bit set during
+  // SEC and PEI phases).
+  //
+  Address = MemoryAddress & ~(SIZE_4KB - 1);
+  if (Address == GetLocalApicBaseAddress ()) {
+    return 0;
+  }
+
+  State = MemEncryptSevGetAddressRangeState (
+            0,
+            MemoryAddress,
+            MemoryLength
+            );
+  if (State == MemEncryptSevAddressRangeUnencrypted) {
+    return 0;
+  }
+
+  //
+  // Any state other than unencrypted is an error, issue a #GP.
+  //
+  GpEvent.Uint64 = 0;
+  GpEvent.Elements.Vector = GP_EXCEPTION;
+  GpEvent.Elements.Type   = GHCB_EVENT_INJECTION_TYPE_EXCEPTION;
+  GpEvent.Elements.Valid  = 1;
+
+  return GpEvent.Uint64;
+}
+
 /**
   Handle an MMIO event.
 
@@ -653,6 +709,11 @@ MmioExit (
       return UnsupportedExit (Ghcb, Regs, InstructionData);
     }
 
+    Status = ValidateMmioMemory (Ghcb, InstructionData->Ext.RmData, Bytes);
+    if (Status != 0) {
+      return Status;
+    }
+
     ExitInfo1 = InstructionData->Ext.RmData;
     ExitInfo2 = Bytes;
     CopyMem (Ghcb->SharedBuffer, &InstructionData->Ext.RegData, Bytes);
@@ -683,6 +744,11 @@ MmioExit (
     InstructionData->ImmediateSize = Bytes;
     InstructionData->End += Bytes;
 
+    Status = ValidateMmioMemory (Ghcb, InstructionData->Ext.RmData, Bytes);
+    if (Status != 0) {
+      return Status;
+    }
+
     ExitInfo1 = InstructionData->Ext.RmData;
     ExitInfo2 = Bytes;
     CopyMem (Ghcb->SharedBuffer, InstructionData->Immediate, Bytes);
@@ -717,6 +783,11 @@ MmioExit (
       return UnsupportedExit (Ghcb, Regs, InstructionData);
     }
 
+    Status = ValidateMmioMemory (Ghcb, InstructionData->Ext.RmData, Bytes);
+    if (Status != 0) {
+      return Status;
+    }
+
     ExitInfo1 = InstructionData->Ext.RmData;
     ExitInfo2 = Bytes;
 
@@ -748,6 +819,11 @@ MmioExit (
   case 0xB7:
     Bytes = (Bytes != 0) ? Bytes : 2;
 
+    Status = ValidateMmioMemory (Ghcb, InstructionData->Ext.RmData, Bytes);
+    if (Status != 0) {
+      return Status;
+    }
+
     ExitInfo1 = InstructionData->Ext.RmData;
     ExitInfo2 = Bytes;
 
@@ -774,6 +850,11 @@ MmioExit (
   case 0xBF:
     Bytes = (Bytes != 0) ? Bytes : 2;
 
+    Status = ValidateMmioMemory (Ghcb, InstructionData->Ext.RmData, Bytes);
+    if (Status != 0) {
+      return Status;
+    }
+
     ExitInfo1 = InstructionData->Ext.RmData;
     ExitInfo2 = Bytes;
 

OvmfPkg/OvmfPkgX64.dsc

@@ -266,6 +266,7 @@
   CpuExceptionHandlerLib|UefiCpuPkg/Library/CpuExceptionHandlerLib/SecPeiCpuExceptionHandlerLib.inf
 !endif
   VmgExitLib|OvmfPkg/Library/VmgExitLib/SecVmgExitLib.inf
+  MemEncryptSevLib|OvmfPkg/Library/BaseMemEncryptSevLib/SecMemEncryptSevLib.inf
 
 [LibraryClasses.common.PEI_CORE]
   HobLib|MdePkg/Library/PeiHobLib/PeiHobLib.inf