[PATCH 07/12] OvmfPkg/VmgExitLib: Check for an explicit DR7 cached value

["Lendacky, Thomas" <thomas.lendacky@amd.com>]

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

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

Check the DR7 cached indicator against a specific value. This makes it
harder for a hypervisor to just write random data into that field in an
attempt to use an invalid DR7 value.

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>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c b/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
index 1671db3a01b1..5149ab2bc989 100644
--- a/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
+++ b/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
@@ -128,10 +128,13 @@ UINT64
 
 //
 // Per-CPU data mapping structure
+//   Use UINT32 for cached indicators and compare to a specific value
+//   so that the hypervisor can't indicate a value is cached by just
+//   writing random data to that area.
 //
 typedef struct {
-  BOOLEAN  Dr7Cached;
-  UINT64   Dr7;
+  UINT32  Dr7Cached;
+  UINT64  Dr7;
 } SEV_ES_PER_CPU_DATA;
 
 
@@ -1489,7 +1492,7 @@ Dr7WriteExit (
   }
 
   SevEsData->Dr7 = *Register;
-  SevEsData->Dr7Cached = TRUE;
+  SevEsData->Dr7Cached = 1;
 
   return 0;
 }
@@ -1533,7 +1536,7 @@ Dr7ReadExit (
   // If there is a cached valued for DR7, return that. Otherwise return the
   // DR7 standard reset value of 0x400 (no debug breakpoints set).
   //
-  *Register = (SevEsData->Dr7Cached) ? SevEsData->Dr7 : 0x400;
+  *Register = (SevEsData->Dr7Cached == 1) ? SevEsData->Dr7 : 0x400;
 
   return 0;
 }
-- 
2.28.0