[RFC PATCH v3 32/43] OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Bypass flash detection with SEV-ES is enabled

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

The flash detection routine will attempt to determine how the flash
device behaves (e.g. ROM, RAM, Flash). But when SEV-ES is enabled and
the flash device behaves as a ROM device (meaning it is marked read-only
by the hypervisor), this check may result in an infinite nested page fault
because of the attempted write. Since the instruction cannot be emulated
when SEV-ES is enabled, the RIP is never advanced, resulting in repeated
nested page faults.

When SEV-ES is enabled, exit the flash detection early and assume that
the FD behaves as Flash. This will result in QemuFlashWrite() being called
to store EFI variables, which will also result in an infinite nested page
fault when the write is performed. In this case, update QemuFlashWrite()
to use the VmgMmioWrite function from the VmgExitLib library to have the
hypervisor perform the write without having to emulate the instruction.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 OvmfPkg/OvmfPkgIa32.dsc                       |  1 +
 OvmfPkg/OvmfPkgIa32X64.dsc                    |  1 +
 OvmfPkg/OvmfPkgX64.dsc                        |  1 +
 .../FvbServicesRuntimeDxe.inf                 |  2 +
 .../QemuFlash.c                               | 38 +++++++++++++++++--
 5 files changed, 40 insertions(+), 3 deletions(-)

diff --git a/OvmfPkg/OvmfPkgIa32.dsc b/OvmfPkg/OvmfPkgIa32.dsc
index 56670eefde6b..ff2814c6246e 100644
--- a/OvmfPkg/OvmfPkgIa32.dsc
+++ b/OvmfPkg/OvmfPkgIa32.dsc
@@ -320,6 +320,7 @@ [LibraryClasses.common.DXE_RUNTIME_DRIVER]
   BaseCryptLib|CryptoPkg/Library/BaseCryptLib/RuntimeCryptLib.inf
   PciLib|OvmfPkg/Library/DxePciLibI440FxQ35/DxePciLibI440FxQ35.inf
   QemuFwCfgS3Lib|OvmfPkg/Library/QemuFwCfgS3Lib/DxeQemuFwCfgS3LibFwCfg.inf
+  VmgExitLib|UefiCpuPkg/Library/VmgExitLib/VmgExitLib.inf
 
 [LibraryClasses.common.UEFI_DRIVER]
   PcdLib|MdePkg/Library/DxePcdLib/DxePcdLib.inf
diff --git a/OvmfPkg/OvmfPkgIa32X64.dsc b/OvmfPkg/OvmfPkgIa32X64.dsc
index 9897e6889573..212952cfaacd 100644
--- a/OvmfPkg/OvmfPkgIa32X64.dsc
+++ b/OvmfPkg/OvmfPkgIa32X64.dsc
@@ -325,6 +325,7 @@ [LibraryClasses.common.DXE_RUNTIME_DRIVER]
   BaseCryptLib|CryptoPkg/Library/BaseCryptLib/RuntimeCryptLib.inf
   PciLib|OvmfPkg/Library/DxePciLibI440FxQ35/DxePciLibI440FxQ35.inf
   QemuFwCfgS3Lib|OvmfPkg/Library/QemuFwCfgS3Lib/DxeQemuFwCfgS3LibFwCfg.inf
+  VmgExitLib|UefiCpuPkg/Library/VmgExitLib/VmgExitLib.inf
 
 [LibraryClasses.common.UEFI_DRIVER]
   PcdLib|MdePkg/Library/DxePcdLib/DxePcdLib.inf
diff --git a/OvmfPkg/OvmfPkgX64.dsc b/OvmfPkg/OvmfPkgX64.dsc
index 59c4f9207fc3..8331fc0b663e 100644
--- a/OvmfPkg/OvmfPkgX64.dsc
+++ b/OvmfPkg/OvmfPkgX64.dsc
@@ -325,6 +325,7 @@ [LibraryClasses.common.DXE_RUNTIME_DRIVER]
   BaseCryptLib|CryptoPkg/Library/BaseCryptLib/RuntimeCryptLib.inf
   PciLib|OvmfPkg/Library/DxePciLibI440FxQ35/DxePciLibI440FxQ35.inf
   QemuFwCfgS3Lib|OvmfPkg/Library/QemuFwCfgS3Lib/DxeQemuFwCfgS3LibFwCfg.inf
+  VmgExitLib|UefiCpuPkg/Library/VmgExitLib/VmgExitLib.inf
 
 [LibraryClasses.common.UEFI_DRIVER]
   PcdLib|MdePkg/Library/DxePcdLib/DxePcdLib.inf
diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf
index ca6326e833ed..0b7741ac07f8 100644
--- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf
+++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf
@@ -38,6 +38,7 @@ [Sources]
 [Packages]
   MdePkg/MdePkg.dec
   MdeModulePkg/MdeModulePkg.dec
+  UefiCpuPkg/UefiCpuPkg.dec
   OvmfPkg/OvmfPkg.dec
 
 [LibraryClasses]
@@ -52,6 +53,7 @@ [LibraryClasses]
   UefiBootServicesTableLib
   UefiDriverEntryPoint
   UefiRuntimeLib
+  VmgExitLib
 
 [Guids]
   gEfiEventVirtualAddressChangeGuid   # ALWAYS_CONSUMED
diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c
index c81c58972bf2..4f3bf690fcad 100644
--- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c
+++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c
@@ -9,7 +9,9 @@
 
 #include <Library/BaseMemoryLib.h>
 #include <Library/DebugLib.h>
+#include <Library/MemEncryptSevLib.h>
 #include <Library/PcdLib.h>
+#include <Library/VmgExitLib.h>
 
 #include "QemuFlash.h"
 
@@ -80,6 +82,20 @@ QemuFlashDetected (
 
   DEBUG ((EFI_D_INFO, "QEMU Flash: Attempting flash detection at %p\n", Ptr));
 
+  if (MemEncryptSevEsIsEnabled()) {
+    //
+    // When SEV-ES is enabled, the check below can result in an infinite
+    // loop with respect to a nested page fault. When the FD behaves as
+    // a ROM, the nested page table entry is read-only. The check below
+    // will cause a nested page fault that cannot be emulated, causing
+    // the instruction to retried over and over. For SEV-ES, assume that
+    // the FD does not behave as FLASH.
+    //
+    DEBUG ((DEBUG_INFO,
+      "QEMU Flash: SEV-ES enabled, assuming FD behaves as FLASH\n"));
+    return TRUE;
+  }
+
   OriginalUint8 = *Ptr;
   *Ptr = CLEAR_STATUS_CMD;
   ProbeUint8 = *Ptr;
@@ -147,6 +163,21 @@ QemuFlashRead (
 }
 
 
+STATIC
+VOID
+QemuFlashPtrWrite (
+  IN        volatile UINT8                      *Ptr,
+  IN        UINT8                               Value
+  )
+{
+  if (MemEncryptSevEsIsEnabled()) {
+    VmgMmioWrite ((UINT8 *) Ptr, &Value, 1);
+  } else {
+    *Ptr = Value;
+  }
+}
+
+
 /**
   Write to QEMU Flash
 
@@ -181,8 +212,9 @@ QemuFlashWrite (
   //
   Ptr = QemuFlashPtr (Lba, Offset);
   for (Loop = 0; Loop < *NumBytes; Loop++) {
-    *Ptr = WRITE_BYTE_CMD;
-    *Ptr = Buffer[Loop];
+    QemuFlashPtrWrite (Ptr, WRITE_BYTE_CMD);
+    QemuFlashPtrWrite (Ptr, Buffer[Loop]);
+
     Ptr++;
   }
 
@@ -190,7 +222,7 @@ QemuFlashWrite (
   // Restore flash to read mode
   //
   if (*NumBytes > 0) {
-    *(Ptr - 1) = READ_ARRAY_CMD;
+    QemuFlashPtrWrite (Ptr - 1, READ_ARRAY_CMD);
   }
 
   return EFI_SUCCESS;
-- 
2.17.1