OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Bypass flash detection with SEV-ES

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198

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 VMGEXIT MMIO write support to have the hypervisor perform the
write without having to emulate the instruction.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>

OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf

@@ -38,6 +38,7 @@
 [Packages]
   MdePkg/MdePkg.dec
   MdeModulePkg/MdeModulePkg.dec
+  UefiCpuPkg/UefiCpuPkg.dec
   OvmfPkg/OvmfPkg.dec
 
 [LibraryClasses]
@@ -52,6 +53,7 @@
   UefiBootServicesTableLib
   UefiDriverEntryPoint
   UefiRuntimeLib
+  VmgExitLib
 
 [Guids]
   gEfiEventVirtualAddressChangeGuid   # ALWAYS_CONSUMED

OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c

@@ -9,6 +9,7 @@
 
 #include <Library/BaseMemoryLib.h>
 #include <Library/DebugLib.h>
+#include <Library/MemEncryptSevLib.h>
 #include <Library/PcdLib.h>
 
 #include "QemuFlash.h"
@@ -80,6 +81,21 @@ QemuFlashDetected (
 
   DEBUG ((DEBUG_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 memslot is mapped
+    // read-only, 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, acknowledge that
+    // the FD appears as ROM and not as FLASH, but report FLASH anyway because
+    // FLASH behavior can be simulated using VMGEXIT.
+    //
+    DEBUG ((DEBUG_INFO,
+      "QEMU Flash: SEV-ES enabled, assuming FD behaves as FLASH\n"));
+    return TRUE;
+  }
+
   OriginalUint8 = *Ptr;
   *Ptr = CLEAR_STATUS_CMD;
   ProbeUint8 = *Ptr;
@@ -181,8 +197,9 @@ 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 +207,7 @@ QemuFlashWrite (
   // Restore flash to read mode
   //
   if (*NumBytes > 0) {
-    *(Ptr - 1) = READ_ARRAY_CMD;
+    QemuFlashPtrWrite (Ptr - 1, READ_ARRAY_CMD);
   }
 
   return EFI_SUCCESS;

OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.h

@@ -89,5 +89,18 @@ QemuFlashBeforeProbe (
   IN  UINTN                   FdBlockCount
   );
 
+/**
+  Write to QEMU Flash
+
+  @param[in] Ptr    Pointer to the location to write.
+  @param[in] Value  The value to write.
+
+**/
+VOID
+QemuFlashPtrWrite (
+  IN        volatile UINT8    *Ptr,
+  IN        UINT8             Value
+  );
+
 #endif
 

OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c

@@ -10,6 +10,9 @@
 **/
 
 #include <Library/UefiRuntimeLib.h>
+#include <Library/MemEncryptSevLib.h>
+#include <Library/VmgExitLib.h>
+#include <Register/Amd/Msr.h>
 
 #include "QemuFlash.h"
 
@@ -32,3 +35,40 @@ QemuFlashBeforeProbe (
   // Do nothing
   //
 }
+
+/**
+  Write to QEMU Flash
+
+  @param[in] Ptr    Pointer to the location to write.
+  @param[in] Value  The value to write.
+
+**/
+VOID
+QemuFlashPtrWrite (
+  IN        volatile UINT8    *Ptr,
+  IN        UINT8             Value
+  )
+{
+  if (MemEncryptSevEsIsEnabled ()) {
+    MSR_SEV_ES_GHCB_REGISTER  Msr;
+    GHCB                      *Ghcb;
+
+    Msr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);
+    Ghcb = Msr.Ghcb;
+
+    //
+    // Writing to flash is emulated by the hypervisor through the use of write
+    // protection. This won't work for an SEV-ES guest because the write won't
+    // be recognized as a true MMIO write, which would result in the required
+    // #VC exception. Instead, use the the VMGEXIT MMIO write support directly
+    // to perform the update.
+    //
+    VmgInit (Ghcb);
+    Ghcb->SharedBuffer[0] = Value;
+    Ghcb->SaveArea.SwScratch = (UINT64) (UINTN) Ghcb->SharedBuffer;
+    VmgExit (Ghcb, SVM_EXIT_MMIO_WRITE, (UINT64) (UINTN) Ptr, 1);
+    VmgDone (Ghcb);
+  } else {
+    *Ptr = Value;
+  }
+}

OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashSmm.c

@@ -46,3 +46,19 @@ QemuFlashBeforeProbe (
              );
   ASSERT_EFI_ERROR (Status);
 }
+
+/**
+  Write to QEMU Flash
+
+  @param[in] Ptr    Pointer to the location to write.
+  @param[in] Value  The value to write.
+
+**/
+VOID
+QemuFlashPtrWrite (
+  IN        volatile UINT8    *Ptr,
+  IN        UINT8             Value
+  )
+{
+  *Ptr = Value;
+}