[PATCH v1 3/4] OvmfPkg: IommuDxe: Zero the shared page(s) on Unmap()

[Brijesh Singh <brijesh.singh@amd.com>]

To support the Map(), we allocate bounce buffer with C-bit cleared,
the buffer is referred as a DeviceAddress. Typically, DeviceAddress
is used as communication block between guest and hypervisor. When
guest is done with communication block, it calls Unmap().The Unmap()
free's the DeviceAddress, if we do not clear the content of shared
communication block during Unmap() then data remains readble to the
hypervisor for an unpredicatable time. Let's zero the bounce buffer
after we are done using it.

I did some benchmark and did not see any measure perform impact of
zeroing the page(s).

Suggested-by: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
---
 OvmfPkg/IoMmuDxe/AmdSevIoMmu.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/OvmfPkg/IoMmuDxe/AmdSevIoMmu.c b/OvmfPkg/IoMmuDxe/AmdSevIoMmu.c
index 5ae54482fffe..04e3725ff7e6 100644
--- a/OvmfPkg/IoMmuDxe/AmdSevIoMmu.c
+++ b/OvmfPkg/IoMmuDxe/AmdSevIoMmu.c
@@ -67,8 +67,7 @@ SetBufferAsEncDec (
   // buffer matches with same encryption mask.
   //
   if (!Enc) {
-    Status = MemEncryptSevClearPageEncMask (0, MapInfo->DeviceAddress,
-               MapInfo->NumberOfPages, TRUE);
+    Status = MemEncryptSevClearPageEncMask (0, TempBuffer, MapInfo->NumberOfPages, TRUE);
     ASSERT_EFI_ERROR (Status);
   }
 
@@ -79,7 +78,7 @@ SetBufferAsEncDec (
   //
   CopyMem (
     (VOID *) (UINTN) TempBuffer,
-    (VOID *) (UINTN)MapInfo->HostAddress,
+    (VOID *) (UINTN) MapInfo->HostAddress,
     MapInfo->NumberOfBytes);
 
   //
@@ -109,11 +108,8 @@ SetBufferAsEncDec (
   //
   // Restore the encryption mask of the intermediate buffer
   //
-  if (!Enc) {
-    Status = MemEncryptSevSetPageEncMask (0, MapInfo->DeviceAddress,
-               MapInfo->NumberOfPages, TRUE);
-    ASSERT_EFI_ERROR (Status);
-  }
+  Status = MemEncryptSevSetPageEncMask (0, TempBuffer, MapInfo->NumberOfPages, TRUE);
+  ASSERT_EFI_ERROR (Status);
 
   //
   // Free the intermediate buffer
@@ -386,6 +382,12 @@ IoMmuUnmap (
   ASSERT_EFI_ERROR(Status);
 
   //
+  // Zero the shared memory so that hypervisor no longer able to get intelligentable
+  // data.
+  //
+  SetMem ((VOID *) (UINTN)MapInfo->DeviceAddress, MapInfo->NumberOfBytes, 0);
+
+  //
   // Free the bounce buffer
   //
   gBS->FreePages (MapInfo->DeviceAddress, MapInfo->NumberOfPages);
-- 
2.7.4