[PATCH] MdeModulePkg Variable: Fix Timestamp zeroing issue on APPEND_WRITE

[Star Zeng <star.zeng@intel.com>]

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=415

When SetVariable() to a time based auth variable with APPEND_WRITE
attribute, and if the EFI_VARIABLE_AUTHENTICATION_2.TimeStamp in
the input Data is earlier than current value, it will cause timestamp
zeroing.

This issue may bring time based auth variable downgrade problem.
For example:
A vendor released three certs at 2014, 2015, and 2016, and system
integrated the 2016 cert. User can SetVariable() with 2015 cert and
APPEND_WRITE attribute to cause timestamp zeroing first, then
SetVariable() with 2014 cert to downgrade the cert.

This patch fixes this issue.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
---
 MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c b/MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c
index a2d61c8cd618..8e8db71bd201 100644
--- a/MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c
+++ b/MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c
@@ -2462,6 +2462,8 @@ UpdateVariable (
         if (Variable->CurrPtr != NULL) {
           if (VariableCompareTimeStampInternal (&(((AUTHENTICATED_VARIABLE_HEADER *) CacheVariable->CurrPtr)->TimeStamp), TimeStamp)) {
             CopyMem (&AuthVariable->TimeStamp, TimeStamp, sizeof (EFI_TIME));
+          } else {
+            CopyMem (&AuthVariable->TimeStamp, &(((AUTHENTICATED_VARIABLE_HEADER *) CacheVariable->CurrPtr)->TimeStamp), sizeof (EFI_TIME));
           }
         }
       }
-- 
2.7.0.windows.1