SecurityPkg OpalPasswordSupportLib: Add check to avoid potential buffer overflow.

Current code not check the CommunicationBuffer size before use it. Attacker can
read beyond the end of the (untrusted) commbuffer into controlled memory. Attacker
can get access outside of valid SMM memory regions. This patch add check before
use it.

bugz: https://bugzilla.tianocore.org/show_bug.cgi?id=198

Cc: Yao Jiewen <jiewen.yao@intel.com>
Cc: Wu Hao <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Yao Jiewen <jiewen.yao@intel.com>

SecurityPkg/Include/Library/OpalPasswordSupportLib.h

@@ -19,6 +19,7 @@ WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
 #include <Protocol/DevicePath.h>
 #include <Library/TcgStorageOpalLib.h>
 
+#define OPAL_PASSWORD_MAX_LENGTH         32
 
 #pragma pack(1)
 
@@ -76,7 +77,7 @@ typedef struct {
 typedef struct {
   LIST_ENTRY                 Link;
 
-  UINT8                      Password[32];
+  UINT8                      Password[OPAL_PASSWORD_MAX_LENGTH];
   UINT8                      PasswordLength;
 
   EFI_DEVICE_PATH_PROTOCOL   OpalDevicePath;