[PATCH 2/9] SecurityPkg/DxeImageVerificationLib: reject CertStack.CertNumber==0 per DBX(CVE-2019-14575)

["Wang, Jian J" <jian.j.wang@intel.com>]

In case the signers' certificate stack, retrieved from the PE/COFF image's
Authenticode blob, has zero elements (= there are zero signer certificates),
then we should consider the image forbidden by DBX, not accepted by DBX.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
---
 .../Library/DxeImageVerificationLib/DxeImageVerificationLib.c   | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c b/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c
index 74dbffa122..5dcd6efed5 100644
--- a/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c
+++ b/SecurityPkg/Library/DxeImageVerificationLib/DxeImageVerificationLib.c
@@ -1326,7 +1326,7 @@ IsForbiddenByDbx (
   //       UINT8  Certn[];
   //
   Pkcs7GetSigners (AuthData, AuthDataSize, &CertBuffer, &BufferLength, &TrustedCert, &TrustedCertLength);
-  if ((BufferLength == 0) || (CertBuffer == NULL)) {
+  if ((BufferLength == 0) || (CertBuffer == NULL) || (*CertBuffer) == 0) {
     IsForbidden = TRUE;
     goto Done;
   }
-- 
2.24.0.windows.2