Check the first 4 bytes of the certificate

A non-DER encoding x509 certificate may be mistakenly enrolled into
db or MokList. This commit checks the first 4 bytes of the certificate
to ensure that it's DER encoding.

This commit also removes the iteration of the x509 signature list.
Per UEFI SPEC, each x509 signature list contains only one x509 certificate.
Besides, the size of certificate is incorrect. The size of the header must
be substracted from the signature size.

Signed-off-by: Gary Ching-Pang Lin <glin@suse.com>

1 files changed, 31 insertions, 14 deletions

diff --git a/shim.c b/shim.c
index d8699f98..cd26ce6a 100644
--- a/shim.c
+++ b/shim.c
@@ -226,44 +226,61 @@ static EFI_STATUS relocate_coff (PE_COFF_LOADER_IMAGE_CONTEXT *context,
 	return EFI_SUCCESS;
 }
 
+static BOOLEAN verify_x509(UINT8 *Cert, UINTN CertSize)
+{
+	UINTN length;
+
+	if (!Cert || CertSize < 4)
+		return FALSE;
+
+	/*
+	 * A DER encoding x509 certificate starts with SEQUENCE(0x30),
+	 * the number of length bytes, and the number of value bytes.
+	 * The size of a x509 certificate is usually between 127 bytes
+	 * and 64KB. For convenience, assume the number of value bytes
+	 * is 2, i.e. the second byte is 0x82.
+	 */
+	if (Cert[0] != 0x30 || Cert[1] != 0x82)
+		return FALSE;
+
+	length = Cert[2]<<8 | Cert[3];
+	if (length != (CertSize - 4))
+		return FALSE;
+
+	return TRUE;
+}
+
 static CHECK_STATUS check_db_cert_in_ram(EFI_SIGNATURE_LIST *CertList,
 					 UINTN dbsize,
 					 WIN_CERTIFICATE_EFI_PKCS *data,
 					 UINT8 *hash)
 {
 	EFI_SIGNATURE_DATA *Cert;
-	UINTN CertCount, Index;
+	UINTN CertSize;
 	BOOLEAN IsFound = FALSE;
 	EFI_GUID CertType = X509_GUID;
 
 	while ((dbsize > 0) && (dbsize >= CertList->SignatureListSize)) {
 		if (CompareGuid (&CertList->SignatureType, &CertType) == 0) {
-			CertCount = (CertList->SignatureListSize - sizeof (EFI_SIGNATURE_LIST) - CertList->SignatureHeaderSize) / CertList->SignatureSize;
 			Cert = (EFI_SIGNATURE_DATA *) ((UINT8 *) CertList + sizeof (EFI_SIGNATURE_LIST) + CertList->SignatureHeaderSize);
-			for (Index = 0; Index < CertCount; Index++) {
+			CertSize = CertList->SignatureSize - sizeof(EFI_GUID);
+			if (verify_x509(Cert->SignatureData, CertSize)) {
 				IsFound = AuthenticodeVerify (data->CertData,
 							      data->Hdr.dwLength - sizeof(data->Hdr),
 							      Cert->SignatureData,
-							      CertList->SignatureSize,
+							      CertSize,
 							      hash, SHA256_DIGEST_SIZE);
 				if (IsFound)
-					break;
-
-				Cert = (EFI_SIGNATURE_DATA *) ((UINT8 *) Cert + CertList->SignatureSize);
+					return DATA_FOUND;
+			} else if (verbose) {
+				console_notify(L"Not a DER encoding x.509 Certificate");
 			}
-
 		}
 
-		if (IsFound)
-			break;
-
 		dbsize -= CertList->SignatureListSize;
 		CertList = (EFI_SIGNATURE_LIST *) ((UINT8 *) CertList + CertList->SignatureListSize);
 	}
 
-	if (IsFound)
-		return DATA_FOUND;
-
 	return DATA_NOT_FOUND;
 }