From 5f18e2e3643524c6b6b38c44c6ce4eabdcfd59d1 Mon Sep 17 00:00:00 2001
From: Gary Ching-Pang Lin <glin@suse.com>
Date: Tue, 27 May 2014 17:42:00 +0800
Subject: [PATCH 30/74] 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>
---
 MokManager.c | 23 +++++++++++++++++++++--
 shim.c       | 45 +++++++++++++++++++++++++++++++--------------
 2 files changed, 52 insertions(+), 16 deletions(-)

diff --git a/MokManager.c b/MokManager.c
index 3da61f4..c9fbbac 100644
--- a/MokManager.c
+++ b/MokManager.c
@@ -1306,11 +1306,30 @@ static INTN mok_pw_prompt (void *MokPW, UINTN MokPWSize) {
 	return -1;
 }
 
-static BOOLEAN verify_certificate(void *cert, UINTN size)
+static BOOLEAN verify_certificate(UINT8 *cert, UINTN size)
 {
 	X509 *X509Cert;
-	if (!cert || size == 0)
+	UINTN length;
+	if (!cert || size < 0)
+		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) {
+		console_notify(L"Not a DER encoding X509 certificate");
 		return FALSE;
+	}
+
+	length = (cert[2]<<8 | cert[3]);
+	if (length != (size - 4)) {
+		console_notify(L"Invalid X509 certificate: Inconsistent size");
+		return FALSE;
+	}
 
 	if (!(X509ConstructCertificate(cert, size, (UINT8 **) &X509Cert)) ||
 	    X509Cert == NULL) {
diff --git a/shim.c b/shim.c
index d8699f9..cd26ce6 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;
 }
 
-- 
1.9.3