Add constant time memcmp_ct function

core/tee/tee_authenc.c

@@ -25,6 +25,7 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <string_ext.h>
 #include <tee/tee_authenc.h>
 #include <tee/tee_cipher.h>
 #include <tee/tee_mac.h>
@@ -413,7 +414,7 @@ TEE_Result tee_authenc_dec_final(
 		goto out;
 	}
 
-	if (memcmp(dst_tag, tag, tag_len) != 0)
+	if (buf_compare_ct(dst_tag, tag, tag_len) != 0)
 		res = TEE_ERROR_MAC_INVALID;
 	else
 		res = TEE_SUCCESS;

core/tee/tee_hash.c

@@ -29,6 +29,7 @@
 #include <tee/tee_hash.h>
 #include <kernel/tee_core_trace.h>
 #include "tee_ltc_wrapper.h"
+#include <string_ext.h>
 
 #define MAX_DIGEST 64
 
@@ -197,7 +198,7 @@ TEE_Result tee_hash_check(
 	if (res != TEE_SUCCESS)
 		return res;
 
-	if (memcmp(digest, hash, hash_size) != 0)
+	if (buf_compare_ct(digest, hash, hash_size) != 0)
 		return TEE_ERROR_SECURITY;
 
 	return TEE_SUCCESS;

core/tee/tee_rpmb.c

@@ -27,6 +27,7 @@
 #include <sys/types.h>
 #include <stdlib.h>
 #include <string.h>
+#include <string_ext.h>
 #include <kernel/tee_core_trace.h>
 #include <tee_api_types.h>
 #include <kernel/tee_common_otp.h>
@@ -657,8 +658,8 @@ static TEE_Result tee_rpmb_resp_unpack_verify(struct rpmb_data_frame *datafrm,
 	}
 
 	if (rawdata->nonce != NULL) {
-		if (memcmp(rawdata->nonce, lastfrm.nonce, RPMB_NONCE_SIZE)
-				!= 0) {
+		if (buf_compare_ct(rawdata->nonce, lastfrm.nonce,
+				   RPMB_NONCE_SIZE) != 0) {
 			DMSG("%s: nonce mismatched", __func__);
 			return TEE_ERROR_SECURITY;
 		}
@@ -688,9 +689,9 @@ static TEE_Result tee_rpmb_resp_unpack_verify(struct rpmb_data_frame *datafrm,
 				return res;
 		}
 
-		if (memcmp(rawdata->key_mac,
-			   (datafrm + nbr_frms - 1)->key_mac,
-			   RPMB_KEY_MAC_SIZE) != 0) {
+		if (buf_compare_ct(rawdata->key_mac,
+				   (datafrm + nbr_frms - 1)->key_mac,
+				   RPMB_KEY_MAC_SIZE) != 0) {
 			DMSG("%s: MAC mismatched:", __func__);
 #ifdef ENABLE_RPMB_DATA_DUMP
 			HEX_PRINT_BUF((uint8_t *)rawdata->key_mac, 32);

lib/libutils/ext/buf_compare_ct.c

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2014, Linaro Limited
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ * this list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ * this list of conditions and the following disclaimer in the documentation
+ * and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+#include <string_ext.h>
+
+int buf_compare_ct(const void *s1, const void *s2, size_t n)
+{
+	int res = 0;
+	unsigned char *c1 = (unsigned char *)s1;
+	unsigned char *c2 = (unsigned char *)s2;
+
+	while (n--) {
+		res |= (*c1 ^ *c2);
+		c1++;
+		c2++;
+	}
+
+	return res;
+}

lib/libutils/ext/include/string_ext.h

@@ -43,4 +43,14 @@
 size_t strlcpy(char *dst, const char *src, size_t size);
 size_t strlcat(char *dst, const char *src, size_t size);
 
+/*
+ * This memory compare function will compare two buffers in a constant time.
+ *
+ * Note that this function will not have same kind of return values as the
+ * traditional libc memcmp which return either less than or greater than zero
+ * depending on which string that is lexically greater. This function will
+ * return 0 if it is a match, otherwise it will return a non-zero value.
+ */
+int buf_compare_ct(const void *s1, const void *s2, size_t n);
+
 #endif /* STRING_EXT_H */

lib/libutils/ext/sub.mk

@@ -2,3 +2,4 @@ global-incdirs-y += include
 
 srcs-y += strlcat.c
 srcs-y += strlcpy.c
+srcs-y += buf_compare_ct.c