crypto: crypto_memneq - add equality testing of memory regions w/o timing leaks

When comparing MAC hashes, AEAD authentication tags, or other hash
values in the context of authentication or integrity checking, it
is important not to leak timing information to a potential attacker,
i.e. when communication happens over a network.

Bytewise memory comparisons (such as memcmp) are usually optimized so
that they return a nonzero value as soon as a mismatch is found. E.g,
on x86_64/i5 for 512 bytes this can be ~50 cyc for a full mismatch
and up to ~850 cyc for a full match (cold). This early-return behavior
can leak timing information as a side channel, allowing an attacker to
iteratively guess the correct result.

This patch adds a new method crypto_memneq ("memory not equal to each
other") to the crypto API that compares memory areas of the same length
in roughly "constant time" (cache misses could change the timing, but
since they don't reveal information about the content of the strings
being compared, they are effectively benign). Iow, best and worst case
behaviour take the same amount of time to complete (in contrast to
memcmp).

Note that crypto_memneq (unlike memcmp) can only be used to test for
equality or inequality, NOT for lexicographical order. This, however,
is not an issue for its use-cases within the crypto API.

We tried to locate all of the places in the crypto API where memcmp was
being used for authentication or integrity checking, and convert them
over to crypto_memneq.

crypto_memneq is declared noinline, placed in its own source file,
and compiled with optimizations that might increase code size disabled
("Os") because a smart compiler (or LTO) might notice that the return
value is always compared against zero/nonzero, and might then
reintroduce the same early-return optimization that we are trying to
avoid.

Using #pragma or __attribute__ optimization annotations of the code
for disabling optimization was avoided as it seems to be considered
broken or unmaintained for long time in GCC [1]. Therefore, we work
around that by specifying the compile flag for memneq.o directly in
the Makefile. We found that this seems to be most appropriate.

As we use ("Os"), this patch also provides a loop-free "fast-path" for
frequently used 16 byte digests. Similarly to kernel library string
functions, leave an option for future even further optimized architecture
specific assembler implementations.

This was a joint work of James Yonan and Daniel Borkmann. Also thanks
for feedback from Florian Weimer on this and earlier proposals [2].

  [1] http://gcc.gnu.org/ml/gcc/2012-07/msg00211.html
  [2] https://lkml.org/lkml/2013/2/10/131

Signed-off-by: James Yonan <james@openvpn.net>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Cc: Florian Weimer <fw@deneb.enyo.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

7 files changed, 157 insertions, 13 deletions

diff --git a/crypto/Makefile b/crypto/Makefile
index 580af97..d6a401c 100644
--- a/crypto/Makefile
+++ b/crypto/Makefile
@@ -2,8 +2,13 @@
 # Cryptographic API
 #
 
+# memneq MUST be built with -Os or -O0 to prevent early-return optimizations
+# that will defeat memneq's actual purpose to prevent timing attacks.
+CFLAGS_REMOVE_memneq.o := -O1 -O2 -O3
+CFLAGS_memneq.o := -Os
+
 obj-$(CONFIG_CRYPTO) += crypto.o
-crypto-y := api.o cipher.o compress.o
+crypto-y := api.o cipher.o compress.o memneq.o
 
 obj-$(CONFIG_CRYPTO_WORKQUEUE) += crypto_wq.o
 
diff --git a/crypto/asymmetric_keys/rsa.c b/crypto/asymmetric_keys/rsa.c
index 4a6a069..1912b9b 100644
--- a/crypto/asymmetric_keys/rsa.c
+++ b/crypto/asymmetric_keys/rsa.c
@@ -13,6 +13,7 @@
 #include <linux/module.h>
 #include <linux/kernel.h>
 #include <linux/slab.h>
+#include <crypto/algapi.h>
 #include "public_key.h"
 
 MODULE_LICENSE("GPL");
@@ -189,12 +190,12 @@ static int RSA_verify(const u8 *H, const u8 *EM, size_t k, size_t hash_size,
 		}
 	}
 
-	if (memcmp(asn1_template, EM + T_offset, asn1_size) != 0) {
+	if (crypto_memneq(asn1_template, EM + T_offset, asn1_size) != 0) {
 		kleave(" = -EBADMSG [EM[T] ASN.1 mismatch]");
 		return -EBADMSG;
 	}
 
-	if (memcmp(H, EM + T_offset + asn1_size, hash_size) != 0) {
+	if (crypto_memneq(H, EM + T_offset + asn1_size, hash_size) != 0) {
 		kleave(" = -EKEYREJECTED [EM[T] hash mismatch]");
 		return -EKEYREJECTED;
 	}
diff --git a/crypto/authenc.c b/crypto/authenc.c
index ffce19d..2b3f4ab 100644
--- a/crypto/authenc.c
+++ b/crypto/authenc.c
@@ -188,7 +188,7 @@ static void authenc_verify_ahash_update_done(struct crypto_async_request *areq,
 	scatterwalk_map_and_copy(ihash, areq_ctx->sg, areq_ctx->cryptlen,
 				 authsize, 0);
 
-	err = memcmp(ihash, ahreq->result, authsize) ? -EBADMSG : 0;
+	err = crypto_memneq(ihash, ahreq->result, authsize) ? -EBADMSG : 0;
 	if (err)
 		goto out;
 
@@ -227,7 +227,7 @@ static void authenc_verify_ahash_done(struct crypto_async_request *areq,
 	scatterwalk_map_and_copy(ihash, areq_ctx->sg, areq_ctx->cryptlen,
 				 authsize, 0);
 
-	err = memcmp(ihash, ahreq->result, authsize) ? -EBADMSG : 0;
+	err = crypto_memneq(ihash, ahreq->result, authsize) ? -EBADMSG : 0;
 	if (err)
 		goto out;
 
@@ -462,7 +462,7 @@ static int crypto_authenc_verify(struct aead_request *req,
 	ihash = ohash + authsize;
 	scatterwalk_map_and_copy(ihash, areq_ctx->sg, areq_ctx->cryptlen,
 				 authsize, 0);
-	return memcmp(ihash, ohash, authsize) ? -EBADMSG : 0;
+	return crypto_memneq(ihash, ohash, authsize) ? -EBADMSG : 0;
 }
 
 static int crypto_authenc_iverify(struct aead_request *req, u8 *iv,
diff --git a/crypto/authencesn.c b/crypto/authencesn.c
index ab53762..c569d58 100644
--- a/crypto/authencesn.c
+++ b/crypto/authencesn.c
@@ -247,7 +247,7 @@ static void authenc_esn_verify_ahash_update_done(struct crypto_async_request *ar
 	scatterwalk_map_and_copy(ihash, areq_ctx->sg, areq_ctx->cryptlen,
 				 authsize, 0);
 
-	err = memcmp(ihash, ahreq->result, authsize) ? -EBADMSG : 0;
+	err = crypto_memneq(ihash, ahreq->result, authsize) ? -EBADMSG : 0;
 	if (err)
 		goto out;
 
@@ -296,7 +296,7 @@ static void authenc_esn_verify_ahash_update_done2(struct crypto_async_request *a
 	scatterwalk_map_and_copy(ihash, areq_ctx->sg, areq_ctx->cryptlen,
 				 authsize, 0);
 
-	err = memcmp(ihash, ahreq->result, authsize) ? -EBADMSG : 0;
+	err = crypto_memneq(ihash, ahreq->result, authsize) ? -EBADMSG : 0;
 	if (err)
 		goto out;
 
@@ -336,7 +336,7 @@ static void authenc_esn_verify_ahash_done(struct crypto_async_request *areq,
 	scatterwalk_map_and_copy(ihash, areq_ctx->sg, areq_ctx->cryptlen,
 				 authsize, 0);
 
-	err = memcmp(ihash, ahreq->result, authsize) ? -EBADMSG : 0;
+	err = crypto_memneq(ihash, ahreq->result, authsize) ? -EBADMSG : 0;
 	if (err)
 		goto out;
 
@@ -568,7 +568,7 @@ static int crypto_authenc_esn_verify(struct aead_request *req)
 	ihash = ohash + authsize;
 	scatterwalk_map_and_copy(ihash, areq_ctx->sg, areq_ctx->cryptlen,
 				 authsize, 0);
-	return memcmp(ihash, ohash, authsize) ? -EBADMSG : 0;
+	return crypto_memneq(ihash, ohash, authsize) ? -EBADMSG : 0;
 }
 
 static int crypto_authenc_esn_iverify(struct aead_request *req, u8 *iv,
diff --git a/crypto/ccm.c b/crypto/ccm.c
index 499c917..3e05499 100644
--- a/crypto/ccm.c
+++ b/crypto/ccm.c
@@ -363,7 +363,7 @@ static void crypto_ccm_decrypt_done(struct crypto_async_request *areq,
 
 	if (!err) {
 		err = crypto_ccm_auth(req, req->dst, cryptlen);
-		if (!err && memcmp(pctx->auth_tag, pctx->odata, authsize))
+		if (!err && crypto_memneq(pctx->auth_tag, pctx->odata, authsize))
 			err = -EBADMSG;
 	}
 	aead_request_complete(req, err);
@@ -422,7 +422,7 @@ static int crypto_ccm_decrypt(struct aead_request *req)
 		return err;
 
 	/* verify */
-	if (memcmp(authtag, odata, authsize))
+	if (crypto_memneq(authtag, odata, authsize))
 		return -EBADMSG;
 
 	return err;
diff --git a/crypto/gcm.c b/crypto/gcm.c
index 43e1fb0..b4f0179 100644
--- a/crypto/gcm.c
+++ b/crypto/gcm.c
@@ -582,7 +582,7 @@ static int crypto_gcm_verify(struct aead_request *req,
 
 	crypto_xor(auth_tag, iauth_tag, 16);
 	scatterwalk_map_and_copy(iauth_tag, req->src, cryptlen, authsize, 0);
-	return memcmp(iauth_tag, auth_tag, authsize) ? -EBADMSG : 0;
+	return crypto_memneq(iauth_tag, auth_tag, authsize) ? -EBADMSG : 0;
 }
 
 static void gcm_decrypt_done(struct crypto_async_request *areq, int err)
diff --git a/crypto/memneq.c b/crypto/memneq.c
new file mode 100644
index 0000000..cd01622
--- /dev/null
+++ b/crypto/memneq.c
@@ -0,0 +1,138 @@
+/*
+ * Constant-time equality testing of memory regions.
+ *
+ * Authors:
+ *
+ *   James Yonan <james@openvpn.net>
+ *   Daniel Borkmann <dborkman@redhat.com>
+ *
+ * This file is provided under a dual BSD/GPLv2 license.  When using or
+ * redistributing this file, you may do so under either license.
+ *
+ * GPL LICENSE SUMMARY
+ *
+ * Copyright(c) 2013 OpenVPN Technologies, Inc. All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St - Fifth Floor, Boston, MA 02110-1301 USA.
+ * The full GNU General Public License is included in this distribution
+ * in the file called LICENSE.GPL.
+ *
+ * BSD LICENSE
+ *
+ * Copyright(c) 2013 OpenVPN Technologies, Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ *   * Redistributions of source code must retain the above copyright
+ *     notice, this list of conditions and the following disclaimer.
+ *   * 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.
+ *   * Neither the name of OpenVPN Technologies nor the names of its
+ *     contributors may be used to endorse or promote products derived
+ *     from this software without specific prior written permission.
+ *
+ * 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
+ * OWNER 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 <crypto/algapi.h>
+
+#ifndef __HAVE_ARCH_CRYPTO_MEMNEQ
+
+/* Generic path for arbitrary size */
+static inline unsigned long
+__crypto_memneq_generic(const void *a, const void *b, size_t size)
+{
+	unsigned long neq = 0;
+
+#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
+	while (size >= sizeof(unsigned long)) {
+		neq |= *(unsigned long *)a ^ *(unsigned long *)b;
+		a += sizeof(unsigned long);
+		b += sizeof(unsigned long);
+		size -= sizeof(unsigned long);
+	}
+#endif /* CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS */
+	while (size > 0) {
+		neq |= *(unsigned char *)a ^ *(unsigned char *)b;
+		a += 1;
+		b += 1;
+		size -= 1;
+	}
+	return neq;
+}
+
+/* Loop-free fast-path for frequently used 16-byte size */
+static inline unsigned long __crypto_memneq_16(const void *a, const void *b)
+{
+#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
+	if (sizeof(unsigned long) == 8)
+		return ((*(unsigned long *)(a)   ^ *(unsigned long *)(b))
+		      | (*(unsigned long *)(a+8) ^ *(unsigned long *)(b+8)));
+	else if (sizeof(unsigned int) == 4)
+		return ((*(unsigned int *)(a)    ^ *(unsigned int *)(b))
+                      | (*(unsigned int *)(a+4)  ^ *(unsigned int *)(b+4))
+		      | (*(unsigned int *)(a+8)  ^ *(unsigned int *)(b+8))
+	              | (*(unsigned int *)(a+12) ^ *(unsigned int *)(b+12)));
+	else
+#endif /* CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS */
+		return ((*(unsigned char *)(a)    ^ *(unsigned char *)(b))
+		      | (*(unsigned char *)(a+1)  ^ *(unsigned char *)(b+1))
+		      | (*(unsigned char *)(a+2)  ^ *(unsigned char *)(b+2))
+		      | (*(unsigned char *)(a+3)  ^ *(unsigned char *)(b+3))
+		      | (*(unsigned char *)(a+4)  ^ *(unsigned char *)(b+4))
+		      | (*(unsigned char *)(a+5)  ^ *(unsigned char *)(b+5))
+		      | (*(unsigned char *)(a+6)  ^ *(unsigned char *)(b+6))
+		      | (*(unsigned char *)(a+7)  ^ *(unsigned char *)(b+7))
+		      | (*(unsigned char *)(a+8)  ^ *(unsigned char *)(b+8))
+		      | (*(unsigned char *)(a+9)  ^ *(unsigned char *)(b+9))
+		      | (*(unsigned char *)(a+10) ^ *(unsigned char *)(b+10))
+		      | (*(unsigned char *)(a+11) ^ *(unsigned char *)(b+11))
+		      | (*(unsigned char *)(a+12) ^ *(unsigned char *)(b+12))
+		      | (*(unsigned char *)(a+13) ^ *(unsigned char *)(b+13))
+		      | (*(unsigned char *)(a+14) ^ *(unsigned char *)(b+14))
+		      | (*(unsigned char *)(a+15) ^ *(unsigned char *)(b+15)));
+}
+
+/* Compare two areas of memory without leaking timing information,
+ * and with special optimizations for common sizes.  Users should
+ * not call this function directly, but should instead use
+ * crypto_memneq defined in crypto/algapi.h.
+ */
+noinline unsigned long __crypto_memneq(const void *a, const void *b,
+				       size_t size)
+{
+	switch (size) {
+	case 16:
+		return __crypto_memneq_16(a, b);
+	default:
+		return __crypto_memneq_generic(a, b, size);
+	}
+}
+EXPORT_SYMBOL(__crypto_memneq);
+
+#endif /* __HAVE_ARCH_CRYPTO_MEMNEQ */