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authorJames Yonan <james@openvpn.net>2013-09-26 02:20:39 -0600
committerHerbert Xu <herbert@gondor.apana.org.au>2013-10-07 14:17:06 +0800
commit6bf37e5aa90f18baf5acf4874bca505dd667c37f (patch)
tree7972d4009f662cf78abdbea3b4040316d46503c1 /crypto/memneq.c
parentd319fe2a0af3509f959d5195fb8916accbf14857 (diff)
downloadlinux-stericsson-6bf37e5aa90f18baf5acf4874bca505dd667c37f.tar.gz
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>
Diffstat (limited to 'crypto/memneq.c')
-rw-r--r--crypto/memneq.c138
1 files changed, 138 insertions, 0 deletions
diff --git a/crypto/memneq.c b/crypto/memneq.c
new file mode 100644
index 000000000000..cd0162221c14
--- /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 */