xref: /netbsd-src/crypto/external/bsd/heimdal/dist/lib/roken/ct.c (revision d3273b5b76f5afaafe308cead5511dbb8df8c5e9)
1 /*	$NetBSD: ct.c,v 1.2 2017/01/28 21:31:50 christos Exp $	*/
2 
3 /*
4  * Copyright (c) 2009 Kungliga Tekniska Högskolan
5  * (Royal Institute of Technology, Stockholm, Sweden).
6  * All rights reserved.
7  *
8  * Redistribution and use in source and binary forms, with or without
9  * modification, are permitted provided that the following conditions
10  * are met:
11  *
12  * 1. Redistributions of source code must retain the above copyright
13  *    notice, this list of conditions and the following disclaimer.
14  *
15  * 2. Redistributions in binary form must reproduce the above copyright
16  *    notice, this list of conditions and the following disclaimer in the
17  *    documentation and/or other materials provided with the distribution.
18  *
19  * 3. Neither the name of the Institute nor the names of its contributors
20  *    may be used to endorse or promote products derived from this software
21  *    without specific prior written permission.
22  *
23  * THIS SOFTWARE IS PROVIDED BY THE INSTITUTE AND CONTRIBUTORS ``AS IS'' AND
24  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
25  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
26  * ARE DISCLAIMED.  IN NO EVENT SHALL THE INSTITUTE OR CONTRIBUTORS BE LIABLE
27  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
28  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
29  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
30  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
31  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
32  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
33  * SUCH DAMAGE.
34  */
35 
36 #include <config.h>
37 #include <krb5/roken.h>
38 
39 /**
40  * Constant time compare to memory regions. The reason for making it
41  * constant time is to make sure that timeing information leak from
42  * where in the function the diffrence is.
43  *
44  * ct_memcmp() can't be used to order memory regions like memcmp(),
45  * for example, use ct_memcmp() with qsort().
46  *
47  * We use volatile to avoid optimizations where the compiler and/or
48  * linker turn this ct_memcmp() into a plain memcmp().  The pointers
49  * themselves are also marked volatile (not just the memory pointed at)
50  * because in some GCC versions there is a bug which can be worked
51  * around by doing this.
52  *
53  * @param p1 memory region 1 to compare
54  * @param p2 memory region 2 to compare
55  * @param len length of memory
56  *
57  * @return 0 when the memory regions are equal, non zero if not
58  *
59  * @ingroup roken
60  */
61 
62 int
ct_memcmp(const volatile void * volatile p1,const volatile void * volatile p2,size_t len)63 ct_memcmp(const volatile void * volatile p1,
64           const volatile void * volatile p2,
65           size_t len)
66 {
67     /*
68      * There's no need for s1 and s2 to be volatile; only p1 and p2 have
69      * to be in order to work around GCC bugs.
70      *
71      * However, s1 and s2 do have to point to volatile, as we don't know
72      * if the object was originally defined as volatile, and if it was
73      * then we'd get undefined behavior here if s1/s2 were declared to
74      * point to non-volatile memory.
75      */
76     const volatile unsigned char *s1 = p1;
77     const volatile unsigned char *s2 = p2;
78     size_t i;
79     int r = 0;
80 
81     for (i = 0; i < len; i++)
82 	r |= (s1[i] ^ s2[i]);
83     return !!r;
84 }
85