1/**
2 *  Constant-time functions
3 *
4 *  Copyright The Mbed TLS Contributors
5 *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
6 */
7
8/*
9 * The following functions are implemented without using comparison operators, as those
10 * might be translated to branches by some compilers on some platforms.
11 */
12
13#include <stdint.h>
14#include <limits.h>
15
16#include "common.h"
17#include "constant_time_internal.h"
18#include "mbedtls/constant_time.h"
19#include "mbedtls/error.h"
20#include "mbedtls/platform_util.h"
21
22#include <string.h>
23
24#if !defined(MBEDTLS_CT_ASM)
25/*
26 * Define an object with the value zero, such that the compiler cannot prove that it
27 * has the value zero (because it is volatile, it "may be modified in ways unknown to
28 * the implementation").
29 */
30volatile mbedtls_ct_uint_t mbedtls_ct_zero = 0;
31#endif
32
33/*
34 * Define MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS where assembly is present to
35 * perform fast unaligned access to volatile data.
36 *
37 * This is needed because mbedtls_get_unaligned_uintXX etc don't support volatile
38 * memory accesses.
39 *
40 * Some of these definitions could be moved into alignment.h but for now they are
41 * only used here.
42 */
43#if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS) && \
44    ((defined(MBEDTLS_CT_ARM_ASM) && (UINTPTR_MAX == 0xfffffffful)) || \
45    defined(MBEDTLS_CT_AARCH64_ASM))
46/* We check pointer sizes to avoid issues with them not matching register size requirements */
47#define MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS
48
49static inline uint32_t mbedtls_get_unaligned_volatile_uint32(volatile const unsigned char *p)
50{
51    /* This is UB, even where it's safe:
52     *    return *((volatile uint32_t*)p);
53     * so instead the same thing is expressed in assembly below.
54     */
55    uint32_t r;
56#if defined(MBEDTLS_CT_ARM_ASM)
57    asm volatile ("ldr %0, [%1]" : "=r" (r) : "r" (p) :);
58#elif defined(MBEDTLS_CT_AARCH64_ASM)
59    asm volatile ("ldr %w0, [%1]" : "=r" (r) : MBEDTLS_ASM_AARCH64_PTR_CONSTRAINT(p) :);
60#else
61#error "No assembly defined for mbedtls_get_unaligned_volatile_uint32"
62#endif
63    return r;
64}
65#endif /* defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS) &&
66          (defined(MBEDTLS_CT_ARM_ASM) || defined(MBEDTLS_CT_AARCH64_ASM)) */
67
68int mbedtls_ct_memcmp(const void *a,
69                      const void *b,
70                      size_t n)
71{
72    size_t i = 0;
73    /*
74     * `A` and `B` are cast to volatile to ensure that the compiler
75     * generates code that always fully reads both buffers.
76     * Otherwise it could generate a test to exit early if `diff` has all
77     * bits set early in the loop.
78     */
79    volatile const unsigned char *A = (volatile const unsigned char *) a;
80    volatile const unsigned char *B = (volatile const unsigned char *) b;
81    uint32_t diff = 0;
82
83#if defined(MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS)
84    for (; (i + 4) <= n; i += 4) {
85        uint32_t x = mbedtls_get_unaligned_volatile_uint32(A + i);
86        uint32_t y = mbedtls_get_unaligned_volatile_uint32(B + i);
87        diff |= x ^ y;
88    }
89#endif
90
91    for (; i < n; i++) {
92        /* Read volatile data in order before computing diff.
93         * This avoids IAR compiler warning:
94         * 'the order of volatile accesses is undefined ..' */
95        unsigned char x = A[i], y = B[i];
96        diff |= x ^ y;
97    }
98
99
100#if (INT_MAX < INT32_MAX)
101    /* We don't support int smaller than 32-bits, but if someone tried to build
102     * with this configuration, there is a risk that, for differing data, the
103     * only bits set in diff are in the top 16-bits, and would be lost by a
104     * simple cast from uint32 to int.
105     * This would have significant security implications, so protect against it. */
106#error "mbedtls_ct_memcmp() requires minimum 32-bit ints"
107#else
108    /* The bit-twiddling ensures that when we cast uint32_t to int, we are casting
109     * a value that is in the range 0..INT_MAX - a value larger than this would
110     * result in implementation defined behaviour.
111     *
112     * This ensures that the value returned by the function is non-zero iff
113     * diff is non-zero.
114     */
115    return (int) ((diff & 0xffff) | (diff >> 16));
116#endif
117}
118
119#if defined(MBEDTLS_NIST_KW_C)
120
121int mbedtls_ct_memcmp_partial(const void *a,
122                              const void *b,
123                              size_t n,
124                              size_t skip_head,
125                              size_t skip_tail)
126{
127    unsigned int diff = 0;
128
129    volatile const unsigned char *A = (volatile const unsigned char *) a;
130    volatile const unsigned char *B = (volatile const unsigned char *) b;
131
132    size_t valid_end = n - skip_tail;
133
134    for (size_t i = 0; i < n; i++) {
135        unsigned char x = A[i], y = B[i];
136        unsigned int d = x ^ y;
137        mbedtls_ct_condition_t valid = mbedtls_ct_bool_and(mbedtls_ct_uint_ge(i, skip_head),
138                                                           mbedtls_ct_uint_lt(i, valid_end));
139        diff |= mbedtls_ct_uint_if_else_0(valid, d);
140    }
141
142    /* Since we go byte-by-byte, the only bits set will be in the bottom 8 bits, so the
143     * cast from uint to int is safe. */
144    return (int) diff;
145}
146
147#endif
148
149#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
150
151void mbedtls_ct_memmove_left(void *start, size_t total, size_t offset)
152{
153    volatile unsigned char *buf = start;
154    for (size_t i = 0; i < total; i++) {
155        mbedtls_ct_condition_t no_op = mbedtls_ct_uint_gt(total - offset, i);
156        /* The first `total - offset` passes are a no-op. The last
157         * `offset` passes shift the data one byte to the left and
158         * zero out the last byte. */
159        for (size_t n = 0; n < total - 1; n++) {
160            unsigned char current = buf[n];
161            unsigned char next    = buf[n+1];
162            buf[n] = mbedtls_ct_uint_if(no_op, current, next);
163        }
164        buf[total-1] = mbedtls_ct_uint_if_else_0(no_op, buf[total-1]);
165    }
166}
167
168#endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */
169
170void mbedtls_ct_memcpy_if(mbedtls_ct_condition_t condition,
171                          unsigned char *dest,
172                          const unsigned char *src1,
173                          const unsigned char *src2,
174                          size_t len)
175{
176#if defined(MBEDTLS_CT_SIZE_64)
177    const uint64_t mask     = (uint64_t) condition;
178    const uint64_t not_mask = (uint64_t) ~mbedtls_ct_compiler_opaque(condition);
179#else
180    const uint32_t mask     = (uint32_t) condition;
181    const uint32_t not_mask = (uint32_t) ~mbedtls_ct_compiler_opaque(condition);
182#endif
183
184    /* If src2 is NULL, setup src2 so that we read from the destination address.
185     *
186     * This means that if src2 == NULL && condition is false, the result will be a
187     * no-op because we read from dest and write the same data back into dest.
188     */
189    if (src2 == NULL) {
190        src2 = dest;
191    }
192
193    /* dest[i] = c1 == c2 ? src[i] : dest[i] */
194    size_t i = 0;
195#if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS)
196#if defined(MBEDTLS_CT_SIZE_64)
197    for (; (i + 8) <= len; i += 8) {
198        uint64_t a = mbedtls_get_unaligned_uint64(src1 + i) & mask;
199        uint64_t b = mbedtls_get_unaligned_uint64(src2 + i) & not_mask;
200        mbedtls_put_unaligned_uint64(dest + i, a | b);
201    }
202#else
203    for (; (i + 4) <= len; i += 4) {
204        uint32_t a = mbedtls_get_unaligned_uint32(src1 + i) & mask;
205        uint32_t b = mbedtls_get_unaligned_uint32(src2 + i) & not_mask;
206        mbedtls_put_unaligned_uint32(dest + i, a | b);
207    }
208#endif /* defined(MBEDTLS_CT_SIZE_64) */
209#endif /* MBEDTLS_EFFICIENT_UNALIGNED_ACCESS */
210    for (; i < len; i++) {
211        dest[i] = (src1[i] & mask) | (src2[i] & not_mask);
212    }
213}
214
215void mbedtls_ct_memcpy_offset(unsigned char *dest,
216                              const unsigned char *src,
217                              size_t offset,
218                              size_t offset_min,
219                              size_t offset_max,
220                              size_t len)
221{
222    size_t offsetval;
223
224    for (offsetval = offset_min; offsetval <= offset_max; offsetval++) {
225        mbedtls_ct_memcpy_if(mbedtls_ct_uint_eq(offsetval, offset), dest, src + offsetval, NULL,
226                             len);
227    }
228}
229
230#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
231
232void mbedtls_ct_zeroize_if(mbedtls_ct_condition_t condition, void *buf, size_t len)
233{
234    uint32_t mask = (uint32_t) ~condition;
235    uint8_t *p = (uint8_t *) buf;
236    size_t i = 0;
237#if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS)
238    for (; (i + 4) <= len; i += 4) {
239        mbedtls_put_unaligned_uint32((void *) (p + i),
240                                     mbedtls_get_unaligned_uint32((void *) (p + i)) & mask);
241    }
242#endif
243    for (; i < len; i++) {
244        p[i] = p[i] & mask;
245    }
246}
247
248#endif /* defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT) */
249