Line data Source code
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 : */
30 : volatile 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 :
49 : static 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 :
68 673 : int mbedtls_ct_memcmp(const void *a,
69 : const void *b,
70 : size_t n)
71 : {
72 673 : 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 : */
79 673 : volatile const unsigned char *A = (volatile const unsigned char *) a;
80 673 : volatile const unsigned char *B = (volatile const unsigned char *) b;
81 673 : 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 159649 : 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 158976 : unsigned char x = A[i], y = B[i];
96 158976 : 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 : */
115 673 : return (int) ((diff & 0xffff) | (diff >> 16));
116 : #endif
117 : }
118 :
119 : #if defined(MBEDTLS_NIST_KW_C)
120 :
121 : int 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 :
151 0 : void mbedtls_ct_memmove_left(void *start, size_t total, size_t offset)
152 : {
153 0 : volatile unsigned char *buf = start;
154 0 : for (size_t i = 0; i < total; i++) {
155 0 : 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 0 : for (size_t n = 0; n < total - 1; n++) {
160 0 : unsigned char current = buf[n];
161 0 : unsigned char next = buf[n+1];
162 0 : buf[n] = mbedtls_ct_uint_if(no_op, current, next);
163 : }
164 0 : buf[total-1] = mbedtls_ct_uint_if_else_0(no_op, buf[total-1]);
165 : }
166 0 : }
167 :
168 : #endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */
169 :
170 169860 : void 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 169860 : const uint64_t mask = (uint64_t) condition;
178 169860 : 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 169860 : if (src2 == NULL) {
190 169860 : src2 = dest;
191 : }
192 :
193 : /* dest[i] = c1 == c2 ? src[i] : dest[i] */
194 169860 : size_t i = 0;
195 : #if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS)
196 : #if defined(MBEDTLS_CT_SIZE_64)
197 3886397 : for (; (i + 8) <= len; i += 8) {
198 3716537 : uint64_t a = mbedtls_get_unaligned_uint64(src1 + i) & mask;
199 3716537 : uint64_t b = mbedtls_get_unaligned_uint64(src2 + i) & not_mask;
200 3716537 : 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 169860 : for (; i < len; i++) {
211 0 : dest[i] = (src1[i] & mask) | (src2[i] & not_mask);
212 : }
213 169860 : }
214 :
215 0 : void 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 0 : for (offsetval = offset_min; offsetval <= offset_max; offsetval++) {
225 0 : mbedtls_ct_memcpy_if(mbedtls_ct_uint_eq(offsetval, offset), dest, src + offsetval, NULL,
226 : len);
227 : }
228 0 : }
229 :
230 : #if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
231 :
232 0 : void mbedtls_ct_zeroize_if(mbedtls_ct_condition_t condition, void *buf, size_t len)
233 : {
234 0 : uint32_t mask = (uint32_t) ~condition;
235 0 : uint8_t *p = (uint8_t *) buf;
236 0 : size_t i = 0;
237 : #if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS)
238 0 : for (; (i + 4) <= len; i += 4) {
239 0 : mbedtls_put_unaligned_uint32((void *) (p + i),
240 0 : mbedtls_get_unaligned_uint32((void *) (p + i)) & mask);
241 : }
242 : #endif
243 0 : for (; i < len; i++) {
244 0 : p[i] = p[i] & mask;
245 : }
246 0 : }
247 :
248 : #endif /* defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT) */
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