mbedtls/library/constant_time.c

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/**
* Constant-time functions
*
* Copyright The Mbed TLS Contributors
* SPDX-License-Identifier: Apache-2.0
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may
* not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
/*
* The following functions are implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
#include <limits.h>
#include "common.h"
#include "constant_time_internal.h"
#include "mbedtls/constant_time.h"
#include "mbedtls/error.h"
#include "mbedtls/platform_util.h"
#include <string.h>
#if defined(MBEDTLS_USE_PSA_CRYPTO)
#define PSA_TO_MBEDTLS_ERR(status) PSA_TO_MBEDTLS_ERR_LIST(status, \
psa_to_ssl_errors, \
psa_generic_status_to_mbedtls)
#endif
#if !defined(MBEDTLS_CT_ASM)
/*
* Define an object with the value zero, such that the compiler cannot prove that it
* has the value zero (because it is volatile, it "may be modified in ways unknown to
* the implementation").
*/
volatile mbedtls_ct_uint_t mbedtls_ct_zero = 0;
#endif
/*
* Define MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS where assembly is present to
* perform fast unaligned access to volatile data.
*
* This is needed because mbedtls_get_unaligned_uintXX etc don't support volatile
* memory accesses.
*
* Some of these definitions could be moved into alignment.h but for now they are
* only used here.
*/
#if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS) && \
(defined(MBEDTLS_CT_ARM_ASM) || defined(MBEDTLS_CT_AARCH64_ASM))
#define MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS
static inline uint32_t mbedtls_get_unaligned_volatile_uint32(volatile const unsigned char *p)
{
/* This is UB, even where it's safe:
* return *((volatile uint32_t*)p);
* so instead the same thing is expressed in assembly below.
*/
uint32_t r;
#if defined(MBEDTLS_CT_ARM_ASM)
asm volatile ("ldr %0, [%1]" : "=r" (r) : "r" (p) :);
#elif defined(MBEDTLS_CT_AARCH64_ASM)
asm volatile ("ldr %w0, [%1]" : "=r" (r) : "r" (p) :);
#else
#error No assembly defined for mbedtls_get_unaligned_volatile_uint32
#endif
return r;
}
#endif /* defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS) &&
(defined(MBEDTLS_CT_ARM_ASM) || defined(MBEDTLS_CT_AARCH64_ASM)) */
int mbedtls_ct_memcmp(const void *a,
const void *b,
size_t n)
{
size_t i = 0;
/*
* `A` and `B` are cast to volatile to ensure that the compiler
* generates code that always fully reads both buffers.
* Otherwise it could generate a test to exit early if `diff` has all
* bits set early in the loop.
*/
volatile const unsigned char *A = (volatile const unsigned char *) a;
volatile const unsigned char *B = (volatile const unsigned char *) b;
uint32_t diff = 0;
#if defined(MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS)
for (; (i + 4) <= n; i += 4) {
uint32_t x = mbedtls_get_unaligned_volatile_uint32(A + i);
uint32_t y = mbedtls_get_unaligned_volatile_uint32(B + i);
diff |= x ^ y;
}
#endif
for (; i < n; i++) {
/* Read volatile data in order before computing diff.
* This avoids IAR compiler warning:
* 'the order of volatile accesses is undefined ..' */
unsigned char x = A[i], y = B[i];
diff |= x ^ y;
}
return (int) diff;
}
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
void mbedtls_ct_memmove_left(void *start, size_t total, size_t offset)
{
/* Iterate over the array, reading each byte once and writing each byte once. */
for (size_t i = 0; i < total; i++) {
/* Each iteration, read one byte, and write it to start[i].
*
* The source address will either be the "true" source address, if it's in the range
* where data is getting moved, or (if the source address is off the end of the
* array), it will wrap back to the start.
*
* If the source address is out of range, mask it to zero.
*/
// The address that we will read from
// TODO: if offset is marked as secret, this upsets Memsan.
size_t j = i + offset;
// Is the address off the end of the array?
mbedtls_ct_condition_t not_dummy = mbedtls_ct_bool_lt(j, total);
// Bring read address into range
j = j % total;
// Read a byte
uint8_t b = ((uint8_t *) start)[j];
// Set it to zero if it's out of range
b = mbedtls_ct_uint_if0(not_dummy, b);
// Write the byte to start[i]
((uint8_t *) start)[i] = b;
}
}
#endif /* MBEDTLS_PKCS1_V15 && MBEDTLS_RSA_C && ! MBEDTLS_RSA_ALT */
void mbedtls_ct_memcpy_if(mbedtls_ct_condition_t condition,
unsigned char *dest,
const unsigned char *src1,
const unsigned char *src2,
size_t len)
{
const uint32_t mask = (uint32_t) condition;
const uint32_t not_mask = (uint32_t) ~mbedtls_ct_compiler_opaque(condition);
/* If src2 is NULL and condition == 0, then this function has no effect.
* In this case, copy from dest back into dest. */
if (src2 == NULL) {
src2 = dest;
}
/* dest[i] = c1 == c2 ? src[i] : dest[i] */
size_t i = 0;
#if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS)
for (; (i + 4) <= len; i += 4) {
uint32_t a = mbedtls_get_unaligned_uint32(src1 + i) & mask;
uint32_t b = mbedtls_get_unaligned_uint32(src2 + i) & not_mask;
mbedtls_put_unaligned_uint32(dest + i, a | b);
}
#endif /* MBEDTLS_EFFICIENT_UNALIGNED_ACCESS */
for (; i < len; i++) {
dest[i] = (src1[i] & mask) | (src2[i] & not_mask);
}
}
void mbedtls_ct_memcpy_offset(unsigned char *dest,
const unsigned char *src,
size_t offset,
size_t offset_min,
size_t offset_max,
size_t len)
{
size_t offsetval;
for (offsetval = offset_min; offsetval <= offset_max; offsetval++) {
mbedtls_ct_memcpy_if(mbedtls_ct_bool_eq(offsetval, offset), dest, src + offsetval, NULL,
len);
}
}
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
void mbedtls_ct_zeroize_if(mbedtls_ct_condition_t condition, void *buf, size_t len)
{
uint32_t mask = (uint32_t) ~condition;
uint8_t *p = (uint8_t *) buf;
size_t i = 0;
#if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS)
for (; (i + 4) <= len; i += 4) {
mbedtls_put_unaligned_uint32((void *) (p + i),
mbedtls_get_unaligned_uint32((void *) (p + i)) & mask);
}
#endif
for (; i < len; i++) {
p[i] = p[i] & mask;
}
}
#endif /* defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT) */