/**
* Constant-time functions
*
* Copyright The Mbed TLS Contributors
* SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
*/
/*
* The following functions are implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
#include <stdint.h>
#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_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) && (UINTPTR_MAX == 0xfffffffful)) || \
defined(MBEDTLS_CT_AARCH64_ASM))
/* We check pointer sizes to avoid issues with them not matching register size requirements */
#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) : MBEDTLS_ASM_AARCH64_PTR_CONSTRAINT(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;
}
#if (INT_MAX < INT32_MAX)
/* We don't support int smaller than 32-bits, but if someone tried to build
* with this configuration, there is a risk that, for differing data, the
* only bits set in diff are in the top 16-bits, and would be lost by a
* simple cast from uint32 to int.
* This would have significant security implications, so protect against it. */
#error "mbedtls_ct_memcmp() requires minimum 32-bit ints"
#else
/* The bit-twiddling ensures that when we cast uint32_t to int, we are casting
* a value that is in the range 0..INT_MAX - a value larger than this would
* result in implementation defined behaviour.
*
* This ensures that the value returned by the function is non-zero iff
* diff is non-zero.
*/
return (int) ((diff & 0xffff) | (diff >> 16));
#endif
}
#if defined(MBEDTLS_NIST_KW_C)
int mbedtls_ct_memcmp_partial(const void *a,
const void *b,
size_t n,
size_t skip_head,
size_t skip_tail)
{
unsigned int diff = 0;
volatile const unsigned char *A = (volatile const unsigned char *) a;
volatile const unsigned char *B = (volatile const unsigned char *) b;
size_t valid_end = n - skip_tail;
for (size_t i = 0; i < n; i++) {
unsigned char x = A[i], y = B[i];
unsigned int d = x ^ y;
mbedtls_ct_condition_t valid = mbedtls_ct_bool_and(mbedtls_ct_uint_ge(i, skip_head),
mbedtls_ct_uint_lt(i, valid_end));
diff |= mbedtls_ct_uint_if_else_0(valid, d);
}
/* Since we go byte-by-byte, the only bits set will be in the bottom 8 bits, so the
* cast from uint to int is safe. */
return (int) diff;
}
#endif
#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)
{
volatile unsigned char *buf = start;
for (size_t i = 0; i < total; i++) {
mbedtls_ct_condition_t no_op = mbedtls_ct_uint_gt(total - offset, i);
/* The first `total - offset` passes are a no-op. The last
* `offset` passes shift the data one byte to the left and
* zero out the last byte. */
for (size_t n = 0; n < total - 1; n++) {
unsigned char current = buf[n];
unsigned char next = buf[n+1];
buf[n] = mbedtls_ct_uint_if(no_op, current, next);
}
buf[total-1] = mbedtls_ct_uint_if_else_0(no_op, buf[total-1]);
}
}
#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)
{
#if defined(MBEDTLS_CT_SIZE_64)
const uint64_t mask = (uint64_t) condition;
const uint64_t not_mask = (uint64_t) ~mbedtls_ct_compiler_opaque(condition);
#else
const uint32_t mask = (uint32_t) condition;
const uint32_t not_mask = (uint32_t) ~mbedtls_ct_compiler_opaque(condition);
#endif
/* If src2 is NULL, setup src2 so that we read from the destination address.
*
* This means that if src2 == NULL && condition is false, the result will be a
* no-op because we read from dest and write the same data 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)
#if defined(MBEDTLS_CT_SIZE_64)
for (; (i + 8) <= len; i += 8) {
uint64_t a = mbedtls_get_unaligned_uint64(src1 + i) & mask;
uint64_t b = mbedtls_get_unaligned_uint64(src2 + i) & not_mask;
mbedtls_put_unaligned_uint64(dest + i, a | b);
}
#else
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 /* defined(MBEDTLS_CT_SIZE_64) */
#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_uint_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) */