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/*
* Helper functions for constant time operations
* Copyright (c) 2019, The Linux Foundation
*
* This software may be distributed under the terms of the BSD license.
* See README for more details.
*
* These helper functions can be used to implement logic that needs to minimize
* externally visible differences in execution path by avoiding use of branches,
* avoiding early termination or other time differences, and forcing same memory
* access pattern regardless of values.
*/
#ifndef CONST_TIME_H
#define CONST_TIME_H
#if defined(__clang__)
#define NO_UBSAN_UINT_OVERFLOW \
__attribute__((no_sanitize("unsigned-integer-overflow")))
#else
#define NO_UBSAN_UINT_OVERFLOW
#endif
/**
* const_time_fill_msb - Fill all bits with MSB value
* @param val Input value
* @return Value with all the bits set to the MSB of the input val
*/
static inline unsigned int const_time_fill_msb(unsigned int val)
{
/* Move the MSB to LSB and multiple by -1 to fill in all bits. */
return (val >> (sizeof(val) * 8 - 1)) * ~0U;
}
/* @return -1 if val is zero; 0 if val is not zero */
static inline unsigned int const_time_is_zero(unsigned int val)
NO_UBSAN_UINT_OVERFLOW
{
/* Set MSB to 1 for 0 and fill rest of bits with the MSB value */
return const_time_fill_msb(~val & (val - 1));
}
/* @return -1 if a == b; 0 if a != b */
static inline unsigned int const_time_eq(unsigned int a, unsigned int b)
{
return const_time_is_zero(a ^ b);
}
/* @return -1 if a == b; 0 if a != b */
static inline unsigned char const_time_eq_u8(unsigned int a, unsigned int b)
{
return (unsigned char) const_time_eq(a, b);
}
/**
* const_time_eq_bin - Constant time memory comparison
* @param a First buffer to compare
* @param b Second buffer to compare
* @param len Number of octets to compare
* @return -1 if buffers are equal, 0 if not
*
* This function is meant for comparing passwords or hash values where
* difference in execution time or memory access pattern could provide external
* observer information about the location of the difference in the memory
* buffers. The return value does not behave like memcmp(), i.e.,
* const_time_eq_bin() cannot be used to sort items into a defined order. Unlike
* memcmp(), the execution time of const_time_eq_bin() does not depend on the
* contents of the compared memory buffers, but only on the total compared
* length.
*/
static inline unsigned int const_time_eq_bin(const void *a, const void *b,
size_t len)
{
const unsigned char *aa = a;
const unsigned char *bb = b;
size_t i;
unsigned char res = 0;
for (i = 0; i < len; i++)
res |= aa[i] ^ bb[i];
return const_time_is_zero(res);
}
/**
* const_time_select - Constant time unsigned int selection
* @param mask 0 (false) or -1 (true) to identify which value to select
* @param true_val Value to select for the true case
* @param false_val Value to select for the false case
* @return true_val if mask == -1, false_val if mask == 0
*/
static inline unsigned int const_time_select(unsigned int mask,
unsigned int true_val,
unsigned int false_val)
{
return (mask & true_val) | (~mask & false_val);
}
/**
* const_time_select_int - Constant time int selection
* @param mask 0 (false) or -1 (true) to identify which value to select
* @param true_val Value to select for the true case
* @param false_val Value to select for the false case
* @return true_val if mask == -1, false_val if mask == 0
*/
static inline int const_time_select_int(unsigned int mask, int true_val,
int false_val)
{
return (int) const_time_select(mask, (unsigned int) true_val,
(unsigned int) false_val);
}
/**
* const_time_select_u8 - Constant time u8 selection
* @param mask 0 (false) or -1 (true) to identify which value to select
* @param true_val Value to select for the true case
* @param false_val Value to select for the false case
* @return true_val if mask == -1, false_val if mask == 0
*/
static inline unsigned char const_time_select_u8(unsigned char mask, unsigned char true_val, unsigned char false_val)
{
return (unsigned char) const_time_select(mask, true_val, false_val);
}
/**
* const_time_select_s8 - Constant time s8 selection
* @param mask 0 (false) or -1 (true) to identify which value to select
* @param true_val Value to select for the true case
* @param false_val Value to select for the false case
* @return true_val if mask == -1, false_val if mask == 0
*/
static inline char const_time_select_s8(char mask, char true_val, char false_val)
{
return (char) const_time_select(mask, (unsigned int) true_val,
(unsigned int) false_val);
}
/**
* const_time_select_bin - Constant time binary buffer selection copy
* @param mask 0 (false) or -1 (true) to identify which value to copy
* @param true_val Buffer to copy for the true case
* @param false_val Buffer to copy for the false case
* @param len Number of octets to copy
* @param dst Destination buffer for the copy
*
* This function copies the specified buffer into the destination buffer using
* operations with identical memory access pattern regardless of which buffer
* is being copied.
*/
static inline void const_time_select_bin(unsigned char mask, const unsigned char *true_val,
const unsigned char *false_val, size_t len,
unsigned char *dst)
{
size_t i;
for (i = 0; i < len; i++)
dst[i] = const_time_select_u8(mask, true_val[i], false_val[i]);
}
static inline int const_time_memcmp(const void *a, const void *b, size_t len)
{
const unsigned char *aa = a;
const unsigned char *bb = b;
int diff, res = 0;
unsigned int mask;
if (len == 0)
return 0;
do {
len--;
diff = (int) aa[len] - (int) bb[len];
mask = const_time_is_zero((unsigned int) diff);
res = const_time_select_int(mask, res, diff);
} while (len);
return res;
}
#endif /* CONST_TIME_H */
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