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Improve implementation of mbedtls_ct_int_if

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Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
This commit is contained in:
Dave Rodgman 2023-09-21 21:06:48 +01:00
parent 61f1beaccf
commit 530c3da698
1 changed files with 36 additions and 11 deletions
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47
library/constant_time_impl.h
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@ -331,6 +331,42 @@ static inline unsigned char mbedtls_ct_uchar_in_range_if(unsigned char low,
return (unsigned char) (~(low_mask | high_mask)) & to;
}
static inline int mbedtls_ct_int_if(mbedtls_ct_condition_t condition, int if1, int if0)
{
/*
* Use memcpy to avoid unsigned-to-signed conversion UB.
*
* Although this looks inefficient, in practice (for gcc and clang),
* the sizeof test is eliminated at compile-time, and identical code is generated
* as for mbedtls_ct_uint_if (for -Os and -O1 or higher).
*/
int result;
if (sizeof(int) <= sizeof(unsigned int)) {
unsigned int uif1 = 0, uif0 = 0;
memcpy(&uif1, &if1, sizeof(if1));
memcpy(&uif0, &if0, sizeof(if0));
unsigned int uresult = mbedtls_ct_uint_if(condition, uif1, uif0);
memcpy(&result, &uresult, sizeof(result));
} else {
mbedtls_ct_memcpy_if(condition, (unsigned char *) &result, (const unsigned char *) &if1,
(const unsigned char *) &if0, sizeof(result));
}
return result;
}
static inline int mbedtls_ct_int_if_else_0(mbedtls_ct_condition_t condition, int if1)
{
if (sizeof(int) <= sizeof(unsigned int)) {
unsigned int uif1 = 0;
memcpy(&uif1, &if1, sizeof(if1));
unsigned int uresult = mbedtls_ct_uint_if_else_0(condition, uif1);
int result;
memcpy(&result, &uresult, sizeof(result));
return result;
} else {
return mbedtls_ct_int_if(condition, if1, 0);
}
}
/* ============================================================================
* Everything below here is trivial wrapper functions
@ -397,17 +433,6 @@ static inline mbedtls_mpi_uint mbedtls_ct_mpi_uint_if_else_0(mbedtls_ct_conditio
#endif /* MBEDTLS_BIGNUM_C */
static inline int mbedtls_ct_int_if(mbedtls_ct_condition_t condition, int if1, int if0)
{
int a[2] = {if0, if1};
return a[mbedtls_ct_uint_if_else_0(condition, 1)];
}
static inline int mbedtls_ct_int_if_else_0(mbedtls_ct_condition_t condition, int if1)
{
return mbedtls_ct_int_if(condition, if1, 0);
}
static inline mbedtls_ct_condition_t mbedtls_ct_uint_eq(mbedtls_ct_uint_t x,
mbedtls_ct_uint_t y)
{