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Avoid UB caused by conversion to int

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Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard 2021-06-07 09:51:00 +02:00
parent 448f13584d
commit 3ae4ae445f
1 changed files with 42 additions and 7 deletions
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49
library/bignum.c
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@ -237,6 +237,45 @@ void mbedtls_mpi_swap( mbedtls_mpi *X, mbedtls_mpi *Y )
memcpy( Y, &T, sizeof( mbedtls_mpi ) );
}
/**
* Select between two sign values in constant-time.
*
* This is functionally equivalent to second ? a : b but uses only bit
* operations in order to avoid branches.
*
* \param[in] a The first sign; must be either +1 or -1.
* \param[in] b The second sign; must be either +1 or -1.
* \param[in] second Must be either 1 (return b) or 0 (return a).
*
* \return The selected sign value.
*/
static int mpi_safe_cond_select_sign( int a, int b, unsigned char second )
{
/* In order to avoid questions about what we can reasonnably assume about
* the representations of signed integers, move everything to unsigned
* by taking advantage of the fact that a and b are either +1 or -1. */
unsigned ua = a + 1;
unsigned ub = b + 1;
/* MSVC has a warning about unary minus on unsigned integer types,
* but this is well-defined and precisely what we want to do here. */
#if defined(_MSC_VER)
#pragma warning( push )
#pragma warning( disable : 4146 )
#endif
/* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
const unsigned mask = -second;
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
/* select ua or ub */
unsigned ur = ( ua & ~mask ) | ( ub & mask );
/* ur is now 0 or 2, convert back to -1 or +1 */
return( (int) ur - 1 );
}
/*
* Conditionally assign dest = src, without leaking information
* about whether the assignment was made or not.
@ -277,7 +316,6 @@ int mbedtls_mpi_safe_cond_assign( mbedtls_mpi *X, const mbedtls_mpi *Y, unsigned
{
int ret = 0;
size_t i;
unsigned int mask;
mbedtls_mpi_uint limb_mask;
MPI_VALIDATE_RET( X != NULL );
MPI_VALIDATE_RET( Y != NULL );
@ -292,7 +330,6 @@ int mbedtls_mpi_safe_cond_assign( mbedtls_mpi *X, const mbedtls_mpi *Y, unsigned
/* make sure assign is 0 or 1 in a time-constant manner */
assign = (assign | (unsigned char)-assign) >> 7;
/* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
mask = -assign;
limb_mask = -assign;
#if defined(_MSC_VER)
@ -301,7 +338,7 @@ int mbedtls_mpi_safe_cond_assign( mbedtls_mpi *X, const mbedtls_mpi *Y, unsigned
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, Y->n ) );
X->s = ( X->s & ~mask ) | ( Y->s & mask );
X->s = mpi_safe_cond_select_sign( X->s, Y->s, assign );
mpi_safe_cond_assign( Y->n, X->p, Y->p, assign );
@ -322,7 +359,6 @@ int mbedtls_mpi_safe_cond_swap( mbedtls_mpi *X, mbedtls_mpi *Y, unsigned char sw
{
int ret, s;
size_t i;
unsigned int sign_mask;
mbedtls_mpi_uint limb_mask;
mbedtls_mpi_uint tmp;
MPI_VALIDATE_RET( X != NULL );
@ -341,7 +377,6 @@ int mbedtls_mpi_safe_cond_swap( mbedtls_mpi *X, mbedtls_mpi *Y, unsigned char sw
/* make sure swap is 0 or 1 in a time-constant manner */
swap = (swap | (unsigned char)-swap) >> 7;
/* all-bits 1 if swap is 1, all-bits 0 if swap is 0 */
sign_mask = -swap;
limb_mask = -swap;
#if defined(_MSC_VER)
@ -352,8 +387,8 @@ int mbedtls_mpi_safe_cond_swap( mbedtls_mpi *X, mbedtls_mpi *Y, unsigned char sw
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( Y, X->n ) );
s = X->s;
X->s = ( X->s & ~sign_mask ) | ( Y->s & sign_mask );
Y->s = ( Y->s & ~sign_mask ) | ( s & sign_mask );
X->s = mpi_safe_cond_select_sign( X->s, Y->s, swap );
Y->s = mpi_safe_cond_select_sign( Y->s, s, swap );
for( i = 0; i < X->n; i++ )