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Factor repeated code in ssl_cf functions

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Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard 2020-08-25 10:01:00 +02:00
parent 2ddec4306f
commit 6e2a9a7faa
1 changed files with 28 additions and 37 deletions
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65
library/ssl_msg.c
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@ -1044,6 +1044,25 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
} }
#if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC) #if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
/*
* Turn a bit into a mask:
* - if bit == 1, return the all-bits 1 mask, aka (size_t) -1
* - if bit == 0, return the all-bits 0 mask, aka 0
*/
static size_t mbedtls_ssl_cf_mask_from_bit( size_t bit )
{
/* 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
return -bit;
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
}
/* /*
* Constant-flow mask generation for "less than" comparison: * Constant-flow mask generation for "less than" comparison:
* - if x < y, return all bits 1, that is (size_t) -1 * - if x < y, return all bits 1, that is (size_t) -1
@ -1052,7 +1071,7 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
* Use only bit operations to avoid branches that could be used by some * Use only bit operations to avoid branches that could be used by some
* compilers on some platforms to translate comparison operators. * compilers on some platforms to translate comparison operators.
*/ */
static size_t mbedtls_ssl_cf_mask_lt(size_t x, size_t y) static size_t mbedtls_ssl_cf_mask_lt( size_t x, size_t y )
{ {
/* This has the msb set if and only if x < y */ /* This has the msb set if and only if x < y */
const size_t sub = x - y; const size_t sub = x - y;
@ -1060,17 +1079,8 @@ static size_t mbedtls_ssl_cf_mask_lt(size_t x, size_t y)
/* sub1 = (x < y) in {0, 1} */ /* sub1 = (x < y) in {0, 1} */
const size_t sub1 = sub >> ( sizeof( sub ) * 8 - 1 ); const size_t sub1 = sub >> ( sizeof( sub ) * 8 - 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
/* mask = (x < y) ? 0xff... : 0x00... */ /* mask = (x < y) ? 0xff... : 0x00... */
const size_t mask = -sub1; const size_t mask = mbedtls_ssl_cf_mask_from_bit( sub1 );
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
return( mask ); return( mask );
} }
@ -1083,9 +1093,9 @@ static size_t mbedtls_ssl_cf_mask_lt(size_t x, size_t y)
* Use only bit operations to avoid branches that could be used by some * Use only bit operations to avoid branches that could be used by some
* compilers on some platforms to translate comparison operators. * compilers on some platforms to translate comparison operators.
*/ */
static size_t mbedtls_ssl_cf_mask_ge(size_t x, size_t y) static size_t mbedtls_ssl_cf_mask_ge( size_t x, size_t y )
{ {
return( ~mbedtls_ssl_cf_mask_lt(x, y) ); return( ~mbedtls_ssl_cf_mask_lt( x, y ) );
} }
/* /*
@ -1095,7 +1105,7 @@ static size_t mbedtls_ssl_cf_mask_ge(size_t x, size_t y)
* Use only bit operations to avoid branches that could be used by some * Use only bit operations to avoid branches that could be used by some
* compilers on some platforms to translate comparison operators. * compilers on some platforms to translate comparison operators.
*/ */
static size_t mbedtls_ssl_cf_bool_eq(size_t x, size_t y) static size_t mbedtls_ssl_cf_bool_eq( size_t x, size_t y )
{ {
/* diff = 0 if x == y, non-zero otherwise */ /* diff = 0 if x == y, non-zero otherwise */
const size_t diff = x ^ y; const size_t diff = x ^ y;
@ -1134,32 +1144,13 @@ static void mbedtls_ssl_cf_memcpy_if_eq( unsigned char *dst,
size_t len, size_t len,
size_t c1, size_t c2 ) size_t c1, size_t c2 )
{ {
/* diff = 0 if c1 == c2, non-zero otherwise */ /* mask = c1 == c2 ? 0xff : 0x00 */
const size_t diff = c1 ^ c2; const size_t equal = mbedtls_ssl_cf_bool_eq( c1, c2 );
const unsigned char mask = mbedtls_ssl_cf_mask_from_bit( equal );
/* 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
/* diff_msb's most significant bit is equal to c1 != c2 */
const size_t diff_msb = ( diff | -diff );
/* diff1 = (c1 != c2) in {0, 1} */
const size_t diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 );
/* mask = c1 != c2 ? 0xff : 0x00 */
const unsigned char mask = (unsigned char) -diff1;
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
/* dst[i] = c1 != c2 ? dst[i] : src[i] */ /* dst[i] = c1 != c2 ? dst[i] : src[i] */
for( size_t i = 0; i < len; i++ ) for( size_t i = 0; i < len; i++ )
dst[i] = ( dst[i] & mask ) | ( src[i] & ~mask ); dst[i] = ( dst[i] & ~mask ) | ( src[i] & mask );
} }
/* /*