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Support negative zero as MPI test input

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The bignum module does not officially support "negative zero" (an
mbedtls_mpi object with s=-1 and all limbs zero). However, we have a
history of bugs where a function that should produce an official
zero (with s=1), produces a negative zero in some circumstances. So it's
good to check that the bignum functions are robust when passed a negative
zero as input. And for that, we need a way to construct a negative zero
from test case arguments.

There are checks that functions don't produce negative zeros as output in
the test suite. Skip those checks if there's a negative zero input: we
don't want functions to _create_ negative zeros, but we don't mind if
they _propagate_ negative zeros.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine
2022-11-09 21:08:44 +01:00
parent 806c9588ef
commit ca6e8aac58
4 changed files with 84 additions and 11 deletions
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28
tests/src/helpers.c
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@@ -89,6 +89,10 @@ void mbedtls_test_set_step( unsigned long step )
mbedtls_test_info.step = step;
}
#if defined(MBEDTLS_BIGNUM_C)
unsigned mbedtls_test_case_uses_negative_0 = 0;
#endif
void mbedtls_test_info_reset( void )
{
mbedtls_test_info.result = MBEDTLS_TEST_RESULT_SUCCESS;
@@ -98,6 +102,9 @@ void mbedtls_test_info_reset( void )
mbedtls_test_info.filename = 0;
memset( mbedtls_test_info.line1, 0, sizeof( mbedtls_test_info.line1 ) );
memset( mbedtls_test_info.line2, 0, sizeof( mbedtls_test_info.line2 ) );
#if defined(MBEDTLS_BIGNUM_C)
mbedtls_test_case_uses_negative_0 = 0;
#endif
}
int mbedtls_test_equal( const char *test, int line_no, const char* filename,
@@ -396,6 +403,15 @@ exit:
int mbedtls_test_read_mpi( mbedtls_mpi *X, const char *s )
{
int negative = 0;
/* Always set the sign bit to -1 if the input has a minus sign, even for 0.
* This creates an invalid representation, which mbedtls_mpi_read_string()
* avoids but we want to be able to create that in test data. */
if( s[0] == '-' )
{
++s;
negative = 1;
}
/* mbedtls_mpi_read_string() currently retains leading zeros.
* It always allocates at least one limb for the value 0. */
if( s[0] == 0 )
@@ -403,7 +419,15 @@ int mbedtls_test_read_mpi( mbedtls_mpi *X, const char *s )
mbedtls_mpi_free( X );
return( 0 );
}
else
return( mbedtls_mpi_read_string( X, 16, s ) );
int ret = mbedtls_mpi_read_string( X, 16, s );
if( ret != 0 )
return( ret );
if( negative )
{
if( mbedtls_mpi_cmp_int( X, 0 ) == 0 )
++mbedtls_test_case_uses_negative_0;
X->s = -1;
}
return( 0 );
}
#endif