From ca6e8aac587966c726c25723c67c3680edc57ef5 Mon Sep 17 00:00:00 2001 From: Gilles Peskine Date: Wed, 9 Nov 2022 21:08:44 +0100 Subject: [PATCH] Support negative zero as MPI test input 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 --- tests/include/test/helpers.h | 26 +++++++++++++++++----- tests/src/helpers.c | 28 ++++++++++++++++++++++-- tests/suites/test_suite_bignum.function | 17 +++++++++++--- tests/suites/test_suite_bignum.misc.data | 24 ++++++++++++++++++++ 4 files changed, 84 insertions(+), 11 deletions(-) diff --git a/tests/include/test/helpers.h b/tests/include/test/helpers.h index e0e6fd27fe..568d5e5273 100644 --- a/tests/include/test/helpers.h +++ b/tests/include/test/helpers.h @@ -295,13 +295,19 @@ int mbedtls_test_read_mpi_core( mbedtls_mpi_uint **pX, size_t *plimbs, /** Read an MPI from a hexadecimal string. * - * Like mbedtls_mpi_read_string(), but size the resulting bignum based - * on the number of digits in the string. In particular, construct a - * bignum with 0 limbs for an empty string, and a bignum with leading 0 - * limbs if the string has sufficiently many leading 0 digits. + * Like mbedtls_mpi_read_string(), but with tighter guarantees around + * edge cases. * - * This is important so that the "0 (null)" and "0 (1 limb)" and - * "leading zeros" test cases do what they claim. + * - This function guarantees that if \p s begins with '-' then the sign + * bit of the result will be negative, even if the value is 0. + * When this function encounters such a "negative 0", it + * increments #mbedtls_test_read_mpi. + * - The size of the result is exactly the minimum number of limbs needed + * to fit the digits in the input. In particular, this function constructs + * a bignum with 0 limbs for an empty string, and a bignum with leading 0 + * limbs if the string has sufficiently many leading 0 digits. + * This is important so that the "0 (null)" and "0 (1 limb)" and + * "leading zeros" test cases do what they claim. * * \param[out] X The MPI object to populate. It must be initialized. * \param[in] s The null-terminated hexadecimal string to read from. @@ -309,6 +315,14 @@ int mbedtls_test_read_mpi_core( mbedtls_mpi_uint **pX, size_t *plimbs, * \return \c 0 on success, an \c MBEDTLS_ERR_MPI_xxx error code otherwise. */ int mbedtls_test_read_mpi( mbedtls_mpi *X, const char *s ); + +/** Nonzero if the current test case had an input parsed with + * mbedtls_test_read_mpi() that is a negative 0 (`"-"`, `"-0"`, `"-00"`, etc., + * constructing a result with the sign bit set to -1 and the value being + * all-limbs-0, which is not a valid representation in #mbedtls_mpi but is + * tested for robustness). + */ +extern unsigned mbedtls_test_case_uses_negative_0; #endif /* MBEDTLS_BIGNUM_C */ #endif /* TEST_HELPERS_H */ diff --git a/tests/src/helpers.c b/tests/src/helpers.c index cc23fd7c4d..7c83714f19 100644 --- a/tests/src/helpers.c +++ b/tests/src/helpers.c @@ -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 diff --git a/tests/suites/test_suite_bignum.function b/tests/suites/test_suite_bignum.function index 5c3d776f09..b75f534f46 100644 --- a/tests/suites/test_suite_bignum.function +++ b/tests/suites/test_suite_bignum.function @@ -13,10 +13,21 @@ * constructing the value. */ static int sign_is_valid( const mbedtls_mpi *X ) { + /* Only +1 and -1 are valid sign bits, not e.g. 0 */ if( X->s != 1 && X->s != -1 ) - return( 0 ); // invalid sign bit, e.g. 0 - if( mbedtls_mpi_bitlen( X ) == 0 && X->s != 1 ) - return( 0 ); // negative zero + return( 0 ); + + /* The value 0 must be represented with the sign +1. A "negative zero" + * with s=-1 is an invalid representation. Forbid that. As an exception, + * we sometimes test the robustness of library functions when given + * a negative zero input. If a test case has a negative zero as input, + * we don't mind if the function has a negative zero output. */ + if( ! mbedtls_test_case_uses_negative_0 && + mbedtls_mpi_bitlen( X ) == 0 && X->s != 1 ) + { + return( 0 ); + } + return( 1 ); } diff --git a/tests/suites/test_suite_bignum.misc.data b/tests/suites/test_suite_bignum.misc.data index 0b8aa334ac..818f3613b0 100644 --- a/tests/suites/test_suite_bignum.misc.data +++ b/tests/suites/test_suite_bignum.misc.data @@ -1144,6 +1144,18 @@ mpi_div_mpi:"":"1":"":"":0 Test mbedtls_mpi_div_mpi: 0 (null) / -1 mpi_div_mpi:"":"-1":"":"":0 +Test mbedtls_mpi_div_mpi: -0 (null) / 1 +mpi_div_mpi:"-":"1":"":"":0 + +Test mbedtls_mpi_div_mpi: -0 (null) / -1 +mpi_div_mpi:"-":"-1":"":"":0 + +Test mbedtls_mpi_div_mpi: -0 (null) / 42 +mpi_div_mpi:"-":"2a":"":"":0 + +Test mbedtls_mpi_div_mpi: -0 (null) / -42 +mpi_div_mpi:"-":"-2a":"":"":0 + Test mbedtls_mpi_div_mpi #1 mpi_div_mpi:"9e22d6da18a33d1ef28d2a82242b3f6e9c9742f63e5d440f58a190bfaf23a7866e67589adb80":"22":"4a6abf75b13dc268ea9cc8b5b6aaf0ac85ecd437a4e0987fb13cf8d2acc57c0306c738c1583":"1a":0 @@ -1204,6 +1216,18 @@ mpi_mod_mpi:"":"1":"":0 Test mbedtls_mpi_mod_mpi: 0 (null) % -1 mpi_mod_mpi:"":"-1":"":MBEDTLS_ERR_MPI_NEGATIVE_VALUE +Test mbedtls_mpi_mod_mpi: -0 (null) % 1 +mpi_mod_mpi:"-":"1":"":0 + +Test mbedtls_mpi_mod_mpi: -0 (null) % -1 +mpi_mod_mpi:"-":"-1":"":MBEDTLS_ERR_MPI_NEGATIVE_VALUE + +Test mbedtls_mpi_mod_mpi: -0 (null) % 42 +mpi_mod_mpi:"-":"2a":"":0 + +Test mbedtls_mpi_mod_mpi: -0 (null) % -42 +mpi_mod_mpi:"-":"-2a":"":MBEDTLS_ERR_MPI_NEGATIVE_VALUE + Base test mbedtls_mpi_mod_int #1 mpi_mod_int:"3e8":"d":"c":0