Fix undefined behavior with the most negative mbedtls_mpi_sint
When x is the most negative value of a two's complement type,
`(unsigned_type)(-x)` has undefined behavior, whereas `-(unsigned_type)x`
has well-defined behavior and does what was intended.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_bignum.function b/tests/suites/test_suite_bignum.function
index 5c3d776..3238467 100644
--- a/tests/suites/test_suite_bignum.function
+++ b/tests/suites/test_suite_bignum.function
@@ -1447,6 +1447,150 @@
}
/* END_CASE */
+/* BEGIN_CASE */
+void most_negative_mpi_sint( )
+{
+ /* Ad hoc tests for n = -p = -2^(biL-1) as a mbedtls_mpi_sint. We
+ * guarantee that mbedtls_mpi_sint is a two's complement type, so this
+ * is a valid value. However, negating it (`-n`) has undefined behavior
+ * (although in practice `-n` evaluates to the value n).
+ *
+ * This function has ad hoc tests for this value. It's separated from other
+ * functions because the test framework makes it hard to pass this value
+ * into test cases.
+ *
+ * In the comments here:
+ * - biL = number of bits in limbs
+ * - p = 2^(biL-1) (smallest positive value not in mbedtls_mpi_sint range)
+ * - n = -2^(biL-1) (largest negative value in mbedtls_mpi_sint range)
+ */
+
+ mbedtls_mpi A, R, X;
+ mbedtls_mpi_init( &A );
+ mbedtls_mpi_init( &R );
+ mbedtls_mpi_init( &X );
+
+ const size_t biL = 8 * sizeof( mbedtls_mpi_sint );
+ mbedtls_mpi_uint most_positive_plus_1 = (mbedtls_mpi_uint) 1 << ( biL - 1 );
+ const mbedtls_mpi_sint most_positive = most_positive_plus_1 - 1;
+ const mbedtls_mpi_sint most_negative = - most_positive - 1;
+ TEST_EQUAL( (mbedtls_mpi_uint) most_negative,
+ (mbedtls_mpi_uint) 1 << ( biL - 1 ) );
+ TEST_EQUAL( (mbedtls_mpi_uint) most_negative << 1, 0 );
+
+ /* Test mbedtls_mpi_lset() */
+ TEST_EQUAL( mbedtls_mpi_lset( &A, most_negative ), 0 );
+ TEST_EQUAL( A.s, -1 );
+ TEST_EQUAL( A.n, 1 );
+ TEST_EQUAL( A.p[0], most_positive_plus_1 );
+
+ /* Test mbedtls_mpi_cmp_int(): -p == -p */
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &A, most_negative ), 0 );
+
+ /* Test mbedtls_mpi_cmp_int(): -(p+1) < -p */
+ A.p[0] = most_positive_plus_1 + 1;
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &A, most_negative ), -1 );
+
+ /* Test mbedtls_mpi_cmp_int(): -(p-1) > -p */
+ A.p[0] = most_positive_plus_1 - 1;
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &A, most_negative ), 1 );
+
+ /* Test mbedtls_mpi_add_int(): (p-1) + (-p) */
+ TEST_EQUAL( mbedtls_mpi_lset( &A, most_positive ), 0 );
+ TEST_EQUAL( mbedtls_mpi_add_int( &X, &A, most_negative ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &X, -1 ), 0 );
+
+ /* Test mbedtls_mpi_add_int(): (0) + (-p) */
+ TEST_EQUAL( mbedtls_mpi_lset( &A, 0 ), 0 );
+ TEST_EQUAL( mbedtls_mpi_add_int( &X, &A, most_negative ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &X, most_negative ), 0 );
+
+ /* Test mbedtls_mpi_add_int(): (-p) + (-p) */
+ TEST_EQUAL( mbedtls_mpi_lset( &A, most_negative ), 0 );
+ TEST_EQUAL( mbedtls_mpi_add_int( &X, &A, most_negative ), 0 );
+ TEST_EQUAL( X.s, -1 );
+ TEST_EQUAL( X.n, 2 );
+ TEST_EQUAL( X.p[0], 0 );
+ TEST_EQUAL( X.p[1], 1 );
+
+ /* Test mbedtls_mpi_sub_int(): (p) - (-p) */
+ mbedtls_mpi_free( &X );
+ TEST_EQUAL( mbedtls_mpi_lset( &A, most_positive ), 0 );
+ TEST_EQUAL( mbedtls_mpi_sub_int( &X, &A, most_negative ), 0 );
+ TEST_EQUAL( X.s, 1 );
+ TEST_EQUAL( X.n, 1 );
+ TEST_EQUAL( X.p[0], ~(mbedtls_mpi_uint)0 );
+
+ /* Test mbedtls_mpi_sub_int(): (0) - (-p) */
+ TEST_EQUAL( mbedtls_mpi_lset( &A, 0 ), 0 );
+ TEST_EQUAL( mbedtls_mpi_sub_int( &X, &A, most_negative ), 0 );
+ TEST_EQUAL( X.s, 1 );
+ TEST_EQUAL( X.n, 1 );
+ TEST_EQUAL( X.p[0], most_positive_plus_1 );
+
+ /* Test mbedtls_mpi_sub_int(): (-p) - (-p) */
+ TEST_EQUAL( mbedtls_mpi_lset( &A, most_negative ), 0 );
+ TEST_EQUAL( mbedtls_mpi_sub_int( &X, &A, most_negative ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &X, 0 ), 0 );
+
+ /* Test mbedtls_mpi_div_int(): (-p+1) / (-p) */
+ TEST_EQUAL( mbedtls_mpi_lset( &A, -most_positive ), 0 );
+ TEST_EQUAL( mbedtls_mpi_div_int( &X, &R, &A, most_negative ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &X, 0 ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &R, -most_positive ), 0 );
+
+ /* Test mbedtls_mpi_div_int(): (-p) / (-p) */
+ TEST_EQUAL( mbedtls_mpi_lset( &A, most_negative ), 0 );
+ TEST_EQUAL( mbedtls_mpi_div_int( &X, &R, &A, most_negative ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &X, 1 ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &R, 0 ), 0 );
+
+ /* Test mbedtls_mpi_div_int(): (-2*p) / (-p) */
+ TEST_EQUAL( mbedtls_mpi_shift_l( &A, 1 ), 0 );
+ TEST_EQUAL( mbedtls_mpi_div_int( &X, &R, &A, most_negative ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &X, 2 ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &R, 0 ), 0 );
+
+ /* Test mbedtls_mpi_div_int(): (-2*p+1) / (-p) */
+ TEST_EQUAL( mbedtls_mpi_add_int( &A, &A, 1 ), 0 );
+ TEST_EQUAL( mbedtls_mpi_div_int( &X, &R, &A, most_negative ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &X, 1 ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &R, -most_positive ), 0 );
+
+ /* Test mbedtls_mpi_div_int(): (p-1) / (-p) */
+ TEST_EQUAL( mbedtls_mpi_lset( &A, most_positive ), 0 );
+ TEST_EQUAL( mbedtls_mpi_div_int( &X, &R, &A, most_negative ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &X, 0 ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &R, most_positive ), 0 );
+
+ /* Test mbedtls_mpi_div_int(): (p) / (-p) */
+ TEST_EQUAL( mbedtls_mpi_add_int( &A, &A, 1 ), 0 );
+ TEST_EQUAL( mbedtls_mpi_div_int( &X, &R, &A, most_negative ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &X, -1 ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &R, 0 ), 0 );
+
+ /* Test mbedtls_mpi_div_int(): (2*p) / (-p) */
+ TEST_EQUAL( mbedtls_mpi_shift_l( &A, 1 ), 0 );
+ TEST_EQUAL( mbedtls_mpi_div_int( &X, &R, &A, most_negative ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &X, -2 ), 0 );
+ TEST_EQUAL( mbedtls_mpi_cmp_int( &R, 0 ), 0 );
+
+ /* Test mbedtls_mpi_mod_int(): never valid */
+ TEST_EQUAL( mbedtls_mpi_mod_int( X.p, &A, most_negative ),
+ MBEDTLS_ERR_MPI_NEGATIVE_VALUE );
+
+ /* Test mbedtls_mpi_random(): never valid */
+ TEST_EQUAL( mbedtls_mpi_random( &X, most_negative, &A,
+ mbedtls_test_rnd_std_rand, NULL ),
+ MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
+
+exit:
+ mbedtls_mpi_free( &A );
+ mbedtls_mpi_free( &R );
+ mbedtls_mpi_free( &X );
+}
+/* END_CASE */
+
/* BEGIN_CASE depends_on:MBEDTLS_SELF_TEST */
void mpi_selftest( )
{
diff --git a/tests/suites/test_suite_bignum.misc.data b/tests/suites/test_suite_bignum.misc.data
index 0b8aa33..7aaacbe 100644
--- a/tests/suites/test_suite_bignum.misc.data
+++ b/tests/suites/test_suite_bignum.misc.data
@@ -1934,6 +1934,9 @@
MPI random bad arguments: min > N = 1, 0 limb in upper bound
mpi_random_fail:2:"000000000000000001":MBEDTLS_ERR_MPI_BAD_INPUT_DATA
+Most negative mbedtls_mpi_sint
+most_negative_mpi_sint:
+
MPI Selftest
depends_on:MBEDTLS_SELF_TEST
mpi_selftest: