Fix null pointer dereference in mbedtls_mpi_exp_mod
Fix a null pointer dereference in mbedtls_mpi_exp_mod(X, A, N, E, _RR) when
A is the value 0 represented with 0 limbs.
Make the code a little more robust against similar bugs.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/bignum.c b/library/bignum.c
index c49d63e..40f1239 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -203,7 +203,13 @@
}
/*
- * Copy the contents of Y into X
+ * Copy the contents of Y into X.
+ *
+ * This function is not constant-time. Leading zeros in Y may be removed.
+ *
+ * Ensure that X does not shrink. This is not guaranteed by the public API,
+ * but some code in the bignum module relies on this property, for example
+ * in mbedtls_mpi_exp_mod().
*/
int mbedtls_mpi_copy( mbedtls_mpi *X, const mbedtls_mpi *Y )
{
@@ -217,7 +223,11 @@
if( Y->n == 0 )
{
- mbedtls_mpi_free( X );
+ if( X->n != 0 )
+ {
+ X->s = 1;
+ memset( X->p, 0, X->n * ciL );
+ }
return( 0 );
}
@@ -2175,6 +2185,11 @@
#endif
j = N->n + 1;
+ /* All W[i] and X must have at least N->n limbs for the mpi_montmul()
+ * and mpi_montred() calls later. Here we ensure that W[1] and X are
+ * large enough, and later we'll grow other W[i] to the same length.
+ * They must not be shrunk midway through this function!
+ */
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, j ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &W[1], j ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &T, j * 2 ) );
@@ -2209,10 +2224,18 @@
* W[1] = A * R^2 * R^-1 mod N = A * R mod N
*/
if( mbedtls_mpi_cmp_mpi( A, N ) >= 0 )
+ {
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &W[1], A, N ) );
+ /* This should be a no-op because W[1] is already that large before
+ * mbedtls_mpi_mod_mpi(), but it's necessary to avoid an overflow
+ * in mpi_montmul() below, so let's make sure. */
+ MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &W[1], N->n +1 ) );
+ }
else
MBEDTLS_MPI_CHK( mbedtls_mpi_copy( &W[1], A ) );
+ /* Note that this is safe because W[1] always has at least N->n limbs
+ * (it grew above and was preserved by mbedtls_mpi_copy()). */
mpi_montmul( &W[1], &RR, N, mm, &T );
/*
diff --git a/tests/suites/test_suite_mpi.data b/tests/suites/test_suite_mpi.data
index ea1f27e..630e008 100644
--- a/tests/suites/test_suite_mpi.data
+++ b/tests/suites/test_suite_mpi.data
@@ -1079,19 +1079,15 @@
mbedtls_mpi_exp_mod:10:"-23":10:"-13":10:"29":10:"0":MBEDTLS_ERR_MPI_BAD_INPUT_DATA
Test mbedtls_mpi_exp_mod: 0 (null) ^ 0 (null) mod 9
-depends_on:TEST_TEMPORARILY_DISABLED_BECAUSE_IT_CAUSES_A_CRASH
mbedtls_mpi_exp_mod:16:"":16:"":16:"09":16:"1":0
Test mbedtls_mpi_exp_mod: 0 (null) ^ 0 (1 limb) mod 9
-depends_on:TEST_TEMPORARILY_DISABLED_BECAUSE_IT_CAUSES_A_CRASH
mbedtls_mpi_exp_mod:16:"":16:"00":16:"09":16:"1":0
Test mbedtls_mpi_exp_mod: 0 (null) ^ 1 mod 9
-depends_on:TEST_TEMPORARILY_DISABLED_BECAUSE_IT_CAUSES_A_CRASH
mbedtls_mpi_exp_mod:16:"":16:"01":16:"09":16:"":0
Test mbedtls_mpi_exp_mod: 0 (null) ^ 2 mod 9
-depends_on:TEST_TEMPORARILY_DISABLED_BECAUSE_IT_CAUSES_A_CRASH
mbedtls_mpi_exp_mod:16:"":16:"02":16:"09":16:"":0
Test mbedtls_mpi_exp_mod: 0 (1 limb) ^ 0 (null) mod 9