Merge pull request #3992 from stevew817/feature/ecp_no_fallback

Add a flag for disabling software fallback in ecp.c

ChangeLog.d/no_ecp_fallback.txt

@@ -0,0 +1,4 @@
+Features
+   * Automatic fallback to a software implementation of ECP when
+     MBEDTLS_ECP_xxx_ALT accelerator hooks are in use can now be turned off
+     through setting the new configuration flag MBEDTLS_ECP_NO_FALLBACK.

include/mbedtls/check_config.h

@@ -252,6 +252,10 @@
 #error "MBEDTLS_ECP_NORMALIZE_MXZ_ALT defined, but not all prerequisites"
 #endif
 
+#if defined(MBEDTLS_ECP_NO_FALLBACK) && !defined(MBEDTLS_ECP_INTERNAL_ALT)
+#error "MBEDTLS_ECP_NO_FALLBACK defined, but no alternative implementation enabled"
+#endif
+
 #if defined(MBEDTLS_HAVEGE_C) && !defined(MBEDTLS_TIMING_C)
 #error "MBEDTLS_HAVEGE_C defined, but not all prerequisites"
 #endif

include/mbedtls/config.h

@@ -484,6 +484,11 @@
  * is still present and it is used for group structures not supported by the
  * alternative.
  *
+ * The original implementation can in addition be removed by setting the
+ * MBEDTLS_ECP_NO_FALLBACK option, in which case any function for which the
+ * corresponding MBEDTLS_ECP__FUNCTION_NAME__ALT macro is defined will not be
+ * able to fallback to curves not supported by the alternative implementation.
+ *
  * Any of these options become available by defining MBEDTLS_ECP_INTERNAL_ALT
  * and implementing the following functions:
  *      unsigned char mbedtls_internal_ecp_grp_capable(
@@ -497,21 +502,28 @@
  * called before and after each point operation and provide an opportunity to
  * implement optimized set up and tear down instructions.
  *
- * Example: In case you uncomment MBEDTLS_ECP_INTERNAL_ALT and
+ * Example: In case you set MBEDTLS_ECP_INTERNAL_ALT and
- * MBEDTLS_ECP_DOUBLE_JAC_ALT, mbed TLS will still provide the ecp_double_jac
+ * MBEDTLS_ECP_DOUBLE_JAC_ALT, mbed TLS will still provide the ecp_double_jac()
- * function, but will use your mbedtls_internal_ecp_double_jac if the group is
+ * function, but will use your mbedtls_internal_ecp_double_jac() if the group
- * supported (your mbedtls_internal_ecp_grp_capable function returns 1 when
+ * for the operation is supported by your implementation (i.e. your
- * receives it as an argument). If the group is not supported then the original
+ * mbedtls_internal_ecp_grp_capable() function returns 1 for this group). If the
- * implementation is used. The other functions and the definition of
+ * group is not supported by your implementation, then the original mbed TLS
- * mbedtls_ecp_group and mbedtls_ecp_point will not change, so your
+ * implementation of ecp_double_jac() is used instead, unless this fallback
- * implementation of mbedtls_internal_ecp_double_jac and
+ * behaviour is disabled by setting MBEDTLS_ECP_NO_FALLBACK (in which case
- * mbedtls_internal_ecp_grp_capable must be compatible with this definition.
+ * ecp_double_jac() will return MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE).
+ *
+ * The function prototypes and the definition of mbedtls_ecp_group and
+ * mbedtls_ecp_point will not change based on MBEDTLS_ECP_INTERNAL_ALT, so your
+ * implementation of mbedtls_internal_ecp__function_name__ must be compatible
+ * with their definitions.
  *
  * Uncomment a macro to enable alternate implementation of the corresponding
  * function.
  */
 /* Required for all the functions in this section */
 //#define MBEDTLS_ECP_INTERNAL_ALT
+/* Turn off software fallback for curves not supported in hardware */
+//#define MBEDTLS_ECP_NO_FALLBACK
 /* Support for Weierstrass curves with Jacobi representation */
 //#define MBEDTLS_ECP_RANDOMIZE_JAC_ALT
 //#define MBEDTLS_ECP_ADD_MIXED_ALT

library/ecp.c

@@ -1302,9 +1302,6 @@ cleanup:
  */
 static int ecp_normalize_jac( const mbedtls_ecp_group *grp, mbedtls_ecp_point *pt )
 {
-    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
-    mbedtls_mpi Zi, ZZi;
-
     if( mbedtls_mpi_cmp_int( &pt->Z, 0 ) == 0 )
         return( 0 );
 
@@ -1313,6 +1310,11 @@ static int ecp_normalize_jac( const mbedtls_ecp_group *grp, mbedtls_ecp_point *p
         return( mbedtls_internal_ecp_normalize_jac( grp, pt ) );
 #endif /* MBEDTLS_ECP_NORMALIZE_JAC_ALT */
 
+#if defined(MBEDTLS_ECP_NO_FALLBACK) && defined(MBEDTLS_ECP_NORMALIZE_JAC_ALT)
+    return( MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE );
+#else
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+    mbedtls_mpi Zi, ZZi;
     mbedtls_mpi_init( &Zi ); mbedtls_mpi_init( &ZZi );
 
     /*
@@ -1338,6 +1340,7 @@ cleanup:
     mbedtls_mpi_free( &Zi ); mbedtls_mpi_free( &ZZi );
 
     return( ret );
+#endif /* !defined(MBEDTLS_ECP_NO_FALLBACK) || !defined(MBEDTLS_ECP_NORMALIZE_JAC_ALT) */
 }
 
 /*
@@ -1354,10 +1357,6 @@ cleanup:
 static int ecp_normalize_jac_many( const mbedtls_ecp_group *grp,
                                    mbedtls_ecp_point *T[], size_t T_size )
 {
-    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
-    size_t i;
-    mbedtls_mpi *c, u, Zi, ZZi;
-
     if( T_size < 2 )
         return( ecp_normalize_jac( grp, *T ) );
 
@@ -1366,6 +1365,13 @@ static int ecp_normalize_jac_many( const mbedtls_ecp_group *grp,
         return( mbedtls_internal_ecp_normalize_jac_many( grp, T, T_size ) );
 #endif
 
+#if defined(MBEDTLS_ECP_NO_FALLBACK) && defined(MBEDTLS_ECP_NORMALIZE_JAC_MANY_ALT)
+    return( MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE );
+#else
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+    size_t i;
+    mbedtls_mpi *c, u, Zi, ZZi;
+
     if( ( c = mbedtls_calloc( T_size, sizeof( mbedtls_mpi ) ) ) == NULL )
         return( MBEDTLS_ERR_ECP_ALLOC_FAILED );
 
@@ -1433,6 +1439,7 @@ cleanup:
     mbedtls_free( c );
 
     return( ret );
+#endif /* !defined(MBEDTLS_ECP_NO_FALLBACK) || !defined(MBEDTLS_ECP_NORMALIZE_JAC_MANY_ALT) */
 }
 
 /*
@@ -1477,9 +1484,6 @@ cleanup:
 static int ecp_double_jac( const mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
                            const mbedtls_ecp_point *P )
 {
-    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
-    mbedtls_mpi M, S, T, U;
-
 #if defined(MBEDTLS_SELF_TEST)
     dbl_count++;
 #endif
@@ -1489,6 +1493,12 @@ static int ecp_double_jac( const mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
         return( mbedtls_internal_ecp_double_jac( grp, R, P ) );
 #endif /* MBEDTLS_ECP_DOUBLE_JAC_ALT */
 
+#if defined(MBEDTLS_ECP_NO_FALLBACK) && defined(MBEDTLS_ECP_DOUBLE_JAC_ALT)
+    return( MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE );
+#else
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+    mbedtls_mpi M, S, T, U;
+
     mbedtls_mpi_init( &M ); mbedtls_mpi_init( &S ); mbedtls_mpi_init( &T ); mbedtls_mpi_init( &U );
 
     /* Special case for A = -3 */
@@ -1550,6 +1560,7 @@ cleanup:
     mbedtls_mpi_free( &M ); mbedtls_mpi_free( &S ); mbedtls_mpi_free( &T ); mbedtls_mpi_free( &U );
 
     return( ret );
+#endif /* !defined(MBEDTLS_ECP_NO_FALLBACK) || !defined(MBEDTLS_ECP_DOUBLE_JAC_ALT) */
 }
 
 /*
@@ -1573,9 +1584,6 @@ cleanup:
 static int ecp_add_mixed( const mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
                           const mbedtls_ecp_point *P, const mbedtls_ecp_point *Q )
 {
-    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
-    mbedtls_mpi T1, T2, T3, T4, X, Y, Z;
-
 #if defined(MBEDTLS_SELF_TEST)
     add_count++;
 #endif
@@ -1585,6 +1593,12 @@ static int ecp_add_mixed( const mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
         return( mbedtls_internal_ecp_add_mixed( grp, R, P, Q ) );
 #endif /* MBEDTLS_ECP_ADD_MIXED_ALT */
 
+#if defined(MBEDTLS_ECP_NO_FALLBACK) && defined(MBEDTLS_ECP_ADD_MIXED_ALT)
+    return( MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE );
+#else
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+    mbedtls_mpi T1, T2, T3, T4, X, Y, Z;
+
     /*
      * Trivial cases: P == 0 or Q == 0 (case 1)
      */
@@ -1649,6 +1663,7 @@ cleanup:
     mbedtls_mpi_free( &X ); mbedtls_mpi_free( &Y ); mbedtls_mpi_free( &Z );
 
     return( ret );
+#endif /* !defined(MBEDTLS_ECP_NO_FALLBACK) || !defined(MBEDTLS_ECP_ADD_MIXED_ALT) */
 }
 
 /*
@@ -1661,17 +1676,19 @@ cleanup:
 static int ecp_randomize_jac( const mbedtls_ecp_group *grp, mbedtls_ecp_point *pt,
                 int (*f_rng)(void *, unsigned char *, size_t), void *p_rng )
 {
-    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
-    mbedtls_mpi l, ll;
-    size_t p_size;
-    int count = 0;
-
 #if defined(MBEDTLS_ECP_RANDOMIZE_JAC_ALT)
     if( mbedtls_internal_ecp_grp_capable( grp ) )
         return( mbedtls_internal_ecp_randomize_jac( grp, pt, f_rng, p_rng ) );
 #endif /* MBEDTLS_ECP_RANDOMIZE_JAC_ALT */
 
-    p_size = ( grp->pbits + 7 ) / 8;
+#if defined(MBEDTLS_ECP_NO_FALLBACK) && defined(MBEDTLS_ECP_RANDOMIZE_JAC_ALT)
+    return( MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE );
+#else
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+    mbedtls_mpi l, ll;
+    int count = 0;
+    size_t p_size = ( grp->pbits + 7 ) / 8;
+
     mbedtls_mpi_init( &l ); mbedtls_mpi_init( &ll );
 
     /* Generate l such that 1 < l < p */
@@ -1705,6 +1722,7 @@ cleanup:
     mbedtls_mpi_free( &l ); mbedtls_mpi_free( &ll );
 
     return( ret );
+#endif /* !defined(MBEDTLS_ECP_NO_FALLBACK) || !defined(MBEDTLS_ECP_RANDOMIZE_JAC_ALT) */
 }
 
 /*
@@ -2413,19 +2431,22 @@ cleanup:
  */
 static int ecp_normalize_mxz( const mbedtls_ecp_group *grp, mbedtls_ecp_point *P )
 {
-    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
-
 #if defined(MBEDTLS_ECP_NORMALIZE_MXZ_ALT)
     if( mbedtls_internal_ecp_grp_capable( grp ) )
         return( mbedtls_internal_ecp_normalize_mxz( grp, P ) );
 #endif /* MBEDTLS_ECP_NORMALIZE_MXZ_ALT */
 
+#if defined(MBEDTLS_ECP_NO_FALLBACK) && defined(MBEDTLS_ECP_NORMALIZE_MXZ_ALT)
+    return( MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE );
+#else
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     MBEDTLS_MPI_CHK( mbedtls_mpi_inv_mod( &P->Z, &P->Z, &grp->P ) );
     MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mod( grp, &P->X, &P->X, &P->Z ) );
     MBEDTLS_MPI_CHK( mbedtls_mpi_lset( &P->Z, 1 ) );
 
 cleanup:
     return( ret );
+#endif /* !defined(MBEDTLS_ECP_NO_FALLBACK) || !defined(MBEDTLS_ECP_NORMALIZE_MXZ_ALT) */
 }
 
 /*
@@ -2439,17 +2460,18 @@ cleanup:
 static int ecp_randomize_mxz( const mbedtls_ecp_group *grp, mbedtls_ecp_point *P,
                 int (*f_rng)(void *, unsigned char *, size_t), void *p_rng )
 {
-    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
-    mbedtls_mpi l;
-    size_t p_size;
-    int count = 0;
-
 #if defined(MBEDTLS_ECP_RANDOMIZE_MXZ_ALT)
     if( mbedtls_internal_ecp_grp_capable( grp ) )
         return( mbedtls_internal_ecp_randomize_mxz( grp, P, f_rng, p_rng );
 #endif /* MBEDTLS_ECP_RANDOMIZE_MXZ_ALT */
 
-    p_size = ( grp->pbits + 7 ) / 8;
+#if defined(MBEDTLS_ECP_NO_FALLBACK) && defined(MBEDTLS_ECP_RANDOMIZE_MXZ_ALT)
+    return( MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE );
+#else
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+    mbedtls_mpi l;
+    int count = 0;
+    size_t p_size = ( grp->pbits + 7 ) / 8;
     mbedtls_mpi_init( &l );
 
     /* Generate l such that 1 < l < p */
@@ -2475,6 +2497,7 @@ cleanup:
     mbedtls_mpi_free( &l );
 
     return( ret );
+#endif /* !defined(MBEDTLS_ECP_NO_FALLBACK) || !defined(MBEDTLS_ECP_RANDOMIZE_MXZ_ALT) */
 }
 
 /*
@@ -2497,14 +2520,17 @@ static int ecp_double_add_mxz( const mbedtls_ecp_group *grp,
                                const mbedtls_ecp_point *P, const mbedtls_ecp_point *Q,
                                const mbedtls_mpi *d )
 {
-    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
-    mbedtls_mpi A, AA, B, BB, E, C, D, DA, CB;
-
 #if defined(MBEDTLS_ECP_DOUBLE_ADD_MXZ_ALT)
     if( mbedtls_internal_ecp_grp_capable( grp ) )
         return( mbedtls_internal_ecp_double_add_mxz( grp, R, S, P, Q, d ) );
 #endif /* MBEDTLS_ECP_DOUBLE_ADD_MXZ_ALT */
 
+#if defined(MBEDTLS_ECP_NO_FALLBACK) && defined(MBEDTLS_ECP_DOUBLE_ADD_MXZ_ALT)
+    return( MBEDTLS_ERR_ECP_FEATURE_UNAVAILABLE );
+#else
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+    mbedtls_mpi A, AA, B, BB, E, C, D, DA, CB;
+
     mbedtls_mpi_init( &A ); mbedtls_mpi_init( &AA ); mbedtls_mpi_init( &B );
     mbedtls_mpi_init( &BB ); mbedtls_mpi_init( &E ); mbedtls_mpi_init( &C );
     mbedtls_mpi_init( &D ); mbedtls_mpi_init( &DA ); mbedtls_mpi_init( &CB );
@@ -2534,6 +2560,7 @@ cleanup:
     mbedtls_mpi_free( &D ); mbedtls_mpi_free( &DA ); mbedtls_mpi_free( &CB );
 
     return( ret );
+#endif /* !defined(MBEDTLS_ECP_NO_FALLBACK) || !defined(MBEDTLS_ECP_DOUBLE_ADD_MXZ_ALT) */
 }
 
 /*

library/version_features.c

@@ -225,6 +225,9 @@ static const char * const features[] = {
 #if defined(MBEDTLS_ECP_INTERNAL_ALT)
     "MBEDTLS_ECP_INTERNAL_ALT",
 #endif /* MBEDTLS_ECP_INTERNAL_ALT */
+#if defined(MBEDTLS_ECP_NO_FALLBACK)
+    "MBEDTLS_ECP_NO_FALLBACK",
+#endif /* MBEDTLS_ECP_NO_FALLBACK */
 #if defined(MBEDTLS_ECP_RANDOMIZE_JAC_ALT)
     "MBEDTLS_ECP_RANDOMIZE_JAC_ALT",
 #endif /* MBEDTLS_ECP_RANDOMIZE_JAC_ALT */

programs/test/query_config.c

@@ -658,6 +658,14 @@ int query_config( const char *config )
     }
 #endif /* MBEDTLS_ECP_INTERNAL_ALT */
 
+#if defined(MBEDTLS_ECP_NO_FALLBACK)
+    if( strcmp( "MBEDTLS_ECP_NO_FALLBACK", config ) == 0 )
+    {
+        MACRO_EXPANSION_TO_STR( MBEDTLS_ECP_NO_FALLBACK );
+        return( 0 );
+    }
+#endif /* MBEDTLS_ECP_NO_FALLBACK */
+
 #if defined(MBEDTLS_ECP_RANDOMIZE_JAC_ALT)
     if( strcmp( "MBEDTLS_ECP_RANDOMIZE_JAC_ALT", config ) == 0 )
     {

scripts/config.py

@@ -171,6 +171,7 @@ EXCLUDE_FROM_FULL = frozenset([
     'MBEDTLS_DEPRECATED_REMOVED', # conflicts with deprecated options
     'MBEDTLS_DEPRECATED_WARNING', # conflicts with deprecated options
     'MBEDTLS_ECDH_VARIANT_EVEREST_ENABLED', # influences the use of ECDH in TLS
+    'MBEDTLS_ECP_NO_FALLBACK', # removes internal ECP implementation
     'MBEDTLS_ECP_NO_INTERNAL_RNG', # removes a feature
     'MBEDTLS_ECP_RESTARTABLE', # incompatible with USE_PSA_CRYPTO
     'MBEDTLS_ENTROPY_FORCE_SHA256', # interacts with CTR_DRBG_128_BIT_KEY