Fix missing NULL check in MPI

ChangeLog

@@ -28,6 +28,8 @@ Features
      errors on use of deprecated functions.
 
 Bugfix
+   * mpi_size() and mpi_msb() would segfault when called on an mpi that is
+     initialized but not set (found by pravic).
    * Fix detection of support for getrandom() on Linux (reported by syzzer) by
      doing it at runtime (using uname) rather that compile time.
    * Fix handling of symlinks by "make install" (found by Gaël PORTAY).

include/polarssl/bignum.h

@@ -188,7 +188,9 @@ typedef struct
 mpi;
 
 /**
- * \brief           Initialize one MPI
+ * \brief           Initialize one MPI (make internal references valid)
+ *                  This just makes it ready to be set or freed,
+ *                  but does not define a value for the MPI.
  *
  * \param X         One MPI to initialize.
  */

library/bignum.c

@@ -356,6 +356,9 @@ size_t mpi_msb( const mpi *X )
 {
     size_t i, j;
 
+    if( X->n == 0 )
+        return( 0 );
+
     for( i = X->n - 1; i > 0; i-- )
         if( X->p[i] != 0 )
             break;

tests/suites/test_suite_mpi.data

@@ -1,3 +1,6 @@
+Arguments with no value
+mpi_null:
+
 Base test mpi_read_write_string #1
 mpi_read_write_string:10:"128":10:"128":100:0:0
 

tests/suites/test_suite_mpi.function

@@ -7,6 +7,25 @@
  * END_DEPENDENCIES
  */
 
+/* BEGIN_CASE */
+void mpi_null( )
+{
+    mpi X, Y, Z;
+
+    mpi_init( &X );
+    mpi_init( &Y );
+    mpi_init( &Z );
+
+    TEST_ASSERT( mpi_get_bit( &X, 42 ) == 0 );
+    TEST_ASSERT( mpi_lsb( &X ) == 0 );
+    TEST_ASSERT( mpi_msb( &X ) == 0 );
+    TEST_ASSERT( mpi_size( &X ) == 0 );
+
+exit:
+    mpi_free( &X );
+}
+/* END_CASE */
+
 /* BEGIN_CASE */
 void mpi_read_write_string( int radix_X, char *input_X, int radix_A,
                             char *input_A, int output_size, int result_read,