Call setbuf when reading or writing files: programs

After opening a file containing sensitive data, call mbedtls_setbuf() to
disable buffering. This way, we don't expose sensitive data to a memory
disclosure vulnerability in a buffer outside our control.

This commit adds a call to mbedtls_setbuf() after each call to fopen(),
but only in sample programs that were calling mbedtls_platform_zeroize().
Don't bother protecting stdio buffers in programs where application buffers
weren't protected.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

programs/ssl/ssl_test_common_source.c

@@ -101,6 +101,10 @@ void nss_keylog_export( void *p_expkey,
             goto exit;
         }
 
+        /* Ensure no stdio buffering of secrets, as such buffers cannot be
+         * wiped. */
+        mbedtls_setbuf( f, NULL );
+
         if( fwrite( nss_keylog_line, 1, len, f ) != len )
         {
             fclose( f );