Add setbuf platform function

Add a platform function mbedtls_setbuf(), defaulting to setbuf().

The intent is to allow disabling stdio buffering when reading or writing
files with sensitive data, because this exposes the sensitive data to a
subsequent memory disclosure vulnerability.

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

include/mbedtls/check_config.h

@@ -368,6 +368,20 @@
 #error "MBEDTLS_PLATFORM_EXIT_MACRO and MBEDTLS_PLATFORM_STD_EXIT/MBEDTLS_PLATFORM_EXIT_ALT cannot be defined simultaneously"
 #endif
 
+#if defined(MBEDTLS_PLATFORM_SETBUF_ALT) && !defined(MBEDTLS_PLATFORM_C)
+#error "MBEDTLS_PLATFORM_SETBUF_ALT defined, but not all prerequisites"
+#endif
+
+#if defined(MBEDTLS_PLATFORM_SETBUF_MACRO) && !defined(MBEDTLS_PLATFORM_C)
+#error "MBEDTLS_PLATFORM_SETBUF_MACRO defined, but not all prerequisites"
+#endif
+
+#if defined(MBEDTLS_PLATFORM_SETBUF_MACRO) &&\
+    ( defined(MBEDTLS_PLATFORM_STD_SETBUF) ||\
+        defined(MBEDTLS_PLATFORM_SETBUF_ALT) )
+#error "MBEDTLS_PLATFORM_SETBUF_MACRO and MBEDTLS_PLATFORM_STD_SETBUF/MBEDTLS_PLATFORM_SETBUF_ALT cannot be defined simultaneously"
+#endif
+
 #if defined(MBEDTLS_PLATFORM_TIME_ALT) &&\
     ( !defined(MBEDTLS_PLATFORM_C) ||\
         !defined(MBEDTLS_HAVE_TIME) )

include/mbedtls/mbedtls_config.h

@@ -225,6 +225,7 @@
  * Uncomment a macro to enable alternate implementation of specific base
  * platform function
  */
+//#define MBEDTLS_PLATFORM_SETBUF_ALT
 //#define MBEDTLS_PLATFORM_EXIT_ALT
 //#define MBEDTLS_PLATFORM_TIME_ALT
 //#define MBEDTLS_PLATFORM_FPRINTF_ALT
@@ -3288,6 +3289,7 @@
 //#define MBEDTLS_PLATFORM_STD_MEM_HDR   <stdlib.h> /**< Header to include if MBEDTLS_PLATFORM_NO_STD_FUNCTIONS is defined. Don't define if no header is needed. */
 //#define MBEDTLS_PLATFORM_STD_CALLOC        calloc /**< Default allocator to use, can be undefined */
 //#define MBEDTLS_PLATFORM_STD_FREE            free /**< Default free to use, can be undefined */
+//#define MBEDTLS_PLATFORM_STD_SETBUF      setbuf /**< Default setbuf to use, can be undefined */
 //#define MBEDTLS_PLATFORM_STD_EXIT            exit /**< Default exit to use, can be undefined */
 //#define MBEDTLS_PLATFORM_STD_TIME            time /**< Default time to use, can be undefined. MBEDTLS_HAVE_TIME must be enabled */
 //#define MBEDTLS_PLATFORM_STD_FPRINTF      fprintf /**< Default fprintf to use, can be undefined */
@@ -3305,6 +3307,7 @@
 //#define MBEDTLS_PLATFORM_CALLOC_MACRO        calloc /**< Default allocator macro to use, can be undefined */
 //#define MBEDTLS_PLATFORM_FREE_MACRO            free /**< Default free macro to use, can be undefined */
 //#define MBEDTLS_PLATFORM_EXIT_MACRO            exit /**< Default exit macro to use, can be undefined */
+//#define MBEDTLS_PLATFORM_SETBUF_MACRO      setbuf /**< Default setbuf macro to use, can be undefined */
 //#define MBEDTLS_PLATFORM_TIME_MACRO            time /**< Default time macro to use, can be undefined. MBEDTLS_HAVE_TIME must be enabled */
 //#define MBEDTLS_PLATFORM_TIME_TYPE_MACRO       time_t /**< Default time macro to use, can be undefined. MBEDTLS_HAVE_TIME must be enabled */
 //#define MBEDTLS_PLATFORM_FPRINTF_MACRO      fprintf /**< Default fprintf macro to use, can be undefined */

include/mbedtls/platform.h

@@ -91,6 +91,9 @@ extern "C" {
 #if !defined(MBEDTLS_PLATFORM_STD_FREE)
 #define MBEDTLS_PLATFORM_STD_FREE       free /**< The default \c free function to use. */
 #endif
+#if !defined(MBEDTLS_PLATFORM_STD_SETBUF)
+#define MBEDTLS_PLATFORM_STD_SETBUF   setbuf /**< The default \c setbuf function to use. */
+#endif
 #if !defined(MBEDTLS_PLATFORM_STD_EXIT)
 #define MBEDTLS_PLATFORM_STD_EXIT      exit /**< The default \c exit function to use. */
 #endif
@@ -276,6 +279,58 @@ int mbedtls_platform_set_vsnprintf( int (*vsnprintf_func)( char * s, size_t n,
 #endif /* MBEDTLS_PLATFORM_VSNPRINTF_MACRO */
 #endif /* MBEDTLS_PLATFORM_VSNPRINTF_ALT */
 
+/*
+ * The function pointers for setbuf
+ */
+#if defined(MBEDTLS_PLATFORM_SETBUF_ALT)
+#include <stdio.h>
+/**
+ * \brief                  Function pointer to call for `setbuf()` functionality
+ *                         (changing the internal buffering on stdio calls).
+ *
+ * \note                   The library calls this function to disable
+ *                         buffering when reading or writing sensitive data,
+ *                         to avoid having extra copies of sensitive data
+ *                         remaining in stdio buffers after the file is
+ *                         closed. If this is not a concern, for example if
+ *                         your platform's stdio doesn't have any buffering,
+ *                         you can set mbedtls_setbuf to a function that
+ *                         does nothing.
+ *
+ * \param   setbuf_func    The \c setbuf function implementation.
+ *                         It is always called with `buf` equal to `NULL`.
+ *
+ * \return                 \c 0 on success, negative on error.
+ */
+extern void (*mbedtls_setbuf)( FILE *stream, char *buf );
+
+/**
+ * \brief                  Dynamically configure the function that is called
+ *                         when the mbedtls_setbuf() function is called by the
+ *                         library.
+ *
+ * \param   setbuf_func   The \c setbuf function implementation
+ *
+ * \return                 \c 0
+ */
+int mbedtls_platform_set_setbuf( void (*setbuf_func)(
+                                     FILE *stream, char *buf ) );
+#elif defined(MBEDTLS_PLATFORM_SETBUF_MACRO)
+/**
+ * \brief                  Macro defining the function for the library to
+ *                         call for `setbuf` functionality (changing the
+ *                         internal buffering on stdio calls).
+ *
+ * \note                   See extra comments on the mbedtls_setbuf() function
+ *                         pointer above.
+ *
+ * \return                 \c 0 on success, negative on error.
+ */
+#define mbedtls_setbuf    MBEDTLS_PLATFORM_SETBUF_MACRO
+#else
+#define mbedtls_setbuf    setbuf
+#endif /* MBEDTLS_PLATFORM_SETBUF_ALT / MBEDTLS_PLATFORM_SETBUF_MACRO */
+
 /*
  * The function pointers for exit
  */

library/platform.c

@@ -226,6 +226,28 @@ int mbedtls_platform_set_fprintf( int (*fprintf_func)( FILE *, const char *, ...
 }
 #endif /* MBEDTLS_PLATFORM_FPRINTF_ALT */
 
+#if defined(MBEDTLS_PLATFORM_SETBUF_ALT)
+#if !defined(MBEDTLS_PLATFORM_STD_SETBUF)
+/*
+ * Make dummy function to prevent NULL pointer dereferences
+ */
+static void platform_setbuf_uninit( FILE *stream, char *buf )
+{
+   (( void ) stream);
+   (( void ) buf);
+}
+
+#define MBEDTLS_PLATFORM_STD_SETBUF   platform_setbuf_uninit
+#endif /* !MBEDTLS_PLATFORM_STD_SETBUF */
+void (*mbedtls_setbuf)( FILE *stream, char *buf ) = MBEDTLS_PLATFORM_STD_SETBUF;
+
+int mbedtls_platform_set_setbuf( void (*setbuf_func)( FILE *stream, char *buf ) )
+{
+   mbedtls_setbuf = setbuf_func;
+   return( 0 );
+}
+#endif /* MBEDTLS_PLATFORM_SETBUF_ALT */
+
 #if defined(MBEDTLS_PLATFORM_EXIT_ALT)
 #if !defined(MBEDTLS_PLATFORM_STD_EXIT)
 /*
@@ -288,6 +310,9 @@ int mbedtls_platform_std_nv_seed_read( unsigned char *buf, size_t buf_len )
     if( ( file = fopen( MBEDTLS_PLATFORM_STD_NV_SEED_FILE, "rb" ) ) == NULL )
         return( -1 );
 
+    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
+    mbedtls_setbuf( file, NULL );
+
     if( ( n = fread( buf, 1, buf_len, file ) ) != buf_len )
     {
         fclose( file );
@@ -307,6 +332,9 @@ int mbedtls_platform_std_nv_seed_write( unsigned char *buf, size_t buf_len )
     if( ( file = fopen( MBEDTLS_PLATFORM_STD_NV_SEED_FILE, "w" ) ) == NULL )
         return -1;
 
+    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
+    mbedtls_setbuf( file, NULL );
+
     if( ( n = fwrite( buf, 1, buf_len, file ) ) != buf_len )
     {
         fclose( file );

tests/scripts/all.sh

@@ -2178,6 +2178,7 @@ component_test_no_platform () {
     scripts/config.py unset MBEDTLS_PLATFORM_SNPRINTF_ALT
     scripts/config.py unset MBEDTLS_PLATFORM_TIME_ALT
     scripts/config.py unset MBEDTLS_PLATFORM_EXIT_ALT
+    scripts/config.py unset MBEDTLS_PLATFORM_SETBUF_ALT
     scripts/config.py unset MBEDTLS_PLATFORM_NV_SEED_ALT
     scripts/config.py unset MBEDTLS_ENTROPY_NV_SEED
     scripts/config.py unset MBEDTLS_FS_IO