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Add setbuf platform function

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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>
This commit is contained in:
Gilles Peskine 2022-06-30 17:01:40 +02:00
parent a745c7d439
commit 6497b5a1d1
5 changed files with 101 additions and 0 deletions
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14
include/mbedtls/check_config.h
View file

@ -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) )

3
include/mbedtls/mbedtls_config.h
View file

@ -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 */

55
include/mbedtls/platform.h
View file

@ -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
*/

28
library/platform.c
View file

@ -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 );

1
tests/scripts/all.sh
View file

@ -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