Call setbuf when reading or writing files: library

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(),
except:
* In ctr_drbg.c, in load_file(), because this is only used for DH parameters
  and they are not confidential data.
* In psa_its_file.c, in psa_its_remove(), because the file is only opened
  to check its existence, we don't read data from it.

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

library/ctr_drbg.c

@@ -607,6 +607,9 @@ int mbedtls_ctr_drbg_write_seed_file( mbedtls_ctr_drbg_context *ctx,
     if( ( f = fopen( path, "wb" ) ) == NULL )
         return( MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR );
 
+    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
+    mbedtls_setbuf( f, NULL );
+
     if( ( ret = mbedtls_ctr_drbg_random( ctx, buf,
                                          MBEDTLS_CTR_DRBG_MAX_INPUT ) ) != 0 )
         goto exit;
@@ -640,6 +643,9 @@ int mbedtls_ctr_drbg_update_seed_file( mbedtls_ctr_drbg_context *ctx,
     if( ( f = fopen( path, "rb" ) ) == NULL )
         return( MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR );
 
+    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
+    mbedtls_setbuf( f, NULL );
+
     n = fread( buf, 1, sizeof( buf ), f );
     if( fread( &c, 1, 1, f ) != 0 )
     {

library/dhm.c

@@ -620,6 +620,7 @@ static int load_file( const char *path, unsigned char **buf, size_t *n )
 
     if( ( f = fopen( path, "rb" ) ) == NULL )
         return( MBEDTLS_ERR_DHM_FILE_IO_ERROR );
+    /* The data loaded here is public, so don't bother disabling buffering. */
 
     fseek( f, 0, SEEK_END );
     if( ( size = ftell( f ) ) == -1 )

library/entropy.c

@@ -457,6 +457,9 @@ int mbedtls_entropy_write_seed_file( mbedtls_entropy_context *ctx, const char *p
         goto exit;
     }
 
+    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
+    mbedtls_setbuf( f, NULL );
+
     if( fwrite( buf, 1, MBEDTLS_ENTROPY_BLOCK_SIZE, f ) != MBEDTLS_ENTROPY_BLOCK_SIZE )
     {
         ret = MBEDTLS_ERR_ENTROPY_FILE_IO_ERROR;
@@ -484,6 +487,9 @@ int mbedtls_entropy_update_seed_file( mbedtls_entropy_context *ctx, const char *
     if( ( f = fopen( path, "rb" ) ) == NULL )
         return( MBEDTLS_ERR_ENTROPY_FILE_IO_ERROR );
 
+    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
+    mbedtls_setbuf( f, NULL );
+
     fseek( f, 0, SEEK_END );
     n = (size_t) ftell( f );
     fseek( f, 0, SEEK_SET );

library/entropy_poll.c

@@ -35,7 +35,7 @@
 #if defined(MBEDTLS_TIMING_C)
 #include "mbedtls/timing.h"
 #endif
-#if defined(MBEDTLS_ENTROPY_NV_SEED)
+#if defined(MBEDTLS_ENTROPY_NV_SEED) || !defined(HAVE_SYSCTL_ARND)
 #include "mbedtls/platform.h"
 #endif
 
@@ -195,6 +195,9 @@ int mbedtls_platform_entropy_poll( void *data,
     if( file == NULL )
         return( MBEDTLS_ERR_ENTROPY_SOURCE_FAILED );
 
+    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
+    mbedtls_setbuf( file, NULL );
+
     read_len = fread( output, 1, len, file );
     if( read_len != len )
     {

library/hmac_drbg.c

@@ -436,6 +436,9 @@ int mbedtls_hmac_drbg_write_seed_file( mbedtls_hmac_drbg_context *ctx, const cha
     if( ( f = fopen( path, "wb" ) ) == NULL )
         return( MBEDTLS_ERR_HMAC_DRBG_FILE_IO_ERROR );
 
+    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
+    mbedtls_setbuf( f, NULL );
+
     if( ( ret = mbedtls_hmac_drbg_random( ctx, buf, sizeof( buf ) ) ) != 0 )
         goto exit;
 
@@ -465,6 +468,9 @@ int mbedtls_hmac_drbg_update_seed_file( mbedtls_hmac_drbg_context *ctx, const ch
     if( ( f = fopen( path, "rb" ) ) == NULL )
         return( MBEDTLS_ERR_HMAC_DRBG_FILE_IO_ERROR );
 
+    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
+    mbedtls_setbuf( f, NULL );
+
     n = fread( buf, 1, sizeof( buf ), f );
     if( fread( &c, 1, 1, f ) != 0 )
     {

library/md.c

@@ -605,6 +605,9 @@ int mbedtls_md_file( const mbedtls_md_info_t *md_info, const char *path, unsigne
     if( ( f = fopen( path, "rb" ) ) == NULL )
         return( MBEDTLS_ERR_MD_FILE_IO_ERROR );
 
+    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
+    mbedtls_setbuf( f, NULL );
+
     mbedtls_md_init( &ctx );
 
     if( ( ret = mbedtls_md_setup( &ctx, md_info, 0 ) ) != 0 )

library/pkparse.c

@@ -82,6 +82,9 @@ int mbedtls_pk_load_file( const char *path, unsigned char **buf, size_t *n )
     if( ( f = fopen( path, "rb" ) ) == NULL )
         return( MBEDTLS_ERR_PK_FILE_IO_ERROR );
 
+    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
+    mbedtls_setbuf( f, NULL );
+
     fseek( f, 0, SEEK_END );
     if( ( size = ftell( f ) ) == -1 )
     {

library/psa_its_file.c

@@ -102,6 +102,9 @@ static psa_status_t psa_its_read_file( psa_storage_uid_t uid,
     if( *p_stream == NULL )
         return( PSA_ERROR_DOES_NOT_EXIST );
 
+    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
+    mbedtls_setbuf( *p_stream, NULL );
+
     n = fread( &header, 1, sizeof( header ), *p_stream );
     if( n != sizeof( header ) )
         return( PSA_ERROR_DATA_CORRUPT );
@@ -201,9 +204,13 @@ psa_status_t psa_its_set( psa_storage_uid_t uid,
 
     psa_its_fill_filename( uid, filename );
     stream = fopen( PSA_ITS_STORAGE_TEMP, "wb" );
+
     if( stream == NULL )
         goto exit;
 
+    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
+    mbedtls_setbuf( stream, NULL );
+
     status = PSA_ERROR_INSUFFICIENT_STORAGE;
     n = fwrite( &header, 1, sizeof( header ), stream );
     if( n != sizeof( header ) )