Merge pull request #3935 from paul-elliott-arm/fix_pem_write_2_16

Backport 2.16: Remove Extraneous bytes from buffer post pem write

ChangeLog.d/clean_pem_buffers.txt

@@ -0,0 +1,6 @@
+Bugfix
+  * In PEM writing functions, fill the trailing part of the buffer with null
+    bytes. This guarantees that the corresponding parsing function can read
+    the buffer back, which was the case for mbedtls_x509write_{crt,csr}_pem
+    until this property was inadvertently broken in Mbed TLS 2.19.0.
+    Fixes #3682.

library/pem.c

@@ -508,8 +508,12 @@ int mbedtls_pem_write_buffer( const char *header, const char *footer,
     *p++ = '\0';
     *olen = p - buf;
 
+     /* Clean any remaining data previously written to the buffer */
+    memset( buf + *olen, 0, buf_len - *olen );
+
     mbedtls_free( encode_buf );
     return( 0 );
 }
 #endif /* MBEDTLS_PEM_WRITE_C */
 #endif /* MBEDTLS_PEM_PARSE_C || MBEDTLS_PEM_WRITE_C */
+

tests/suites/test_suite_pkwrite.function

@@ -17,7 +17,7 @@ void pk_write_pubkey_check( char * key_file )
     unsigned char check_buf[5000];
     int ret;
     FILE *f;
-    size_t ilen;
+    size_t ilen, pem_len, buf_index;
 
     memset( buf, 0, sizeof( buf ) );
     memset( check_buf, 0, sizeof( check_buf ) );
@@ -28,12 +28,20 @@ void pk_write_pubkey_check( char * key_file )
     ret = mbedtls_pk_write_pubkey_pem( &key, buf, sizeof( buf ));
     TEST_ASSERT( ret == 0 );
 
+    pem_len = strlen( (char *) buf );
+
+    // check that the rest of the buffer remains clear
+    for( buf_index = pem_len; buf_index < sizeof( buf ); ++buf_index )
+    {
+        TEST_ASSERT( buf[buf_index] == 0 );
+    }
+
     f = fopen( key_file, "r" );
     TEST_ASSERT( f != NULL );
     ilen = fread( check_buf, 1, sizeof( check_buf ), f );
     fclose( f );
 
-    TEST_ASSERT( ilen == strlen( (char *) buf ) );
+    TEST_ASSERT( ilen == pem_len );
     TEST_ASSERT( memcmp( (char *) buf, (char *) check_buf, ilen ) == 0 );
 
 exit:
@@ -49,7 +57,7 @@ void pk_write_key_check( char * key_file )
     unsigned char check_buf[5000];
     int ret;
     FILE *f;
-    size_t ilen;
+    size_t ilen, pem_len, buf_index;
 
     memset( buf, 0, sizeof( buf ) );
     memset( check_buf, 0, sizeof( check_buf ) );
@@ -60,6 +68,14 @@ void pk_write_key_check( char * key_file )
     ret = mbedtls_pk_write_key_pem( &key, buf, sizeof( buf ));
     TEST_ASSERT( ret == 0 );
 
+    pem_len = strlen( (char *) buf );
+
+    // check that the rest of the buffer remains clear
+    for( buf_index = pem_len; buf_index < sizeof( buf ); ++buf_index )
+    {
+        TEST_ASSERT( buf[buf_index] == 0 );
+    }
+
     f = fopen( key_file, "r" );
     TEST_ASSERT( f != NULL );
     ilen = fread( check_buf, 1, sizeof( check_buf ), f );

tests/suites/test_suite_x509write.function

@@ -45,7 +45,7 @@ void x509_csr_check( char * key_file, char * cert_req_check_file, int md_type,
     unsigned char buf[4096];
     unsigned char check_buf[4000];
     int ret;
-    size_t olen = 0, pem_len = 0;
+    size_t olen = 0, pem_len = 0, buf_index;
     int der_len = -1;
     FILE *f;
     const char *subject_name = "C=NL,O=PolarSSL,CN=PolarSSL Server 1";
@@ -71,6 +71,11 @@ void x509_csr_check( char * key_file, char * cert_req_check_file, int md_type,
 
     pem_len = strlen( (char *) buf );
 
+    for( buf_index = pem_len; buf_index < sizeof( buf ); ++buf_index )
+    {
+        TEST_ASSERT( buf[buf_index] == 0 );
+    }
+
     f = fopen( cert_req_check_file, "r" );
     TEST_ASSERT( f != NULL );
     olen = fread( check_buf, 1, sizeof( check_buf ), f );
@@ -113,7 +118,7 @@ void x509_crt_check( char *subject_key_file, char *subject_pwd,
     unsigned char check_buf[5000];
     mbedtls_mpi serial;
     int ret;
-    size_t olen = 0, pem_len = 0;
+    size_t olen = 0, pem_len = 0, buf_index = 0;
     int der_len = -1;
     FILE *f;
     rnd_pseudo_info rnd_info;
@@ -182,6 +187,12 @@ void x509_crt_check( char *subject_key_file, char *subject_pwd,
 
     pem_len = strlen( (char *) buf );
 
+    // check that the rest of the buffer remains clear
+    for( buf_index = pem_len; buf_index < sizeof( buf ); ++buf_index )
+    {
+        TEST_ASSERT( buf[buf_index] == 0 );
+    }
+
     f = fopen( cert_check_file, "r" );
     TEST_ASSERT( f != NULL );
     olen = fread( check_buf, 1, sizeof( check_buf ), f );