Fix buffer overflow in mbedtls_mpi_write_string()

Fix a buffer overflow when writting a string representation of an MPI
number to a buffer in hexadecimal. The problem occurs because hex
digits are written in pairs and this is not accounted for in the
calculation of the required buffer size when the number of digits is
odd.

Author: Andres AG

ChangeLog

@@ -61,6 +61,9 @@ Bugfix
      generated in Visual Studio 2015. Reported by Steve Valliere. #742
    * Fix a resource leak in ssl_cookie, when using MBEDTLS_THREADING_C.
      Raised and fix suggested by Alan Gillingham in the mbed TLS forum. #771
+   * Fix 1 byte buffer overflow in mbedtls_mpi_write_string() when the MPI
+     number to write in hexadecimal is negative and requires an odd number of
+     digits. Found and fixed by Guido Vranken.
 
 = mbed TLS 2.4.1 branch released 2016-12-13
 

library/bignum.c

@@ -534,7 +534,12 @@ int mbedtls_mpi_write_string( const mbedtls_mpi *X, int radix,
     n = mbedtls_mpi_bitlen( X );
     if( radix >=  4 ) n >>= 1;
     if( radix >= 16 ) n >>= 1;
-    n += 3;
+    /*
+     * Round up the buffer length to an even value to ensure that there is
+     * enough room for hexadecimal values that can be represented in an odd
+     * number of digits.
+     */
+    n += 3 + ( ( n + 1 ) & 1 );
 
     if( buflen < n )
     {

tests/suites/test_suite_mpi.data

@@ -46,6 +46,9 @@ mpi_read_write_string:16:"":16:"00":4:0:0
 Test mpi_read_write_string #9 (Empty MPI -> dec)
 mpi_read_write_string:16:"":10:"0":4:0:0
 
+Test mpi_write_string #10 (Negative hex with odd number of digits)
+mpi_read_write_string:16:"-1":16:"":3:0:MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL
+
 Base test mbedtls_mpi_read_binary #1
 mbedtls_mpi_read_binary:"0941379d00fed1491fe15df284dfde4a142f68aa8d412023195cee66883e6290ffe703f4ea5963bf212713cee46b107c09182b5edcd955adac418bf4918e2889af48e1099d513830cec85c26ac1e158b52620e33ba8692f893efbb2f958b4424":10:"56125680981752282334141896320372489490613963693556392520816017892111350604111697682705498319512049040516698827829292076808006940873974979584527073481012636016353913462376755556720019831187364993587901952757307830896531678727717924"