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.
diff --git a/ChangeLog b/ChangeLog
index e22fdee..0407b6a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -56,6 +56,9 @@
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
diff --git a/library/bignum.c b/library/bignum.c
index 4c99e04..8b9082c 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -534,7 +534,12 @@
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 )
{
diff --git a/tests/suites/test_suite_mpi.data b/tests/suites/test_suite_mpi.data
index fa65b5f..da830fe 100644
--- a/tests/suites/test_suite_mpi.data
+++ b/tests/suites/test_suite_mpi.data
@@ -46,6 +46,9 @@
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"