Fix buffer overflow in 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 03bad2c..8b9e4a6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -49,6 +49,9 @@
* Fixed the templates used to generate project and solution files for Visual
Studio 2015 as well as the files themselves, to remove a build warning
generated in Visual Studio 2015. Reported by Steve Valliere. #742
+ * Fix 1 byte buffer overflow in 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 1.3.18 branch 2016-10-17
diff --git a/library/bignum.c b/library/bignum.c
index 4fe841c..afde19b 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -538,7 +538,12 @@
n = mpi_msb( 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( *slen < n )
{
diff --git a/tests/suites/test_suite_mpi.data b/tests/suites/test_suite_mpi.data
index a8da378..81fc73f 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:POLARSSL_ERR_MPI_BUFFER_TOO_SMALL
+
Base test mpi_read_binary #1
mpi_read_binary:"0941379d00fed1491fe15df284dfde4a142f68aa8d412023195cee66883e6290ffe703f4ea5963bf212713cee46b107c09182b5edcd955adac418bf4918e2889af48e1099d513830cec85c26ac1e158b52620e33ba8692f893efbb2f958b4424":10:"56125680981752282334141896320372489490613963693556392520816017892111350604111697682705498319512049040516698827829292076808006940873974979584527073481012636016353913462376755556720019831187364993587901952757307830896531678727717924"