From 58fb49531da29b5a744a0999c4b14eb6d00e0fc8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Manuel=20P=C3=A9gouri=C3=A9-Gonnard?= <mpg2@elzevir.fr>
Date: Mon, 28 Sep 2015 13:48:04 +0200
Subject: [PATCH] Fix potential buffer overflow in mpi_read_string()

Found by Guido Vranken.

Two possible integer overflows (during << 2 or addition in BITS_TO_LIMB())
could result in far too few memory to be allocated, then overflowing the
buffer in the subsequent for loop.

Both integer overflows happen when slen is close to or greater than
SIZE_T_MAX >> 2 (ie 2^30 on a 32 bit system).

Note: one could also avoid those overflows by changing BITS_TO_LIMB(s << 2) to
CHARS_TO_LIMB(s >> 1) but the solution implemented looks more robust with
respect to future code changes.
---
 ChangeLog        | 9 +++++++++
 library/bignum.c | 9 +++++++--
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index c9e48e244..756080325 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,14 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS 2.1.2 released 2015-??-??
+
+Security
+   * Fix potential buffer overflow in mbedtls_mpi_read_string().
+     Found by Guido Vranken. Not exploitable remotely in the context of TLS,
+     but might be in other uses. On 32 bit machines, requires reading a string
+     of close to or larger than 1GB to exploit; on 64 bit machines, would require
+     reading a string of close to or larger than 2^62 bytes.
+
 = mbed TLS 2.1.1 released 2015-09-17
 
 Security
diff --git a/library/bignum.c b/library/bignum.c
index 15cbf73fb..280bbfd85 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -38,6 +38,7 @@
 #include "mbedtls/bn_mul.h"
 
 #include <string.h>
+#include <limits.h>
 
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
@@ -60,9 +61,10 @@ static void mbedtls_zeroize( void *v, size_t n ) {
 
 /*
  * Convert between bits/chars and number of limbs
+ * Divide first in order to avoid potential overflows
  */
-#define BITS_TO_LIMBS(i)  (((i) + biL - 1) / biL)
-#define CHARS_TO_LIMBS(i) (((i) + ciL - 1) / ciL)
+#define BITS_TO_LIMBS(i)  ( (i) / biL + ( (i) % biL != 0 ) )
+#define CHARS_TO_LIMBS(i) ( (i) / ciL + ( (i) % ciL != 0 ) )
 
 /*
  * Initialize one MPI
@@ -409,6 +411,9 @@ int mbedtls_mpi_read_string( mbedtls_mpi *X, int radix, const char *s )
 
     if( radix == 16 )
     {
+        if( slen > SIZE_T_MAX >> 2 )
+            return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
+
         n = BITS_TO_LIMBS( slen << 2 );
 
         MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, n ) );