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Fix potential buffer overflow in mpi_read_string()

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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.
This commit is contained in:
Manuel Pégourié-Gonnard 2015-09-28 13:48:04 +02:00
parent f592e8eaf6
commit 58fb49531d
2 changed files with 16 additions and 2 deletions
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9
ChangeLog
View file

@ -1,5 +1,14 @@
mbed TLS ChangeLog (Sorted per branch, date) 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 = mbed TLS 2.1.1 released 2015-09-17
Security Security

9
library/bignum.c
View file

@ -38,6 +38,7 @@
#include "mbedtls/bn_mul.h" #include "mbedtls/bn_mul.h"
#include <string.h> #include <string.h>
#include <limits.h>
#if defined(MBEDTLS_PLATFORM_C) #if defined(MBEDTLS_PLATFORM_C)
#include "mbedtls/platform.h" #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 * 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 BITS_TO_LIMBS(i) ( (i) / biL + ( (i) % biL != 0 ) )
#define CHARS_TO_LIMBS(i) (((i) + ciL - 1) / ciL) #define CHARS_TO_LIMBS(i) ( (i) / ciL + ( (i) % ciL != 0 ) )
/* /*
* Initialize one MPI * Initialize one MPI
@ -409,6 +411,9 @@ int mbedtls_mpi_read_string( mbedtls_mpi *X, int radix, const char *s )
if( radix == 16 ) if( radix == 16 )
{ {
if( slen > SIZE_T_MAX >> 2 )
return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
n = BITS_TO_LIMBS( slen << 2 ); n = BITS_TO_LIMBS( slen << 2 );
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, n ) ); MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, n ) );