RUS-CERT Advisory 2002-08:01: Incorrect integer overflow detection in C code

Incorrect integer overflow detection in C code

A widely used method of detecting integer overflows results in
undefined behavior according to the C standard.

Who Should Read This Document

This advisory deals with with details of the C programming
language. It is targeted at C programmers.

Systems Affected

 * Systems which run binaries compiled by C compilers which
   perform particular optimizations.

Currently, no such systems are known to RUS-CERT.

Attack Requirements And Impact

Attack requirements and impact depend on the application.

Description

According to the C standard (ISO/IEC 9899:1999, section 6.5,
paragraph 5), overflow in integer expressions results in
undefined behavior:

 If an exceptional condition occurs during the evaluation of
 an expression (that is, if the result is not mathematically
 defined or not in the range of representable values for its
 type), the behavior is undefined.

For unsigned integer types, the C standard later on defines
that arithmetic can never overflow (ibid, parapgraph 9):

 [...] A computation involving unsigned operands can never
 overflow, because a result that cannot be represented by
 the resulting unsigned integer type is reduced modulo the
 number that is one greater than the largest value that can
 be represented by the resulting type.

However, for signed integer types, there is no such guarantee.
As a result, it is not possible to check for signed integer
arithmetic overflow after the overflow occured. This is
problematic especially if such checks are introduced in order
to prevent buffer overflows (see the example below).

RUS-CERT is not aware of a concrete example of a code/compiler
pair which shows the problem. However, compilers will perform
more and more optimizations (such as range tracking for
variables), and this problem might become practical in the
future.

Examples

The following source code excerpt comes from the OpenSSL
library, after the fix for CAN-2002-0659 has been applied
(comments in italics have been added):

static int asn1_get_length(unsigned char **pp, int *inf, long *rl, int max)
{
unsigned char *p= *pp;
long ret=0;
int i;

       if (max-- < 1) return(0);
       if (*p == 0x80)
               {
               *inf=1;
               ret=0;
               p++;
               }
       else
               {
               *inf=0;
               i= *p&0x7f;
               if (*(p++) & 0x80)
                       {
                       if (i > sizeof(long))
                               return 0;
                       if (max-- == 0) return(0);

/* (1) / while (i-- > 0)
{
/ (2) */ ret<<=8L;
ret|= (p++);
if (max-- == 0) return(0);
}
}
else
ret=i;
}
/ (3) */ if (ret < 0)
return 0;
*pp=p;
*rl=ret;
return(1);
}

This routine is used to extract an encoded long value of
varying length, stored with a length prefix and the actual
data in big endian format. Suppose that the encoded integer is
stored using the maximum number of bytes. During the last
iteration of the while loop (1), assignment expression (2)
copies the first byte to the most significant byte in ret. If
the value of this byte exceeds 127, the range of values
representable in type long is exceeded, at least on typical C
implementations, leading to integer overflow and undefined
behavior.

If such an overflow does not occur, the check (3) does not
succeed, and the return statement ist not executed. However,
after undefined behavior has occured, all behavior is allowed
by the C standard, so a conforming implementation may choose
not to execute the return in this case, either. As a result,
the check (3) can be ommitted by a conforming C
implementation. However, this check has been added to
eliminate a security risk – therefore this is obviously
dangerous.

How likely is it that actual compilers will perform such an
optimization in the future (or already do)? Range tracking on
the ret variable could inform compilers that only positive
values are assigned to it. A range tracking feature does not
seem too esoteric, so compiler vendors will probably implement
it one day.

As suggested below, the correct approach uses an unsigned long
(unsigned arithmetic can never overflow, but in this case, the
wraparound is avoided, at least if objects of type unsigned
long contain no padding bits). In addition, the check (3) is
replaced with one against LONG_MAX. The assignment (4') does
not trigger undefined behavior because the check (3') ensures
that the value is representable in type long.

#include <limits.h>

static int asn1_get_length(unsigned char **pp, int *inf, long *rl, int max)
{
unsigned char *p= *pp;
unsigned long ret=0;
int i;

       if (max-- < 1) return(0);
       if (*p == 0x80)
               {
               *inf=1;
               ret=0;
               p++;
               }
       else
               {
               *inf=0;
               i= *p&0x7f;
               if (*(p++) & 0x80)
                       {
                       if (i > sizeof(long))
                               return 0;
                       if (max-- == 0) return(0);

/* (1') / while (i-- > 0)
{
/ (2') */ ret<<=8L;
ret|= (p++);
if (max-- == 0) return(0);
}
}
else
ret=i;
}
/ (3') */ if (ret > LONG_MAX)
return 0;
pp=p;
/ (4') */ *rl=(long)ret;
return(1);
}

The ap_get_chunk_size routine featured a similar overflow bug
(CAN-2002-0392), and again, the developers tried to check
for the overflow after it had occured. However, in this case,
it seems to be less likely that future compiler will optimize
away the crucial check.

About RUS-CERT

RUS-CERT (http://CERT.Uni-Stuttgart.DE/&#41; is the Computer Emergency
Response Team located at the Computing Center (RUS) of the
University of Stuttgart, Germany.

Revision History

This document was published on 2002-08-05.

URL For This Document

http://CERT.Uni-Stuttgart.DE/advisories/c-integer-overflow.php

–
Florian Weimer [email protected]
University of Stuttgart http://CERT.Uni-Stuttgart.DE/people/fw/
RUS-CERT fax +49-711-685-5898