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AUSCERT External Security Bulletin Redistribution
ESB-2008.0366 -- [Win][UNIX/Linux]
Vulnerability Note VU#162289 gcc silently discards some wraparound checks
9 April 2008
AusCERT Security Bulletin Summary
Operating System: Windows
UNIX variants (UNIX, Linux, OSX)
Impact: Reduced Security
CVE Names: CVE-2008-1685
Original Bulletin: http://www.kb.cert.org/vuls/id/162289
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Vulnerability Note VU#162289
gcc silently discards some wraparound checks
Some versions of gcc may silently discard certain checks for overflow.
Applications compiled with these versions of gcc may be vulnerable to
The GNU Compiler Collection provides a compiler suite for a number of
languages, the most prominent being C (gcc). In gcc versions 4.2 and
later, given the types:
gcc will assume that buf+len >= buf.
As a result, code that performs length checks similar to the following:
len = 1<<30;
if(buf+len < buf) /* length check */
are compiled away by these versions of gcc; no object code to perform
the check will appear in the resulting executable program. In the case
where the length test expression is compiled away, a subsequent
manipulation of len could cause an overflow. As a result, applications
that perform such checks as a postcondition may be vulnerable to buffer
Note: this issue does not strictly constitute a vulnerability in gcc
itself. The behavior that gcc exhibits in this case is permitted
by the ISO/IEC 9899:1999 C specification (§6.5.6p8). Rather, this
behavior may introduce vulnerabilities in applications compiled
with the affected versions of gcc. It must be emphasized that
this is a change of behavior for the compiler. Existing code is
particularly susceptible to this change.
An application which performs bounds checks based on an expression such
as the one described above may be vulnerable to buffer overflow if
compiled with gcc versions 4.2 or later. The nature of the resulting
vulnerability would be specific to the application and depends on how
the affected code is used.
Cast objects of type char* to uintptr_t before comparison. The faulty
length check listed above would be written:
if((uintptr_t)buf+len < (uintptr_t)buf)
Alternatively, developers can use size_t on platforms that do not
provide the uintptr_t type. Developers should also follow the
recommendations described in CERT C Secure Coding rule ARR38-C.
Avoid newer versions of gcc
Application developers and vendors of large codebases that cannot be
audited for use of the defective length checks are urged to avoiding
the use of gcc versions 4.2 and later.
Use compiler flags
The gcc compiler provides several flags that may workaround this
condition. The -ftrapv flag will cause a program that uses the
defective test to crash if an overflow occurs. The -fwrapv flag makes
signed arithmetic overflow. It should be noted that both of these
options are somewhat rarely used and it's possible that there are
defects in their implementation. It should also be noted that the use
of -fwrapv may have a negative performance impact on the resulting
program but reduce the likelihood that the offending optimization
No Information Available
Thanks to Russ Cox for discovering this issue and providing additional
information and assistance.
This document was written by Chad R Dougherty.
Date Public 03/30/2008
Date First Published 04/04/2008 09:07:25 AM
Date Last Updated 04/07/2008
US-CERT Technical Alerts
Document Revision 24
If you have feedback, comments, or additional information about this
vulnerability, please send us email.
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