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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 =========================================================================== 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 --------------------------------- Product: gcc Publisher: US-CERT 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 - --------------------------BEGIN INCLUDED TEXT-------------------- Vulnerability Note VU#162289 gcc silently discards some wraparound checks Overview Some versions of gcc may silently discard certain checks for overflow. Applications compiled with these versions of gcc may be vulnerable to buffer overflows. I. Description 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: char *buf; int len; 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 */ [...overflow occurred...] 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 overflows. 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 (ยง6.5.6p8). 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. II. Impact 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. III. Solution Use casts Cast objects of type char* to uintptr_t before comparison. The faulty length check listed above would be written: #include <stdint.h> [...] 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 will occur. Systems Affected No Information Available References https://www.securecoding.cert.org/confluence/x/SgHm Credit Thanks to Russ Cox for discovering this issue and providing additional information and assistance. This document was written by Chad R Dougherty. Other Information Date Public 03/30/2008 Date First Published 04/04/2008 09:07:25 AM Date Last Updated 04/07/2008 CERT Advisory CVE Name US-CERT Technical Alerts Metric 0.00 Document Revision 24 If you have feedback, comments, or additional information about this vulnerability, please send us email. - --------------------------END INCLUDED TEXT-------------------- You have received this e-mail bulletin as a result of your organisation's registration with AusCERT. The mailing list you are subscribed to is maintained within your organisation, so if you do not wish to continue receiving these bulletins you should contact your local IT manager. If you do not know who that is, please send an email to auscert@auscert.org.au and we will forward your request to the appropriate person. NOTE: Third Party Rights This security bulletin is provided as a service to AusCERT's members. 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Previous advisories and external security bulletins can be retrieved from: http://www.auscert.org.au/render.html?cid=1980 If you believe that your computer system has been compromised or attacked in any way, we encourage you to let us know by completing the secure National IT Incident Reporting Form at: http://www.auscert.org.au/render.html?it=3192 =========================================================================== Australian Computer Emergency Response Team The University of Queensland Brisbane Qld 4072 Internet Email: auscert@auscert.org.au Facsimile: (07) 3365 7031 Telephone: (07) 3365 4417 (International: +61 7 3365 4417) AusCERT personnel answer during Queensland business hours which are GMT+10:00 (AEST). On call after hours for member emergencies only. =========================================================================== -----BEGIN PGP SIGNATURE----- Comment: http://www.auscert.org.au/render.html?it=1967 iQCVAwUBR/xffSh9+71yA2DNAQIUiQQAjyjEWMS3dJjvXYRtbA/5toUXv5ni9UQh 2OvQshzqFJbDX6t7cDgkz7e4ELNYIofUh+qlm18DhjORCk3hFOwIlwuEnqwFWb50 mcf4dfFGPIvIVtBZpWovOsGc2j36aYTvZjdQZ5CRTouH7ll2jZ4c2D/6mzTvKYaB Zc7gebURznA= =sVxx -----END PGP SIGNATURE-----