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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

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        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

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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 (§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.

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.

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