Software Engineering Institute

In the C language, given the following types:

        char *buf;
        int len;

some C compilers will assume that buf+len >= buf. As a result, code that performs wrapping checks similar to the following:

len = 1<<30;
[...]
if(buf+len < buf)  /* wrap check */
  [...overflow occurred...]

are optimized out by these compilers; no object code to perform the check will appear in the resulting executable program. In the case where the wrap test expression is optimized out, a subsequent manipulation of len could cause an overflow. As a result, applications that perform such checks may be vulnerable to buffer overflows.

Wrapping checks that use methods similar to the one described above depend on undefined behavior. Conforming implementations are permitted to perform the optimization by the ISO/IEC 9899:1999 C specification (§6.5.6p8) as undefined behavior. Even if a conforming implementation currently generates object code for an undefined behavior, future versions of the compiler are not obligated to do the same; this behavior may be viewed as an opportunity for further optimization. To ensure that such changes to the compiler do not invalidate assumptions, developers should follow the recommendations described in CERT C Secure Coding recommendation MSC15-C and rule ARR38-C to avoid this error. Furthermore, compilers are not required to issue diagnostics for undefined behavior, so there is frequently no easy way to identify undefined behavior in code, particularly during manual code audits.

Note that this issue does not strictly constitute a vulnerability in the compilers themselves. Rather, this behavior may introduce vulnerabilities in applications that include similar code and are compiled with affected compiler implementations. Existing code that relies on the undefined behavior in the wrapping check is particularly susceptible to this behavior.

Multiple implementations are known to perform this optimization. This optimization may be affected by the setting of the optimization level as well as other flags. Additional information about affected implementations can be found in the Systems Affected section of this document.

An application that performs wrapping checks based on an expression such as the one described above may be vulnerable to buffer overflow if compiled with affected compiler implementations. The nature of the resulting vulnerability would be specific to the application and depends on how the affected code is used.

Use casts
Cast objects of type char* to uintptr_t before comparison. The faulty wrapping 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 affected compiler implementations
Application developers and vendors of large codebases that cannot be audited for use of the defective wrapping checks are urged to avoid using compiler implementations that perform the offending optimization. Vendors and developers should carefully evaluate the conditions under which their compiler may perform the offending optimization. In some cases, downgrading the version of the compiler in use or sticking with versions of the compiler that do not perform the offending optimization may mitigate resulting vulnerabilities in applications.