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13th USENIX Security Symposium — Abstract
Pp. 195–206 of the Proceedings
Fixing Races for Fun and Profit: How to use access(2)
Drew Dean, SRI International; Alan J. Hu, University of British Columbia
Abstract
It is well known that it is insecure to use the access(2) system call
in a setuid program to test for the ability of the program's executor
to access a file before opening said file. Although the access(2) call
appears to have been designed exactly for this use, such use is
vulnerable to a race condition. This race condition is a classic example of
a time-of-check-to-time-of-use (TOCTTOU) problem. We prove
the ``folk theorem'' that no portable, deterministic solution exists without
changes to the system call interface, we present a probabilistic
solution, and we examine the effect of increasing CPU speeds on the
exploitability of the attack.
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