The previous scheme made it possible for another user/attacker to cause the
temporary directory creation scheme to panic. All you needed to know was the pid
of the process you wanted to target ('other_pid') and the suffix it was using
(let's pretend it's 'sfx') and then code such as this would, in essence, DOS it:
for i in range(0u, 1001) {
let tp = &Path::new(format!("/tmp/rs-{}-{}-sfx", other_pid, i));
match fs::mkdir(tp, io::USER_RWX) { _ => () }
}
Since the scheme retried only 1000 times to create a temporary directory before
dying, the next time the attacked process called TempDir::new("sfx") after that
would typically cause a panic. Of course, you don't necessarily need an attacker
to cause such a DOS: creating 1000 temporary directories without closing any of
the previous would be enough to DOS yourself.
This patch broadly follows the OpenBSD implementation of mkstemp. It uses the
operating system's random number generator to produce random directory names
that are impractical to guess (and, just in case someone manages to do that, it
retries creating the directory for a long time before giving up; OpenBSD
retries INT_MAX times, although 1<<31 seems enough to thwart even the most
patient attacker).
As a small additional change, this patch also makes the argument that
TempDir::new takes a prefix rather than a suffix. This is because 1) it more
closely matches what mkstemp and friends do 2) if you're going to have a
deterministic part of a filename, you really want it at the beginning so that
shell completion is useful.
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