Re: [Sheflug] Stupid cron question

["Chris J/#6" <nospam [at] nccnet.co.uk>]

> I wouldn't swear to it.. but maybe to get around problems with the
> editor not writing atomicly, and possibility of there being only part of
> a crontab there at the moment cron runs?
> 
> I'm probably wrong of course.. and it's just be to stop the user
> needing permissions to /var/spool/cron/crontabs/.
> 

The temporary file allows crontab to to a basic syntax check of the file 
before installing it as cron. It also prevents the user corrupting their 
current cron. Yes, it has the added side effect of being able to lock the 
crontab directory root read/write only. crontab though needs setuid/setgid 
bits set so it /can/ write to the cron directory. Therefore crontab should 
also be a secure binary against buffer overruns.

> 
> I also recall reading an argument against using pid's in temporary
> filenames, although I can't remember it now, and possibly it was against
> using consecutive pid's.. but I seem to recall it being by one of the
> OpenBSD folks.. 
> 

Yes. You could get a race that allows someone else access to the file. In C to 
get a temporary file using a pid, you could do:
	pid = getpid();
	sprintf (tmpfile, "/tmp/myfile.%d", pid);
	fopen (tmpfile, "rw");
	.
	.
	.

the race is that Mr Evil could get the pid of the program and create 
/tmp/myfile.<pid> with his rights, or symlink /tmp/myfile.pid to somewhere 
else where he could try more attacks against it. THe C library routine 
mktemp(3) suffers the same race, but mkstemp(3) doesn't (see below).

> 
> And seeing as I'm babbling like a cretin anyway.
> 
> Why is it, that every peice of information about creating secure tmp
> files, says to use mkstemp(), but the mkstemp() manpage says not to use it,
> and to use tmpfile()? :)  (and seeing as mkstemp() uses the pid, I guess
> what I was thinking about, must be random pids).
> 

Reading the manpage, it's not a security issue with mkstemp, but a portability 
issue...from said page:

	"Don't use this function, use tmpfile(3) instead. It's
	better defined and more portable."


Chris...


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