Re: Why side-effects on open(2) are evil. (was Re: [RFD w/info-PATCH] device arguments from lookup)

Andries.Brouwer
2001-05-19T16:42:37Z

>> Opening device files often has interesting side effects.

> Too bad. They can be triggered by similar races between attacker
> changing the type of object (file<->symlink) and backup.

Yes. This is a well-known security problem.
Doing
stat("file", &s);
if (action desired) {
action("file");
}
is no good because there is a race.
But doing
fd = open("file", flags);
fstat(fd, &s);
if (action desired) {
f_action(fd);
}
is no good either because the open() has unknown side effects.
It helps to add flags like O_NONBLOCK and perhaps O_NOCTTY,
but that is not quite good enough.

One would like to have a version of the open() call that was
guaranteed free of side effects, and gave a fd only -
perhaps for stat(), perhaps for ioctl().
This guarantee could perhaps be obtained by omitting the
f->f_op->open(inode,f);
call in dentry_open() when the open call is
open("file", O_FDONLY);
Of course it may be that we afterwards decide that fd must
be used, and then it needs upgrading:
fd = f_open(fd, O_RDWR);

Andries

[Such a construction allows various cleanups.
But no doubt it has problems that I have not yet thought of.]