[PATCH] Fix race condition when exec'ing setuid files

[David Laight]

From: Theodore Ts'o
> Sent: 07 October 2022 02:41
> 
> On Thu, Oct 06, 2022 at 01:20:35PM -0700, Kees Cook wrote:
> >
> > So the question, then, is "why are they trying to exec while actively
> > spawning new threads?" That appears to be the core problem here, and as
> > far as I can tell, the kernel has behaved this way for a very long time.
> > I don't think the kernel should fix this, either, because it leads to a
> > very weird state for userspace, where the thread spawner may suddenly
> > die due to the exec happening in another thread. This really looks like
> > something userspace needs to handle correctly (i.e. don't try to exec
> > while actively spawning threads).
> 
> One of the classic failure modes is when a threaded program calls a
> library, and that library might try to do a fork/exec (or call
> system(3) to run some command.  e.g., such as running "lvm create ..."
> or to spawn some kind of helper daemon.
> 
> There are a number of stack overflow questions about this, and there
> are some solutions to _some_ of the problems, such as using
> pthread_atfork(), and knowing that you are about to call fork/exec,
> and use some out of band mechanism to to make sure no threads get
> spawned until the fork/exec is completed --- but if you don't know
> that a library is going to do a fork/exec, well, life is tough.

Or that a library thread is about to create a new thread.

> One technique even advocated by a stack overflow article is "avoid
> using threads whenver possible".  :-/

Doesn't fork() only create the current thread in the new process?
So by the time exec() runs there is a nice single threaded process
with an fd table that isn't shared.

For helpers there is always (a properly implemented) posix_spawn() :-)

	David

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