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Date:   Fri, 7 Oct 2022 11:58:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Theodore Ts'o' <>, Kees Cook <>
CC:     Jorge Merlino <>,
        Christian Brauner <>,
        Eric Biederman <>,
        Jann Horn <>,
        Alexander Viro <>,
        Thomas Gleixner <>,
        Andy Lutomirski <>,
        "Sebastian Andrzej Siewior" <>,
        Andrew Morton <>,
        "" <>,
        "" <>,
        John Johansen <>,
        Paul Moore <>,
        James Morris <>,
        "Serge E. Hallyn" <>,
        Stephen Smalley <>,
        Eric Paris <>,
        Richard Haines <>,
        Casey Schaufler <>,
        Xin Long <>,
        "David S. Miller" <>,
        Todd Kjos <>,
        Ondrej Mosnacek <>,
        Prashanth Prahlad <>,
        Micah Morton <>,
        Fenghua Yu <>,
        Andrei Vagin <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>
Subject: RE: [PATCH] Fix race condition when exec'ing setuid files

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() :-)


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