linux-kernel - Re: [PATCH] Fix race condition when exec'ing setuid files

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Date:   Thu, 6 Oct 2022 21:40:38 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Kees Cook <keescook@...omium.org>
Cc:     Jorge Merlino <jorge.merlino@...onical.com>,
        Christian Brauner <brauner@...nel.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Jann Horn <jannh@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        John Johansen <john.johansen@...onical.com>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>,
        Richard Haines <richard_c_haines@...nternet.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Xin Long <lucien.xin@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Todd Kjos <tkjos@...gle.com>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        Prashanth Prahlad <pprahlad@...hat.com>,
        Micah Morton <mortonm@...omium.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Andrei Vagin <avagin@...il.com>, linux-kernel@...r.kernel.org,
        apparmor@...ts.ubuntu.com, linux-security-module@...r.kernel.org,
        selinux@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] Fix race condition when exec'ing setuid files

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. 

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

	       	       	  	   - Ted