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Date:   Thu, 6 Oct 2022 21:40:38 -0400
From:   "Theodore Ts'o" <tytso@mit.edu>
To:     Kees Cook <keescook@chromium.org>
Cc:     Jorge Merlino <jorge.merlino@canonical.com>,
        Christian Brauner <brauner@kernel.org>,
        Eric Biederman <ebiederm@xmission.com>,
        Jann Horn <jannh@google.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org,
        John Johansen <john.johansen@canonical.com>,
        Paul Moore <paul@paul-moore.com>,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Stephen Smalley <stephen.smalley.work@gmail.com>,
        Eric Paris <eparis@parisplace.org>,
        Richard Haines <richard_c_haines@btinternet.com>,
        Casey Schaufler <casey@schaufler-ca.com>,
        Xin Long <lucien.xin@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Todd Kjos <tkjos@google.com>,
        Ondrej Mosnacek <omosnace@redhat.com>,
        Prashanth Prahlad <pprahlad@redhat.com>,
        Micah Morton <mortonm@chromium.org>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Andrei Vagin <avagin@gmail.com>, linux-kernel@vger.kernel.org,
        apparmor@lists.ubuntu.com, linux-security-module@vger.kernel.org,
        selinux@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] Fix race condition when exec'ing setuid files
Message-ID: <Yz+Dln7AAMU+Oj9X@mit.edu>
References: <202209131456.76A13BC5E4@keescook>
 <202210061301.207A20C8E5@keescook>
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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