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

From: David Laight <David.Laight@ACULAB.COM>
To: 'Theodore Ts'o' <tytso@mit.edu>, 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-mm@kvack.org>,
	"linux-fsdevel@vger.kernel.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" <linux-kernel@vger.kernel.org>,
	"apparmor@lists.ubuntu.com" <apparmor@lists.ubuntu.com>,
	"linux-security-module@vger.kernel.org" 
	<linux-security-module@vger.kernel.org>,
	"selinux@vger.kernel.org" <selinux@vger.kernel.org>,
	"linux-hardening@vger.kernel.org"
	<linux-hardening@vger.kernel.org>
Subject: RE: [PATCH] Fix race condition when exec'ing setuid files
Date: Fri, 7 Oct 2022 11:58:06 +0000	[thread overview]
Message-ID: <f01aae2a5936450f889fa5a7d350d363@AcuMS.aculab.com> (raw)
In-Reply-To: <Yz+Dln7AAMU+Oj9X@mit.edu>

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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