Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
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
Thread-Topic: [PATCH] Fix race condition when exec'ing setuid files
Thread-Index: AQHY2e33//tmcksZekaHt2mlO/Dtmq4C0tkw
Date:   Fri, 7 Oct 2022 11:58:06 +0000
Message-ID: <f01aae2a5936450f889fa5a7d350d363@AcuMS.aculab.com>
References: <202209131456.76A13BC5E4@keescook>
 <202210061301.207A20C8E5@keescook> <Yz+Dln7AAMU+Oj9X@mit.edu>
In-Reply-To: <Yz+Dln7AAMU+Oj9X@mit.edu>
Accept-Language: en-GB, en-US
MIME-Version: 1.0
Content-Language: en-US
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8BIT
Precedence: bulk

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

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)