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Message-ID: <Yz+Dln7AAMU+Oj9X@mit.edu> 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
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