Re: [PATCH 1/2] fs/exec: Explicitly unshare fs_struct on exec

From: Kees Cook <keescook@chromium.org>
To: Jann Horn <jannh@google.com>, Christian Brauner <brauner@kernel.org>
Cc: Eric Biederman <ebiederm@xmission.com>,
	Jorge Merlino <jorge.merlino@canonical.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 1/2] fs/exec: Explicitly unshare fs_struct on exec
Date: Thu, 06 Oct 2022 08:25:01 -0700	[thread overview]
Message-ID: <86CE201B-5632-4BB7-BCF6-7CB2C2895409@chromium.org> (raw)
In-Reply-To: <CAG48ez0sEkmaez9tYqgMXrkREmXZgxC9fdQD3mzF9cGo_=Tfyg@mail.gmail.com>

On October 6, 2022 7:13:37 AM PDT, Jann Horn <jannh@google.com> wrote:
>On Thu, Oct 6, 2022 at 11:05 AM Christian Brauner <brauner@kernel.org> wrote:
>> On Thu, Oct 06, 2022 at 01:27:34AM -0700, Kees Cook wrote:
>> > The check_unsafe_exec() counting of n_fs would not add up under a heavily
>> > threaded process trying to perform a suid exec, causing the suid portion
>> > to fail. This counting error appears to be unneeded, but to catch any
>> > possible conditions, explicitly unshare fs_struct on exec, if it ends up
>>
>> Isn't this a potential uapi break? Afaict, before this change a call to
>> clone{3}(CLONE_FS) followed by an exec in the child would have the
>> parent and child share fs information. So if the child e.g., changes the
>> working directory post exec it would also affect the parent. But after
>> this change here this would no longer be true. So a child changing a
>> workding directoro would not affect the parent anymore. IOW, an exec is
>> accompanied by an unshare(CLONE_FS). Might still be worth trying ofc but
>> it seems like a non-trivial uapi change but there might be few users
>> that do clone{3}(CLONE_FS) followed by an exec.
>
>I believe the following code in Chromium explicitly relies on this
>behavior, but I'm not sure whether this code is in active use anymore:
>
>https://source.chromium.org/chromium/chromium/src/+/main:sandbox/linux/suid/sandbox.c;l=101?q=CLONE_FS&sq=&ss=chromium

Oh yes. I think I had tried to forget this existed. Ugh. Okay, so back to the drawing board, I guess. The counting will need to be fixed...

It's possible we can move the counting after dethread -- it seems the early count was just to avoid setting flags after the point of no return, but it's not an error condition...

-- 
Kees Cook