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Date:   Thu, 06 Oct 2022 08:25:01 -0700
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
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On October 6, 2022 7:13:37 AM PDT, Jann Horn <jannh@google=2Ecom> wrote:
>On Thu, Oct 6, 2022 at 11:05 AM Christian Brauner <brauner@kernel=2Eorg> =
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 hea=
vily
>> > threaded process trying to perform a suid exec, causing the suid port=
ion
>> > to fail=2E This counting error appears to be unneeded, but to catch a=
ny
>> > 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=2E So if the child e=2Eg=2E, chan=
ges the
>> working directory post exec it would also affect the parent=2E But afte=
r
>> this change here this would no longer be true=2E So a child changing a
>> workding directoro would not affect the parent anymore=2E IOW, an exec =
is
>> accompanied by an unshare(CLONE_FS)=2E 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=2E
>
>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=2Echromium=2Eorg/chromium/chromium/src/+/main:sandbox/linu=
x/suid/sandbox=2Ec;l=3D101?q=3DCLONE_FS&sq=3D&ss=3Dchromium

Oh yes=2E I think I had tried to forget this existed=2E Ugh=2E Okay, so ba=
ck to the drawing board, I guess=2E The counting will need to be fixed=2E=
=2E=2E

It's possible we can move the counting after dethread -- it seems the earl=
y count was just to avoid setting flags after the point of no return, but i=
t's not an error condition=2E=2E=2E

--=20
Kees Cook