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From:   Jann Horn <jannh@google.com>
Date:   Tue, 18 Oct 2022 13:19:00 +0200
Message-ID: <CAG48ez1xqguRrWT+KrjkyUHGZPVFDbMM3f__71VE-L38kQri9A@mail.gmail.com>
Subject: Re: [PATCH 1/2] fs/exec: Explicitly unshare fs_struct on exec
To:     Kees Cook <keescook@chromium.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Jorge Merlino <jorge.merlino@canonical.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
        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 Mailing List <linux-kernel@vger.kernel.org>,
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On Tue, Oct 18, 2022 at 9:09 AM Kees Cook <keescook@chromium.org> wrote:
> On Fri, Oct 14, 2022 at 05:35:26PM +0200, Jann Horn wrote:
> > On Fri, Oct 14, 2022 at 5:18 AM Andy Lutomirski <luto@kernel.org> wrote=
:
> > > On Thu, Oct 6, 2022, at 7:13 AM, Jann Horn wrote:
> > > > On Thu, Oct 6, 2022 at 11:05 AM Christian Brauner <brauner@kernel.o=
rg> 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 catc=
h 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 ca=
ll 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., chang=
es the
> > > >> working directory post exec it would also affect the parent. But a=
fter
> > > >> this change here this would no longer be true. So a child changing=
 a
> > > >> workding directoro would not affect the parent anymore. IOW, an ex=
ec is
> > > >> accompanied by an unshare(CLONE_FS). Might still be worth trying o=
fc but
> > > >> it seems like a non-trivial uapi change but there might be few use=
rs
> > > >> 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 anymo=
re:
> > > >
> > > > https://source.chromium.org/chromium/chromium/src/+/main:sandbox/li=
nux/suid/sandbox.c;l=3D101?q=3DCLONE_FS&sq=3D&ss=3Dchromium
> > >
> > > Wait, this is absolutely nucking futs.  On a very quick inspection, t=
he sharable things like this are fs, files, sighand, and io.    files and s=
ighand get unshared, which makes sense.  fs supposedly checks for extra ref=
s and prevents gaining privilege.  io is... ignored!  At least it's not imm=
ediately obvious that io is a problem.
> > >
> > > But seriously, this makes no sense at all.  It should not be possible=
 to exec a program and then, without ptrace, change its cwd out from under =
it.  Do we really need to preserve this behavior?
> >
> > I agree that this is pretty wild.
> >
> > The single user I'm aware of is Chrome, and as far as I know, they use
> > it for establishing their sandbox on systems where unprivileged user
> > namespaces are disabled - see
> > <https://chromium.googlesource.com/chromium/src/+/main/docs/linux/suid_=
sandbox.md>.
> > They also have seccomp-based sandboxing, but IIRC there are some small
> > holes that mean it's still useful for them to be able to set up
> > namespaces, like how sendmsg() on a unix domain socket can specify a
> > file path as the destination address.
> >
> > (By the way, I think maybe Chrome wouldn't need this wacky trick with
> > the shared fs_struct if the "NO_NEW_PRIVS permits chroot()" thing had
> > ever landed that you
> > (https://lore.kernel.org/lkml/0e2f0f54e19bff53a3739ecfddb4ffa9a6dbde4d.=
1327858005.git.luto@amacapital.net/)
> > and Micka=C3=ABl Sala=C3=BCn proposed in the past... or alternatively, =
if there
> > was a way to properly filter all the syscalls that Chrome has to
> > permit for renderers.)
> >
> > (But also, to be clear, I don't speak for Chrome, this is just my
> > understanding of how their stuff works.)
>
> Chrome seems to just want a totally empty filesystem view, yes?
> Let's land the nnp+chroot change. :P Only 10 years late! Then we can
> have Chrome use this and we can unshare fs on exec...

Someone should check with Chrome first though to make sure what I said
accurately represents what they think...