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From:   Jann Horn <jannh@google.com>
Date:   Tue, 16 Mar 2021 20:31:56 +0100
Message-ID: <CAG48ez0UHP=B6MW5ySMOAQ677byzyWkwgPto1RdW6FYJH5b7Zg@mail.gmail.com>
Subject: Re: [PATCH v4 1/1] fs: Allow no_new_privs tasks to call chroot(2)
To:     =?UTF-8?B?TWlja2HDq2wgU2FsYcO8bg==?= <mic@digikod.net>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        James Morris <jmorris@namei.org>,
        Serge Hallyn <serge@hallyn.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Casey Schaufler <casey@schaufler-ca.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Christoph Hellwig <hch@lst.de>,
        David Howells <dhowells@redhat.com>,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        John Johansen <john.johansen@canonical.com>,
        Kees Cook <keescook@chromium.org>,
        Kentaro Takeda <takedakn@nttdata.co.jp>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        =?UTF-8?B?TWlja2HDq2wgU2FsYcO8bg==?= <mic@linux.microsoft.com>
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On Tue, Mar 16, 2021 at 8:26 PM Micka=C3=ABl Sala=C3=BCn <mic@digikod.net> =
wrote:
> On 16/03/2021 20:04, Jann Horn wrote:
> > On Tue, Mar 16, 2021 at 6:02 PM Micka=C3=ABl Sala=C3=BCn <mic@digikod.n=
et> wrote:
> >> One could argue that chroot(2) is useless without a properly populated
> >> root hierarchy (i.e. without /dev and /proc).  However, there are
> >> multiple use cases that don't require the chrooting process to create
> >> file hierarchies with special files nor mount points, e.g.:
> >> * A process sandboxing itself, once all its libraries are loaded, may
> >>   not need files other than regular files, or even no file at all.
> >> * Some pre-populated root hierarchies could be used to chroot into,
> >>   provided for instance by development environments or tailored
> >>   distributions.
> >> * Processes executed in a chroot may not require access to these speci=
al
> >>   files (e.g. with minimal runtimes, or by emulating some special file=
s
> >>   with a LD_PRELOADed library or seccomp).
> >>
> >> Unprivileged chroot is especially interesting for userspace developers
> >> wishing to harden their applications.  For instance, chroot(2) and Yam=
a
> >> enable to build a capability-based security (i.e. remove filesystem
> >> ambient accesses) by calling chroot/chdir with an empty directory and
> >> accessing data through dedicated file descriptors obtained with
> >> openat2(2) and RESOLVE_BENEATH/RESOLVE_IN_ROOT/RESOLVE_NO_MAGICLINKS.
> >
> > I don't entirely understand. Are you writing this with the assumption
> > that a future change will make it possible to set these RESOLVE flags
> > process-wide, or something like that?
>
> No, this scenario is for applications willing to sandbox themselves and
> only use the FDs to access legitimate data.

But if you're chrooted to /proc/self/fdinfo and have an fd to some
directory - let's say /home/user/Downloads - there is nothing that
ensures that you only use that fd with RESOLVE_BENEATH, right? If the
application is compromised, it can do something like openat(fd,
"../.bashrc", O_RDWR), right? Or am I missing something?

> > As long as that doesn't exist, I think that to make this safe, you'd
> > have to do something like the following - let a child process set up a
> > new mount namespace for you, and then chroot() into that namespace's
> > root:
> >
> > struct shared_data {
> >   int root_fd;
> > };
> > int helper_fn(void *args) {
> >   struct shared_data *shared =3D args;
> >   mount("none", "/tmp", "tmpfs", MS_NOSUID|MS_NODEV, "");
> >   mkdir("/tmp/old_root", 0700);
> >   pivot_root("/tmp", "/tmp/old_root");
> >   umount("/tmp/old_root", "");
> >   shared->root_fd =3D open("/", O_PATH);
> > }
> > void setup_chroot() {
> >   struct shared_data shared =3D {};
> >   prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
> >   clone(helper_fn, my_stack,
> > CLONE_VFORK|CLONE_VM|CLONE_FILES|CLONE_NEWUSER|CLONE_NEWNS|SIGCHLD,
> > NULL);
> >   fchdir(shared.root_fd);
> >   chroot(".");
> > }
>
> What about this?
> chdir("/proc/self/fdinfo");
> chroot(".");
> close(all unnecessary FDs);

That breaks down if you can e.g. get a unix domain socket connected to
a process in a different chroot, right? Isn't that a bit too fragile?