From: Jann Horn <jannh@google.com>
To: "Mickaël Salaün" <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>,
"Mickaël Salaün" <mic@linux.microsoft.com>
Subject: Re: [PATCH v4 1/1] fs: Allow no_new_privs tasks to call chroot(2)
Date: Tue, 16 Mar 2021 20:04:09 +0100 [thread overview]
Message-ID: <CAG48ez3=M-5WT73HqmFJr6UHwO0+2FJXxcAgRzp6wcd0P3TN=Q@mail.gmail.com> (raw)
In-Reply-To: <20210316170135.226381-2-mic@digikod.net>
On Tue, Mar 16, 2021 at 6:02 PM Mickaël Salaün <mic@digikod.net> 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 special
> files (e.g. with minimal runtimes, or by emulating some special files
> 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 Yama
> 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?
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 = 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 = open("/", O_PATH);
}
void setup_chroot() {
struct shared_data shared = {};
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(".");
}
[...]
> diff --git a/fs/open.c b/fs/open.c
[...]
> +static inline int current_chroot_allowed(void)
> +{
> + /*
> + * Changing the root directory for the calling task (and its future
> + * children) requires that this task has CAP_SYS_CHROOT in its
> + * namespace, or be running with no_new_privs and not sharing its
> + * fs_struct and not escaping its current root (cf. create_user_ns()).
> + * As for seccomp, checking no_new_privs avoids scenarios where
> + * unprivileged tasks can affect the behavior of privileged children.
> + */
> + if (task_no_new_privs(current) && current->fs->users == 1 &&
this read of current->fs->users should be using READ_ONCE()
> + !current_chrooted())
> + return 0;
> + if (ns_capable(current_user_ns(), CAP_SYS_CHROOT))
> + return 0;
> + return -EPERM;
> +}
[...]
Overall I think this change is a good idea.
next prev parent reply other threads:[~2021-03-16 19:05 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-16 17:01 [PATCH v4 0/1] Unprivileged chroot Mickaël Salaün
2021-03-16 17:01 ` [PATCH v4 1/1] fs: Allow no_new_privs tasks to call chroot(2) Mickaël Salaün
2021-03-16 18:43 ` Kees Cook
2021-03-16 19:04 ` Jann Horn [this message]
2021-03-16 19:24 ` Kees Cook
2021-03-16 19:25 ` Mickaël Salaün
2021-03-16 19:26 ` Mickaël Salaün
2021-03-16 19:31 ` Jann Horn
2021-03-16 20:06 ` Mickaël Salaün
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CAG48ez3=M-5WT73HqmFJr6UHwO0+2FJXxcAgRzp6wcd0P3TN=Q@mail.gmail.com' \
--to=jannh@google.com \
--cc=casey@schaufler-ca.com \
--cc=christian.brauner@ubuntu.com \
--cc=dhowells@redhat.com \
--cc=ebiederm@xmission.com \
--cc=hch@lst.de \
--cc=jmorris@namei.org \
--cc=john.johansen@canonical.com \
--cc=keescook@chromium.org \
--cc=kernel-hardening@lists.openwall.com \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-security-module@vger.kernel.org \
--cc=linux@dominikbrodowski.net \
--cc=luto@amacapital.net \
--cc=mic@digikod.net \
--cc=mic@linux.microsoft.com \
--cc=penguin-kernel@i-love.sakura.ne.jp \
--cc=serge@hallyn.com \
--cc=takedakn@nttdata.co.jp \
--cc=viro@zeniv.linux.org.uk \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).