Re: [PATCH v1 0/1] Unprivileged chroot

From: "Mickaël Salaün" <mic@digikod.net>
To: Casey Schaufler <casey@schaufler-ca.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	James Morris <jmorris@namei.org>, Serge Hallyn <serge@hallyn.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Christoph Hellwig <hch@lst.de>,
	David Howells <dhowells@redhat.com>,
	Dominik Brodowski <linux@dominikbrodowski.net>,
	Eric 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@lists.openwall.com,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-security-module@vger.kernel.org
Subject: Re: [PATCH v1 0/1] Unprivileged chroot
Date: Thu, 11 Mar 2021 11:42:55 +0100	[thread overview]
Message-ID: <a28100f5-19ca-adbf-7056-575f6bfc9dc6@digikod.net> (raw)
In-Reply-To: <0dfd4306-8e7c-239b-2829-d4103395ea44@schaufler-ca.com>

On 10/03/2021 21:59, Casey Schaufler wrote:
> On 3/10/2021 10:17 AM, Mickaël Salaün wrote:
>> On 10/03/2021 18:22, Casey Schaufler wrote:
>>> On 3/10/2021 8:09 AM, Mickaël Salaün wrote:
>>>> Hi,
>>>>
>>>> The chroot system call is currently limited to be used by processes with
>>>> the CAP_SYS_CHROOT capability.  This protects against malicious
>>>> procesess willing to trick SUID-like binaries.  The following patch
>>>> allows unprivileged users to safely use chroot(2).
>>> Mount namespaces have pretty well obsoleted chroot(). CAP_SYS_CHROOT is
>>> one of the few fine grained capabilities. We're still finding edge cases
>>> (e.g. ptrace) where no_new_privs is imperfect. I doesn't seem that there
>>> is a compelling reason to remove the privilege requirement on chroot().
>> What is the link between chroot and ptrace?
> 
> The possibility of sophisticated interactions with no_new_privs.

Would you mind giving some practical examples?

> 
>> What is interesting with CAP_SYS_CHROOT?
> 
> CAP_SYS_CHROOT is specific to chroot. It doesn't give you privilege
> beyond what you expect, unlike CAP_CHOWN or CAP_SYS_ADMIN. Making chroot
> unprivileged is silly when it's possibly the best example of how the
> capability mechanism is supposed to work.

Why would it be silly to make the use of this feature safe for any
processes instead of giving the right (with CAP_SYS_CHROOT) to some
processes to use it unsafely?

> 
>>
>>>> This patch is a follow-up of a previous one sent by Andy Lutomirski some
>>>> time ago:
>>>> https://lore.kernel.org/lkml/0e2f0f54e19bff53a3739ecfddb4ffa9a6dbde4d.1327858005.git.luto@amacapital.net/
>>>>
>>>> This patch can be applied on top of v5.12-rc2 .  I would really
>>>> appreciate constructive reviews.
>>>>
>>>> Regards,
>>>>
>>>> Mickaël Salaün (1):
>>>>   fs: Allow no_new_privs tasks to call chroot(2)
>>>>
>>>>  fs/open.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
>>>>  1 file changed, 61 insertions(+), 3 deletions(-)
>>>>
>>>>
>>>> base-commit: a38fd8748464831584a19438cbb3082b5a2dab15
> 