linux-kernel - Re: [PATCH v1 0/1] Unprivileged chroot

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Date:   Wed, 10 Mar 2021 12:59:21 -0800
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Mickaël Salaün <mic@...ikod.net>,
        Al Viro <viro@...iv.linux.org.uk>,
        James Morris <jmorris@...ei.org>,
        Serge Hallyn <serge@...lyn.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Christoph Hellwig <hch@....de>,
        David Howells <dhowells@...hat.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Eric Biederman <ebiederm@...ssion.com>,
        John Johansen <john.johansen@...onical.com>,
        Kees Cook <keescook@...omium.org>,
        Kentaro Takeda <takedakn@...data.co.jp>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        kernel-hardening@...ts.openwall.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v1 0/1] Unprivileged chroot

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.

> 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.

>
>>> 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