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From: Casey Schaufler <casey@schaufler-ca.com>
To: "Mickaël Salaün" <mic@digikod.net>,
	"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,
	Casey Schaufler <casey@schaufler-ca.com>
Subject: Re: [PATCH v1 0/1] Unprivileged chroot
Date: Wed, 10 Mar 2021 12:59:21 -0800	[thread overview]
Message-ID: <0dfd4306-8e7c-239b-2829-d4103395ea44@schaufler-ca.com> (raw)
In-Reply-To: <e0b03cf2-8e37-6a41-5132-b74566a8f269@digikod.net>

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


  reply	other threads:[~2021-03-10 21:00 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-10 16:09 Mickaël Salaün
2021-03-10 16:10 ` [PATCH v1 1/1] fs: Allow no_new_privs tasks to call chroot(2) Mickaël Salaün
2021-03-10 16:56   ` Eric W. Biederman
2021-03-10 18:13     ` Mickaël Salaün
2021-03-10 17:22 ` [PATCH v1 0/1] Unprivileged chroot Casey Schaufler
2021-03-10 18:17   ` Mickaël Salaün
2021-03-10 20:59     ` Casey Schaufler [this message]
2021-03-11 10:42       ` Mickaël Salaün

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