Re: [RFC PATCH v1 3/5] Yama: Enforces noexec mounts or file executability through O_MAYEXEC

From: "Mickaël Salaün" <mickael.salaun@ssi.gouv.fr>
To: Kees Cook <keescook@chromium.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	James Morris <jmorris@namei.org>
Cc: "Jann Horn" <jannh@google.com>,
	"Mickaël Salaün" <mic@digikod.net>,
	"kernel list" <linux-kernel@vger.kernel.org>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Matthew Garrett" <mjg59@google.com>,
	"Michael Kerrisk-manpages" <mtk.manpages@gmail.com>,
	"Mimi Zohar" <zohar@linux.ibm.com>,
	philippe.trebuchet@ssi.gouv.fr, "Shuah Khan" <shuah@kernel.org>,
	thibaut.sautereau@ssi.gouv.fr, vincent.strubel@ssi.gouv.fr,
	"Perez Yves-Alexis" <yves-alexis.perez@ssi.gouv.fr>,
	"Kernel Hardening" <kernel-hardening@lists.openwall.com>,
	"Linux API" <linux-api@vger.kernel.org>,
	linux-security-module <linux-security-module@vger.kernel.org>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"Andy Lutomirski" <luto@kernel.org>
Subject: Re: [RFC PATCH v1 3/5] Yama: Enforces noexec mounts or file executability through O_MAYEXEC
Date: Wed, 9 Jan 2019 14:41:12 +0100	[thread overview]
Message-ID: <58c59dbb-ae6b-ce06-d679-175b3ed6f652@ssi.gouv.fr> (raw)
In-Reply-To: <CAGXu5jKHPE6WfbXpiwzm=vRbgJ-rCePxsrYmVzZ1+RURp-6nJg@mail.gmail.com>

On 09/01/2019 00:30, Kees Cook wrote:
> On Tue, Jan 8, 2019 at 5:29 AM Mickaël Salaün
> <mickael.salaun@ssi.gouv.fr> wrote:
>>
>>
>> On 03/01/2019 12:17, Jann Horn wrote:
>>> On Thu, Dec 13, 2018 at 3:49 PM Mickaël Salaün
>>> <mickael.salaun@ssi.gouv.fr> wrote:
>>>> On 12/12/2018 18:09, Jann Horn wrote:
>>>>> On Wed, Dec 12, 2018 at 9:18 AM Mickaël Salaün <mic@digikod.net> wrote:
>>>>>> Enable to either propagate the mount options from the underlying VFS
>>>>>> mount to prevent execution, or to propagate the file execute permission.
>>>>>> This may allow a script interpreter to check execution permissions
>>>>>> before reading commands from a file.
>>>>>>
>>>>>> The main goal is to be able to protect the kernel by restricting
>>>>>> arbitrary syscalls that an attacker could perform with a crafted binary
>>>>>> or certain script languages.  It also improves multilevel isolation
>>>>>> by reducing the ability of an attacker to use side channels with
>>>>>> specific code.  These restrictions can natively be enforced for ELF
>>>>>> binaries (with the noexec mount option) but require this kernel
>>>>>> extension to properly handle scripts (e.g., Python, Perl).
> 
> I like this idea, but I think it shouldn't live in Yama (since it is
> currently intended to be a ptrace-policy-only LSM). It was
> _originally_ designed to do various DAC improvements, but the
> agreement was that those should live directly in the VFS instead (i.e.
> the symlink, hardlink and now fifo and regular file defenses).
> 
> This should likely go in similarly. (But if not, it could also be its own LSM.)
> 

I think that Yama is quite handy and make sense here, but I'm fine
putting this knob elsewhere. However, I was thinking, for a future patch
series, to add another sysctl to lock this choice, i.e. generalizing the
way Yama can lock the ptrace_scope.

What matter here is the ability for an LSM to use this O_MAYEXEC flag.
Yama is a good place to showcase this feature and I think it is cleaner
to leverage the LSM framework to put new (optional) security features. I
can easily create a new LSM but it would be pretty similar to Yama...
What do you think about it James and Al?

Side question: wouldn't it be better to use a 0600 mode (instead of
0644) for this kind of sysctl?