linux-kernel - Re: [RFC PATCH v1 3/5] Yama: Enforces noexec mounts or file executability through O

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Message-ID: <58c59dbb-ae6b-ce06-d679-175b3ed6f652@ssi.gouv.fr>
Date:   Wed, 9 Jan 2019 14:41:12 +0100
From:   Mickaël Salaün <mickael.salaun@....gouv.fr>
To:     Kees Cook <keescook@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        James Morris <jmorris@...ei.org>
CC:     Jann Horn <jannh@...gle.com>,
        Mickaël Salaün <mic@...ikod.net>,
        kernel list <linux-kernel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Matthew Garrett <mjg59@...gle.com>,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        <philippe.trebuchet@....gouv.fr>, Shuah Khan <shuah@...nel.org>,
        <thibaut.sautereau@....gouv.fr>, <vincent.strubel@....gouv.fr>,
        Perez Yves-Alexis <yves-alexis.perez@....gouv.fr>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [RFC PATCH v1 3/5] Yama: Enforces noexec mounts or file
 executability through O_MAYEXEC


On 09/01/2019 00:30, Kees Cook wrote:
> On Tue, Jan 8, 2019 at 5:29 AM Mickaël Salaün
> <mickael.salaun@....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@....gouv.fr> wrote:
>>>> On 12/12/2018 18:09, Jann Horn wrote:
>>>>> On Wed, Dec 12, 2018 at 9:18 AM Mickaël Salaün <mic@...ikod.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?