linux-kernel - Re: [PATCH v3 1/4] seccomp: Add sysctl to display available actions

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Date:   Thu, 16 Feb 2017 11:01:52 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Tyler Hicks <tyhicks@...onical.com>
Cc:     Paul Moore <paul@...l-moore.com>, Eric Paris <eparis@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Will Drewry <wad@...omium.org>, linux-audit@...hat.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        John Crispin <john@...ozen.org>
Subject: Re: [PATCH v3 1/4] seccomp: Add sysctl to display available actions

On Thu, Feb 16, 2017 at 10:47 AM, Tyler Hicks <tyhicks@...onical.com> wrote:
> On 02/15/2017 09:14 PM, Andy Lutomirski wrote:
>> On Mon, Feb 13, 2017 at 7:45 PM, Tyler Hicks <tyhicks@...onical.com> wrote:
>>> This patch creates a read-only sysctl containing an ordered list of
>>> seccomp actions that the kernel supports. The ordering, from left to
>>> right, is the lowest action value (kill) to the highest action value
>>> (allow). Currently, a read of the sysctl file would return "kill trap
>>> errno trace allow". The contents of this sysctl file can be useful for
>>> userspace code as well as the system administrator.
>>
>> Would this make more sense as a new seccomp(2) mode a la
>> SECCOMP_HAS_ACTION?  Then sandboxy things that have no fs access could
>> use it.
>>
>
> It would make sense for code that needs to check which actions are
> available. It wouldn't make sense for administrators that need to check
> which actions are available unless libseccomp provided a wrapper utility.
>
> Is this a theoretical concern or do you know of a sandboxed piece of
> code that cannot access the sysctl before constructing a seccomp filter?
>

It's semi-theoretical.  But suppose I unshare namespaces, unmount a
bunch of stuff, then ask libseccomp to install a filter.  (I've
written code that does exactly that.)   libseccomp won't be able to
read the sysctl.

--Andy