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

From: Andy Lutomirski
Date: Thu Feb 16 2017 - 14:02:19 EST

On Thu, Feb 16, 2017 at 10:47 AM, Tyler Hicks <tyhicks@xxxxxxxxxxxxx> wrote:
> On 02/15/2017 09:14 PM, Andy Lutomirski wrote:
>> On Mon, Feb 13, 2017 at 7:45 PM, Tyler Hicks <tyhicks@xxxxxxxxxxxxx> 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.