Re: [PATCH v10 09/12] arch/x86: enable task isolation functionality

[Andy Lutomirski]

On Wed, Mar 9, 2016 at 1:10 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Wed, Mar 9, 2016 at 12:58 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Tue, Mar 8, 2016 at 12:40 PM, Chris Metcalf <cmetcalf@xxxxxxxxxxxx> wrote:
>>> On 03/07/2016 03:55 PM, Andy Lutomirski wrote:
>>>>>>
>>>>>> Let task isolation users who want to detect when they screw up and do
>>>>>> >>a syscall do it with seccomp.
>>>>>
>>>>>
>>>>> >Can you give me more details on what you're imagining here?  Remember
>>>>> >that a key use case is that these applications can remove the syscall
>>>>> >prohibition voluntarily; it's only there to prevent unintended uses
>>>>> >(by third party libraries or just straight-up programming bugs).
>>>>> >As far as I can tell, seccomp does not allow you to go from "less
>>>>> >permissive" to "more permissive" settings at all, which means that as
>>>>> >it exists, it's not a good solution for this use case.
>>>>> >
>>>>> >Or were you thinking about a new seccomp API that allows this?
>>>>
>>>> I was.  This is at least the second time I've wanted a way to ask
>>>> seccomp to allow a layer to be removed.
>>>
>>>
>>> Andy,
>>>
>>> Please take a look at this draft patch that intends to enable seccomp
>>> as something that task isolation can use.
>>
>> Kees, this sounds like it may solve your self-instrumentation problem.
>> Want to take a look?
>
> Errrr... I'm pretty uncomfortable with this. I really would like to
> keep the basic semantics of seccomp is simple as possible: filtering
> only gets more restricted.
>
> This doesn't really solve my self-instrumentation desires since I
> still can't sanely deliver signals. I would need a lot more
> convincing. :)
>

I think you could do it by adding a filter that turns all the unknown
things into SIGSYS, allows sigreturn, and allows the seccomp syscall,
at least in the pop-off-the-filter variant.  Then you add this
removably.

In the SIGSYS handler, you pop off the filter, do your bookkeeping,
update the filter, and push it back on.

--Andy