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

From: Kees Cook
Date: Wed Mar 09 2016 - 16:25:22 EST

On Wed, Mar 9, 2016 at 1:18 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> 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.

The other problem is that this won't work if the third-party code
actually uses seccomp itself... this isn't composable as-is.

>> 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.

No, this won't let the original syscall through. I wanted to be able
to document the syscalls as they happened without needing audit or a
ptrace monitor. I am currently convinced that my desire for this is no
good, and it should just be done with a ptrace monitor...


> --Andy

Kees Cook
Chrome OS & Brillo Security