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

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Date:	Wed, 9 Mar 2016 13:25:12 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Chris Metcalf <cmetcalf@...lanox.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Lameter <cl@...ux.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tejun Heo <tj@...nel.org>,
	Gilad Ben Yossef <giladb@...hip.com>,
	Will Deacon <will.deacon@....com>,
	Rik van Riel <riel@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v10 09/12] arch/x86: enable task isolation functionality

On Wed, Mar 9, 2016 at 1:18 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Wed, Mar 9, 2016 at 1:10 PM, Kees Cook <keescook@...omium.org> wrote:
>> On Wed, Mar 9, 2016 at 12:58 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>> On Tue, Mar 8, 2016 at 12:40 PM, Chris Metcalf <cmetcalf@...lanox.com> 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...

-Kees

>
> --Andy



-- 
Kees Cook
Chrome OS & Brillo Security