[PATCH 3/5] v2 seccomp_filters: Enable ftrace-based system call filtering

[Will Drewry]

On Mon, May 16, 2011 at 7:55 AM, Ingo Molnar <mingo at elte.hu> wrote:
>
> * Will Drewry <wad at chromium.org> wrote:
>
>> I agree with you on many of these points!  However, I don't think that the
>> views around LSMs, perf/ftrace infrastructure, or the current seccomp
>> filtering implementation are necessarily in conflict.  Here is my
>> understanding of how the different worlds fit together and where I see this
>> patchset living, along with where I could see future work going.  Perhaps I'm
>> being a trifle naive, but here goes anyway:
>>
>> 1. LSMs provide a global mechanism for hooking "security relevant"
>> events at a point where all the incoming user-sourced data has been
>> preprocessed and moved into userspace.  The hooks are called every
>> time one of those boundaries are crossed.
>
>> 2. Perf and the ftrace infrastructure provide global function tracing
>> and system call hooks with direct access to the caller's registers
>> (and memory).
>
> No, perf events are not just global but per task as well. Nor are events
> limited to 'tracing' (generating a flow of events into a trace buffer) - they
> can just be themselves as well and count and generate callbacks.

I was looking at the perf_sysenter_enable() call, but clearly there is
more going on :)

> The generic NMI watchdog uses that kind of event model for example, see
> kernel/watchdog.c and how it makes use of struct perf_event abstractions to do
> per CPU events (with no buffrs), or how kernel/hw_breakpoint.c uses it for per
> task events and integrates it with the ptrace hw-breakpoints code.
>
> Ideally Peter's one particular suggestion is right IMO and we'd want to be able
> for a perf_event to just be a list of callbacks, attached to a task and barely
> more than a discoverable, named notifier chain in its slimmest form.
>
> In practice it's fatter than that right now, but we should definitely factor
> out that aspect of it more clearly, both code-wise and API-wise.
> kernel/watchdog.c and kernel/hw_breakpoint.c shows that such factoring out is
> possible and desirable.
>
>> 3. seccomp (as it exists today) provides a global system call entry
>> hook point with a binary per-process decision about whether to provide
>> "secure computing" behavior.
>>
>> When I boil that down to abstractions, I see:
>> A. Globally scoped: LSMs, ftrace/perf
>> B. Locally/process scoped: seccomp
>
> Ok, i see where you got the idea that you needed to cut your surface of
> abstraction at the filter engine / syscall enumeration level - i think you were
> thinking of it in the ftrace model of tracepoints, not in the perf model of
> events.
>
> No, events are generic and as such per task as well, not just global.
>
> I've replied to your other mail with more specific suggestions of how we could
> provide your feature using abstractions that share code more widely. Talking
> specifics will i hope help move the discussion forward! :-)

Agreed.  I'll digest both the watchdog code as well as your other
comments and follow up when I have a fuller picture in my head.

(I have a few initial comments I'll post in response to your other mail.)

Thanks!
will