Agreed.
* The untrusted thread would be restricted by seccomp mode 1 and
would contain the device emulation code.
I think the best strategy would allow for a device to run either in the
untrusted thread or the trusted thread. This makes performance testing a
bit easier and it also makes development a bit more natural.
When you refer to the device running in the trusted thread, are you talking
about the case where you run QEMU without sandboxing support? I think we would
ideally like to add this new support such that if it is not enabled, QEMU will
still run as a single process and decomposition wouldn't occur.
* The trusted helper thread would run beside the untrusted thread,
enabling the untrusted thread to make syscalls beyond read(),
write(), exit(), and sigreturn().
I assume you mean process, not thread BTW?
I do mean thread. When making calls on behalf of the seccomp'd thread, I think
there will be syscalls that must be called from the same address space. That's
where the the trusted helper thread would come into play.
* IPC communication mechanisms:
* An IPC mechanism will be required to enable communication between
untrusted and trusted threads.
* An IPC mechanism will also be required to enable communication
between the main QEMU process and device processes.
IPC is easy. We have tons of infrastructure in QEMU for IPC (virtio,
QMP, etc.). Please don't reinvent the wheel here.
Ok
* The communication mechanisms must provide secure communication,
be low overhead (easy to generate, parse, and validate), and must
play well with sVirt/LSMs.
I don't see how sVirt/LSM fits into this but all of these requirements
are also true for the other big untrusted thread that we interact with
(the guest itself).
My view is that we should view the untrusted thread as an extension of
the guest and that the interfaces between the trusted thread and the
untrusted thread views it simply as another machine type that presents a
different (simpler) hardware abstraction.
Yes this makes sense. I think our biggest concern with IPC is that we don't
introduce a TOCTTOU opportunity for a device to change call parameters after
they've been checked and before the calls is made on behalf of the sandboxed
thread. Shared memory that is writable by both untrusted/trusted thread could
introduce this.
* Some thoughts for IPC mechanisms are Unix sockets, pipes, virtio,
Google Native Client's IMC, and shared memory.
The actual mechanism doesn't really matter I think, but see above comments.
* If seccomp mode 2 support becomes available, decomposition of device
emulation into untrusted/trusted threads may not be necessary. This
could result in improved performance (no IPC overhead between trusted
and untrusted thread) and reduced complexity (no need for trusted
helper thread).
If mode 2 is the Right Answer, then we shouldn't wait for it to become
available. We should make it available by pushing it into the kernel.
If we all agree that if mode 2 existed, it's what we would use, then
that we have the answer to this discussion and we know what we need to
go off and do.
That would seem like the logical approach. I think there may be new mode 2
patches coming soon so we can see how they go over.