Re: [RFC] io_uring: add restrictions to support untrusted applications and guests

[Jann Horn <jannh@google.com>]

+Kees, Christian, Sargun, Aleksa, kernel-hardening for their opinions
on seccomp-related aspects

On Tue, Jun 9, 2020 at 4:24 PM Stefano Garzarella <sgarzare@redhat.com> wrote:
> Hi Jens,
> Stefan and I have a proposal to share with io_uring community.
> Before implementing it we would like to discuss it to receive feedbacks and
> to see if it could be accepted:
>
> Adding restrictions to io_uring
> =====================================
> The io_uring API provides submission and completion queues for performing
> asynchronous I/O operations. The queues are located in memory that is
> accessible to both the host userspace application and the kernel, making it
> possible to monitor for activity through polling instead of system calls. This
> design offers good performance and this makes exposing io_uring to guests an
> attractive idea for improving I/O performance in virtualization.
[...]
> Restrictions
> ------------
> This document proposes io_uring API changes that safely allow untrusted
> applications or guests to use io_uring. io_uring's existing security model is
> that of kernel system call handler code. It is designed to reject invalid
> inputs from host userspace applications. Supporting guests as io_uring API
> clients adds a new trust domain with access to even fewer resources than host
> userspace applications.
>
> Guests do not have direct access to host userspace application file descriptors
> or memory. The host userspace application, a Virtual Machine Monitor (VMM) such
> as QEMU, grants access to a subset of its file descriptors and memory. The
> allowed file descriptors are typically the disk image files belonging to the
> guest. The memory is typically the virtual machine's RAM that the VMM has
> allocated on behalf of the guest.
>
> The following extensions to the io_uring API allow the host application to
> grant access to some of its file descriptors.
>
> These extensions are designed to be applicable to other use cases besides
> untrusted guests and are not virtualization-specific. For example, the
> restrictions can be used to allow only a subset of sqe operations available to
> an application similar to seccomp syscall whitelisting.
>
> An address translation and memory restriction mechanism would also be
> necessary, but we can discuss this later.
>
> The IOURING_REGISTER_RESTRICTIONS opcode
> ----------------------------------------
> The new io_uring_register(2) IOURING_REGISTER_RESTRICTIONS opcode permanently
> installs a feature whitelist on an io_ring_ctx. The io_ring_ctx can then be
> passed to untrusted code with the knowledge that only operations present in the
> whitelist can be executed.

This approach of first creating a normal io_uring instance and then
installing restrictions separately in a second syscall means that it
won't be possible to use seccomp to restrict newly created io_uring
instances; code that should be subject to seccomp restrictions and
uring restrictions would only be able to use preexisting io_uring
instances that have already been configured by trusted code.

So I think that from the seccomp perspective, it might be preferable
to set up these restrictions in the io_uring_setup() syscall. It might
also be a bit nicer from a code cleanliness perspective, since you
won't have to worry about concurrently changing restrictions.