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

[Christian Brauner <christian.brauner@ubuntu.com>]

On Mon, Jun 15, 2020 at 11:04:06AM +0200, Jann Horn wrote:
> +Kees, Christian, Sargun, Aleksa, kernel-hardening for their opinions
> on seccomp-related aspects

Just fyi, I'm on holiday this week so my responses have some
non-significant lag into early next week.

> 
> 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

So from what I can gather from this proposal, this would be a separate
security model for io_uring? I'm not to thrilled about that tbh. (There's
some discussion around extending seccomp - also at kernel summit.)
But doing the whole restriction setup in io_uring_setup() would at least
mean that if seccomp is extended to filter first-level pointers it could
know about all the security restrictions that apply to this io_uring
instance (Which I think you were getting at, Jann?).

Hm, would it make sense that if a task has a seccomp filter installed
that blocks openat syscalls that io_uring should automatically block
openat() calls as well or is the expectation "just block all of io_uring
if you're worried about that"?

Christian