Date: Tue, 16 Jun 2020 13:32:54 +0200 From: Jann Horn <jannh@...gle.com> To: Stefano Garzarella <sgarzare@...hat.com> Cc: Jens Axboe <axboe@...nel.dk>, Kees Cook <keescook@...omium.org>, Christian Brauner <christian.brauner@...ntu.com>, Sargun Dhillon <sargun@...gun.me>, Aleksa Sarai <asarai@...e.de>, Stefan Hajnoczi <stefanha@...hat.com>, Jeff Moyer <jmoyer@...hat.com>, io-uring <io-uring@...r.kernel.org>, kernel list <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com> Subject: Re: [RFC] io_uring: add restrictions to support untrusted applications and guests On Tue, Jun 16, 2020 at 11:13 AM Stefano Garzarella <sgarzare@...hat.com> wrote: > On Mon, Jun 15, 2020 at 11:00:25AM -0600, Jens Axboe wrote: > > On 6/15/20 7:33 AM, Stefano Garzarella wrote: > > > 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 > > >> > > >> On Tue, Jun 9, 2020 at 4:24 PM Stefano Garzarella <sgarzare@...hat.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. > > >> > > > > > > Thank you for these details! > > > > > > It seems feasible to include the restrictions during io_uring_setup(). > > > > > > The only doubt concerns the possibility of allowing the trusted code to > > > do some operations, before passing queues to the untrusted code, for > > > example registering file descriptors, buffers, eventfds, etc. > > > > > > To avoid this, I should include these operations in io_uring_setup(), > > > adding some code that I wanted to avoid by reusing io_uring_register(). > > > > > > If I add restrictions in io_uring_setup() and then add an operation to > > > go into safe mode (e.g. a flag in io_uring_enter()), we would have the same > > > problem, right? > > > > > > Just to be clear, I mean something like this: > > > > > > /* params will include restrictions */ > > > fd = io_uring_setup(entries, params); > > > > > > /* trusted code */ > > > io_uring_register_files(fd, ...); > > > io_uring_register_buffers(fd, ...); > > > io_uring_register_eventfd(fd, ...); > > > > > > /* enable safe mode */ > > > io_uring_enter(fd, ..., IORING_ENTER_ENABLE_RESTRICTIONS); > > > > > > > > > Anyway, including a list of things to register in the 'params', passed > > > to io_uring_setup(), should be feasible, if Jens agree :-) > > > > I wonder how best to deal with this, in terms of ring visibility vs > > registering restrictions. We could potentially start the ring in a > > disabled mode, if asked to. It'd still be visible in terms of having > > the fd installed, but it'd just error requests. That'd leave you with > > time to do the various setup routines needed before then flagging it > > as enabled. My only worry on that would be adding overhead for doing > > that. It'd be cheap enough to check for IORING_SETUP_DISABLED in > > ctx->flags in io_uring_enter(), and return -EBADFD or something if > > that's the case. That doesn't cover the SQPOLL case though, but maybe we > > just don't start the sq thread if IORING_SETUP_DISABLED is set. > > It seems to me a very good approach and easy to implement. In this way > we can reuse io_uring_register() without having to modify too much > io_uring_setup(). > > > > > We'd need a way to clear IORING_SETUP_DISABLED through > > io_uring_register(). When clearing, that could then start the sq thread > > as well, when SQPOLL is set. > > Could we do it using io_uring_enter() since we have a flag field or > do you think it's semantically incorrect? > > @Jann, do you think this could work with seccomp? To clarify that I understood your proposal correctly: Is the idea to have two types of mostly orthogonal restrictions; one type being restrictions on the opcode (supplied in io_uring_setup() and enforced immediately) and the other type being restrictions on io_uring_register() (enabled via IORING_ENTER_ENABLE_RESTRICTIONS)? That sounds fine to me. IORING_ENTER_ENABLE_RESTRICTIONS probably isn't necessary for your usecase though, right? Or is the idea to use that to suppress grace periods during setup in io_uring_register(), or something like that?
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