On Tue, Jun 16, 2020 at 12:01 AM Christian Brauner <christian.brauner@xxxxxxxxxx> 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 > > 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@xxxxxxxxxx> 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?). Yeah. > 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"? I mean, if we could make that automagic, that'd be kinda neat; but I'm slightly worried that an automated translation might end up being slightly inaccurate. (But maybe that's acceptable?)
