On Tue, Jul 21, 2020 at 05:58:48PM +0200, Stefano Garzarella wrote: > On Tue, Jul 21, 2020 at 08:27:34AM -0700, Andy Lutomirski wrote: > > On Fri, Jul 17, 2020 at 1:02 AM Stefano Garzarella wrote: > > > > > > On Thu, Jul 16, 2020 at 08:12:35AM -0700, Kees Cook wrote: > > > > On Thu, Jul 16, 2020 at 03:14:04PM +0200, Stefano Garzarella wrote: > > > > > > access (IIUC) is possible without actually calling any of the io_uring > > > > syscalls. Is that correct? A process would receive an fd (via SCM_RIGHTS, > > > > pidfd_getfd, or soon seccomp addfd), and then call mmap() on it to gain > > > > access to the SQ and CQ, and off it goes? (The only glitch I see is > > > > waking up the worker thread?) > > > > > > It is true only if the io_uring istance is created with SQPOLL flag (not the > > > default behaviour and it requires CAP_SYS_ADMIN). In this case the > > > kthread is created and you can also set an higher idle time for it, so > > > also the waking up syscall can be avoided. > > > > I stared at the io_uring code for a while, and I'm wondering if we're > > approaching this the wrong way. It seems to me that most of the > > complications here come from the fact that io_uring SQEs don't clearly > > belong to any particular security principle. (We have struct creds, > > but we don't really have a task or mm.) But I'm also not convinced > > that io_uring actually supports cross-mm submission except by accident > > -- as it stands, unless a user is very careful to only submit SQEs > > that don't use user pointers, the results will be unpredictable. > > Perhaps we can get away with this: > > > > diff --git a/fs/io_uring.c b/fs/io_uring.c > > index 74bc4a04befa..92266f869174 100644 > > --- a/fs/io_uring.c > > +++ b/fs/io_uring.c > > @@ -7660,6 +7660,20 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, > > fd, u32, to_submit, > > if (!percpu_ref_tryget(&ctx->refs)) > > goto out_fput; > > > > + if (unlikely(current->mm != ctx->sqo_mm)) { > > + /* > > + * The mm used to process SQEs will be current->mm or > > + * ctx->sqo_mm depending on which submission path is used. > > + * It's also unclear who is responsible for an SQE submitted > > + * out-of-process from a security and auditing perspective. > > + * > > + * Until a real usecase emerges and there are clear semantics > > + * for out-of-process submission, disallow it. > > + */ > > + ret = -EACCES; > > + goto out; > > + } > > + > > /* > > * For SQ polling, the thread will do all submissions and completions. > > * Just return the requested submit count, and wake the thread if > > > > If we can do that, then we could bind seccomp-like io_uring filters to > > an mm, and we get obvious semantics that ought to cover most of the > > bases. > > > > Jens, Christoph? > > > > Stefano, what's your intended usecase for your restriction patchset? > > > > Hi Andy, > my use case concerns virtualization. The idea, that I described in the > proposal of io-uring restrictions [1], is to share io_uring CQ and SQ queues > with a guest VM for block operations. > > In the PoC that I realized, there is a block device driver in the guest that > uses io_uring queues coming from the host to submit block requests. > > Since the guest is not trusted, we need restrictions to allow only > a subset of syscalls on a subset of file descriptors and memory. BTW there's only a single mm in the kvm.ko use case. Stefan