On Tue, Jul 21, 2020 at 11:39 AM Jens Axboe <axboe@xxxxxxxxx> wrote: > > On 7/21/20 11:44 AM, Andy Lutomirski wrote: > > On Tue, Jul 21, 2020 at 10:30 AM Jens Axboe <axboe@xxxxxxxxx> wrote: > >> > >> On 7/21/20 11:23 AM, Andy Lutomirski wrote: > >>> On Tue, Jul 21, 2020 at 8:31 AM Jens Axboe <axboe@xxxxxxxxx> wrote: > >>>> > >>>> On 7/21/20 9:27 AM, Andy Lutomirski wrote: > >>>>> On Fri, Jul 17, 2020 at 1:02 AM Stefano Garzarella <sgarzare@xxxxxxxxxx> 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. > >>>> > >>>> How so? > >>> > >>> Unless I've missed something, either current->mm or sqo_mm will be > >>> used depending on which thread ends up doing the IO. (And there might > >>> be similar issues with threads.) Having the user memory references > >>> end up somewhere that is an implementation detail seems suboptimal. > >> > >> current->mm is always used from the entering task - obviously if done > >> synchronously, but also if it needs to go async. The only exception is a > >> setup with SQPOLL, in which case ctx->sqo_mm is the task that set up the > >> ring. SQPOLL requires root privileges to setup, and there's no task > >> entering the io_uring at all necessarily. It'll just submit sqes with > >> the credentials that are registered with the ring. > > > > Really? I admit I haven't fully followed how the code works, but it > > looks like anything that goes through the io_queue_async_work() path > > will use sqo_mm, and can't most requests that end up blocking end up > > there? It looks like, even if SQPOLL is not set, the mm used will > > depend on whether the request ends up blocking and thus getting queued > > for later completion. > > > > Or does some magic I missed make this a nonissue. > > No, you are wrong. The logic works as I described it. Can you enlighten me? I don't see any iov_iter_get_pages() calls or equivalents. If an IO is punted, how does the data end up in the io_uring_enter() caller's mm?
