On 7/21/20 1:44 PM, Andy Lutomirski wrote: > 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? If the SQE needs to be punted to an io-wq worker, then io_prep_async_work() is ultimately called before it's queued with the io-wq worker. That grabs anything we need to successfully process this request, user access and all. io-wq then assumes the right "context" to performn that request. As the async punt is always done on behalf of the task that is submitting the IO (via io_uring_enter()), that is the context that we grab and use for that particular request. You keep looking at ctx->sqo_mm, and I've told you several times that it's only related to the SQPOLL thread. If you don't use SQPOLL, no request will ever use it. -- Jens Axboe
