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. >> This is just one known use case, there may very well be others. Outside >> of SQPOLL, which is special, I don't see a reason to restrict this. >> Given that you may have a fuller understanding of it after the above >> explanation, please clearly state what problem you're seeing that >> warrants a change. > > I see two fundamental issues: > > 1. The above. This may be less of an issue than it seems to me, but, > if you submit io from outside sqo_mm, the mm that ends up being used > depends on whether the IO is completed from io_uring_enter() or from > the workqueue. For something like Postgres, I guess this is okay > because the memory is MAP_ANONYMOUS | MAP_SHARED and the pointers all > point the same place regardless. No that is incorrect. If you disregard SQPOLL, then the 'mm' is always who submitted it. > 2. If you create an io_uring and io_uring_enter() it from a different > mm, it's unclear what seccomp is supposed to do. (Or audit, for that > matter.) Which task did the IO? Which mm did the IO? Whose sandbox > is supposed to be applied? Also doesn't seem like a problem, if you understand the 'mm' logic above. Unless SQPOLL is used, the entering tasks mm will be used. There's no mixing of tasks and mm outside of that. -- Jens Axboe
