strace of io_uring events?

[Andy Lutomirski]

On Tue, Jul 21, 2020 at 11:39 AM Jens Axboe <axboe at kernel.dk> wrote:
>
> On 7/21/20 11:44 AM, Andy Lutomirski wrote:
> > On Tue, Jul 21, 2020 at 10:30 AM Jens Axboe <axboe at kernel.dk> wrote:
> >>
> >> On 7/21/20 11:23 AM, Andy Lutomirski wrote:
> >>> On Tue, Jul 21, 2020 at 8:31 AM Jens Axboe <axboe at kernel.dk> wrote:
> >>>>
> >>>> On 7/21/20 9:27 AM, Andy Lutomirski wrote:
> >>>>> On Fri, Jul 17, 2020 at 1:02 AM Stefano Garzarella <sgarzare at redhat.com> 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?