Re: strace of io_uring events?

[Jens Axboe <axboe@kernel.dk>]

On 7/21/20 11:44 AM, Andy Lutomirski wrote:
> On Tue, Jul 21, 2020 at 10:30 AM Jens Axboe <axboe@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@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@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.

>> 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