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From: Jens Axboe <axboe@kernel.dk>
To: Andy Lutomirski <luto@kernel.org>
Cc: Andres Freund <andres@anarazel.de>,
	Stefano Garzarella <sgarzare@redhat.com>,
	Christoph Hellwig <hch@lst.de>, Kees Cook <keescook@chromium.org>,
	Pavel Begunkov <asml.silence@gmail.com>,
	Miklos Szeredi <miklos@szeredi.hu>,
	Matthew Wilcox <willy@infradead.org>,
	Jann Horn <jannh@google.com>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	strace-devel@lists.strace.io, io-uring@vger.kernel.org,
	Linux API <linux-api@vger.kernel.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Stefan Hajnoczi <stefanha@redhat.com>
Subject: Re: strace of io_uring events?
Date: Tue, 21 Jul 2020 13:48:06 -0600	[thread overview]
Message-ID: <6f0d61a4-f9c1-6978-5897-9e50a8f212f4@kernel.dk> (raw)
In-Reply-To: <CALCETrV_tOziNJOp8xanmCU0yJEHcGQk0TBxeiK4U7AVewkgAw@mail.gmail.com>

On 7/21/20 1:44 PM, Andy Lutomirski wrote:
> On Tue, Jul 21, 2020 at 11:39 AM Jens Axboe <axboe@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@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.
> 
> 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


  reply	other threads:[~2020-07-21 19:48 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-15 11:12 Miklos Szeredi
2020-07-15 14:35 ` Andy Lutomirski
2020-07-15 17:11   ` Matthew Wilcox
2020-07-15 19:42     ` Pavel Begunkov
2020-07-15 20:09       ` Miklos Szeredi
2020-07-15 20:20         ` Pavel Begunkov
2020-07-15 23:07           ` Kees Cook
2020-07-16 13:14             ` Stefano Garzarella
2020-07-16 15:12               ` Kees Cook
2020-07-17  8:01                 ` Stefano Garzarella
2020-07-21 15:27                   ` Andy Lutomirski
2020-07-21 15:31                     ` Jens Axboe
2020-07-21 17:23                       ` Andy Lutomirski
2020-07-21 17:30                         ` Jens Axboe
2020-07-21 17:44                           ` Andy Lutomirski
2020-07-21 18:39                             ` Jens Axboe
2020-07-21 19:44                               ` Andy Lutomirski
2020-07-21 19:48                                 ` Jens Axboe [this message]
2020-07-21 19:56                                 ` Andres Freund
2020-07-21 19:37                         ` Andres Freund
2020-07-21 15:58                     ` Stefano Garzarella
2020-07-23 10:39                       ` Stefan Hajnoczi
2020-07-23 13:37                       ` Colin Walters
2020-07-24  7:25                         ` Stefano Garzarella
2020-07-16 13:17             ` Aleksa Sarai
2020-07-16 15:19               ` Kees Cook
2020-07-17  8:17               ` Cyril Hrubis
2020-07-16 16:24             ` Andy Lutomirski
2020-07-16  0:12     ` tytso

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