Linux-Fsdevel Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Jens Axboe <axboe@kernel.dk>
To: Andy Lutomirski <luto@kernel.org>, Andres Freund <andres@anarazel.de>
Cc: 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 11:30:44 -0600	[thread overview]
Message-ID: <ba989463-c627-8af7-9234-4dc8ac4eea0e@kernel.dk> (raw)
In-Reply-To: <CALCETrXAxFzuRB5EJZR7bbgfrEcNc=9_E7wwhPaZ3YGJ1=DZ0w@mail.gmail.com>

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.

>>> Perhaps we can get away with this:
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index 74bc4a04befa..92266f869174 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -7660,6 +7660,20 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int,
>>> fd, u32, to_submit,
>>>      if (!percpu_ref_tryget(&ctx->refs))
>>>          goto out_fput;
>>>
>>> +    if (unlikely(current->mm != ctx->sqo_mm)) {
>>> +        /*
>>> +         * The mm used to process SQEs will be current->mm or
>>> +         * ctx->sqo_mm depending on which submission path is used.
>>> +         * It's also unclear who is responsible for an SQE submitted
>>> +         * out-of-process from a security and auditing perspective.
>>> +         *
>>> +         * Until a real usecase emerges and there are clear semantics
>>> +         * for out-of-process submission, disallow it.
>>> +         */
>>> +        ret = -EACCES;
>>> +        goto out;
>>> +    }
>>> +
>>>      /*
>>>       * For SQ polling, the thread will do all submissions and completions.
>>>       * Just return the requested submit count, and wake the thread if
>>
>> That'll break postgres that already uses this, also see:
>>
>> commit 73e08e711d9c1d79fae01daed4b0e1fee5f8a275
>> Author: Jens Axboe <axboe@kernel.dk>
>> Date:   Sun Jan 26 09:53:12 2020 -0700
>>
>>     Revert "io_uring: only allow submit from owning task"
>>
>> So no, we can't do that.
>>
> 
> Yikes, I missed that.
> 
> Andres, how final is your Postgres branch?  I'm wondering if we could
> get away with requiring a special flag when creating an io_uring to
> indicate that you intend to submit IO from outside the creating mm.
> 
> Even if we can't make this change, we could plausibly get away with
> tying seccomp-style filtering to sqo_mm.  IOW we'd look up a
> hypothetical sqo_mm->io_uring_filters to filter SQEs even when
> submitted from a different mm.

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.

-- 
Jens Axboe


  reply	other threads:[~2020-07-21 17:30 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 [this message]
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
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=ba989463-c627-8af7-9234-4dc8ac4eea0e@kernel.dk \
    --to=axboe@kernel.dk \
    --cc=andres@anarazel.de \
    --cc=asml.silence@gmail.com \
    --cc=christian.brauner@ubuntu.com \
    --cc=hch@lst.de \
    --cc=io-uring@vger.kernel.org \
    --cc=jannh@google.com \
    --cc=keescook@chromium.org \
    --cc=linux-api@vger.kernel.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=luto@kernel.org \
    --cc=miklos@szeredi.hu \
    --cc=mtk.manpages@gmail.com \
    --cc=sgarzare@redhat.com \
    --cc=stefanha@redhat.com \
    --cc=strace-devel@lists.strace.io \
    --cc=willy@infradead.org \
    --subject='Re: strace of io_uring events?' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).