linux-kernel - Re: [PATCH v4 2/5] userfaultfd: add /dev/userfaultfd for fine grained access control

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Message-ID: <CAJHvVcjs_=vbUbXcm1_vAxatEu9inqkVo_geX7pcW1XqWF=gJw@mail.gmail.com>
Date:   Wed, 20 Jul 2022 13:10:13 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Nadav Amit <namit@...are.com>
Cc:     Peter Xu <peterx@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Dmitry V . Levin" <ldv@...linux.org>,
        Gleb Fotengauer-Malinovskiy <glebfm@...linux.org>,
        Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>, Shuah Khan <shuah@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        zhangyi <yi.zhang@...wei.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v4 2/5] userfaultfd: add /dev/userfaultfd for fine grained
 access control

On Wed, Jul 20, 2022 at 10:42 AM Nadav Amit <namit@...are.com> wrote:
>
> On Jul 19, 2022, at 7:32 PM, Peter Xu <peterx@...hat.com> wrote:
>
> > ⚠ External Email
> >
> > On Tue, Jul 19, 2022 at 11:55:21PM +0000, Nadav Amit wrote:
> >> Anyhow, I do want to clarify a bit about the “cross-process support”
> >> userfaultfd situation. Basically, you can already get cross-process support
> >> today, by using calling userfaultfd() on the controlled process and calling
> >> pidfd_open() from another process. It does work and I do not remember any
> >> issues that it introduced (in contrast, for instance, to io-uring, that
> >> would break if you use userfaultfd+iouring+fork today).
> >
> > Do you mean to base it on pidof_getfd()?
>
> autocorrect? :)
>
> I did refer to pidfd_getfd() as a syscall that can be used today by one
> process to control the address space of another process. I did not intend to
> use it for the actual implementation.
>
> > Just want to mention that this will still need collaboration of the target
> > process as userfaultfd needs to be created explicitly there.  From that POV
> > it's still more similar to general SCM_RIGHTS trick to pass over the fd but
> > just to pass it in a different way.
>
> There are also some tricks you can do with ptrace in order not to need the
> collaboration, but they are admittedly fragile.
>
> > IMHO the core change about having /proc/pid/userfaultfd is skipping that
> > only last step to create the handle.
>
> Yes. The point that I was trying to make is that there are no open issues
> with adding support for remote process control through
> /proc/pid/userfaultfd. This is in contrast, for example, for using io-uring
> with userfaultfd. For instance, if you try to use io-uring TODAY with
> userfaultfd (without the async support that I need to add), and you try to
> monitor the fork event, things would break (the new userfaultfd file
> descriptor after fork would be installed on the io-worker thread).
>
> This is all to say that it is really simple to add support for one process
> monitoring userfaultfd of another process, since I understood that Axel had
> concerned that this might be utterly broken…

Mostly I was worried it would be nontrivial to implement, and it isn't
a use case I plan to use so I was hoping to ignore it and defer it to
some future patches. ;)

But, if it "just works" I'm happy to include it in v5.