linux-kernel - Re: [PATCH v2 0/2] mm/memfd: add ioctl(MEMFD_CHECK_IF

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Message-ID: <CAAi7L5f1f3gHGr_8cCv2jW2Q5pjL9bpDD_4zrpXjXkoRmu95fg@mail.gmail.com>
Date:   Fri, 8 Sep 2023 23:55:52 +0200
From:   Michał Cłapiński <mclapinski@...gle.com>
To:     Jonathan Corbet <corbet@....net>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>, Shuah Khan <shuah@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, Yi Liu <yi.l.liu@...el.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Steve French <stfrench@...rosoft.com>,
        Simon Ser <contact@...rsion.fr>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Marc Dionne <marc.dionne@...istor.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        David Howells <dhowells@...hat.com>,
        Luca Vizzarro <Luca.Vizzarro@....com>,
        Jeff Xu <jeffxu@...gle.com>, Aleksa Sarai <cyphar@...har.com>,
        Kees Cook <keescook@...omium.org>,
        Daniel Verkamp <dverkamp@...omium.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 0/2] mm/memfd: add ioctl(MEMFD_CHECK_IF_ORIGINAL)

On Fri, Sep 8, 2023 at 10:34 PM Jonathan Corbet <corbet@....net> wrote:
>
> Michal Clapinski <mclapinski@...gle.com> writes:
>
> > This change introduces a way to check if an fd points to a memfd's
> > original open fd (the one created by memfd_create).
> >
> > We encountered an issue with migrating memfds in CRIU (checkpoint
> > restore in userspace - it migrates running processes between
> > machines). Imagine a scenario:
> > 1. Create a memfd. By default it's open with O_RDWR and yet one can
> > exec() to it (unlike with regular files, where one would get ETXTBSY).
> > 2. Reopen that memfd with O_RDWR via /proc/self/fd/<fd>.
> >
> > Now those 2 fds are indistinguishable from userspace. You can't exec()
> > to either of them (since the reopen incremented inode->i_writecount)
> > and their /proc/self/fdinfo/ are exactly the same. Unfortunately they
> > are not the same. If you close the second one, the first one becomes
> > exec()able again. If you close the first one, the other doesn't become
> > exec()able. Therefore during migration it does matter which is recreated
> > first and which is reopened but there is no way for CRIU to tell which
> > was first.
>
> So please bear with me...I'll confess that I don't fully understand the
> situation here, so this is probably a dumb question.
>
> It seems like you are adding this "original open" test as a way of
> working around a quirk with the behavior of subsequent opens.  I don't
> *think* that this is part of the intended, documented behavior of
> memfds, it's just something that happens.  You're exposing an artifact
> of the current implementation.

I don't know if the exec()ability of the original memfd was intended,
let alone the non-exec()ability of subsequent opens. But otherwise
yes.

> Given that the two file descriptors are otherwise indistinguishable,
> might a better fix be to make them indistinguishable in this regard as
> well?  Is there a good reason why the second fd doesn't become
> exec()able in this scenario and, if not, perhaps that behavior could be
> changed instead?

It probably could be changed, yes. But I'm worried that would be
broadening the bug that is the exec()ability of memfds. AFAIK no other
fd that is opened as writable can be exec()ed. If maintainers would
prefer this, I could do this.