linux-kernel - Re: [PATCH v2 2/3] shm: add memfd

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Date:	Fri, 13 Jun 2014 12:42:47 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Tony Battersby <tonyb@...ernetics.com>,
	Andy Lutomirsky <luto@...capital.net>,
	Al Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Ryan Lortie <desrt@...rt.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Tejun Heo <tj@...nel.org>, Greg Kroah-Hartman <greg@...ah.com>,
	John Stultz <john.stultz@...aro.org>,
	Kristian Hogsberg <krh@...planet.net>,
	Lennart Poettering <lennart@...ttering.net>,
	Daniel Mack <zonque@...il.com>, Kay Sievers <kay@...y.org>
Subject: Re: [PATCH v2 2/3] shm: add memfd_create() syscall

Hi

On Mon, Jun 2, 2014 at 12:59 PM, Hugh Dickins <hughd@...gle.com> wrote:
> On Fri, 23 May 2014, David Herrmann wrote:
>> On Tue, May 20, 2014 at 4:20 AM, Hugh Dickins <hughd@...gle.com> wrote:
>> > But this does highlight how the "size" arg to memfd_create() is
>> > perhaps redundant.  Why give a size there, when size can be changed
>> > afterwards?  I expect your answer is that many callers want to choose
>> > the size at the beginning, and would prefer to avoid the extra call.
>> > I'm not sure if that's a good enough reason for a redundant argument.
>>
>> At one point in time we might be required to support atomic-sealing.
>> So a memfd_create() call takes the initial seals as upper 32bits in
>> "flags" and sets them before returning the object. If these seals
>> contain SEAL_GROW/SHRINK, we must pass the size during setup (think
>> CLOEXEC with fork()).
>
> That does sound like over-design to me.  You stop short of passing
> in an optional buffer of the data it's to contain, good.
>
> I think it would be a clearer interface without the size, but really
> that's an issue for the linux-api people you'll be Cc'ing next time.
>
> You say "think CLOEXEC with fork()": you have thought about this, I
> have not, please spell out for me what the atomic size guards against.
> Do you want an fd that's not shared across fork?

My thinking was:
Imagine a seal called SEAL_OPEN that prevents against open()
(specifically on /proc/self/fd/). That seal obviously has to be set
before creating the object, otherwise there's a race. Therefore, I'd
need a "seals" argument for memfd_create(). Now imagine there's a
similar seal that has such a race but prevents any following resize.
Then I'd have to set the size during initialization, too.

However, in my opinion SEAL_OPEN does not protect against any real
attack (it only protects you from yourself). Therefore, I never added
it. Furthermore, I couldn't think of any similar situation, so I now
removed the "size" argument and made "flags" just an "unsigned int".
It was just a precaution, but I'm fine with dropping it as we cannot
come up with a real possible race.

Sorry for the confusion. I'll send v3 in a minute.

Thanks
David
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