linux-kernel - Re: [PATCH 0/6] File Sealing & memfd

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Message-ID: <CANq1E4Ro8bsgp1QS_Qf9KKBAMZH+joMW5DmfyZ4OBJuTxdXCZA@mail.gmail.com>
Date:	Thu, 20 Mar 2014 16:12:54 +0100
From:	David Herrmann <dh.herrmann@...il.com>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Karol Lewandowski <k.lewandowsk@...sung.com>,
	Kay Sievers <kay@...y.org>, Daniel Mack <zonque@...il.com>,
	Lennart Poettering <lennart@...ttering.net>,
	Kristian Høgsberg <krh@...planet.net>,
	John Stultz <john.stultz@...aro.org>,
	Greg Kroah-Hartman <greg@...ah.com>, Tejun Heo <tj@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	DRI <dri-devel@...ts.freedesktop.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ryan Lortie <desrt@...rt.ca>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Subject: Re: [PATCH 0/6] File Sealing & memfd_create()

Hi

On Thu, Mar 20, 2014 at 3:41 PM, One Thousand Gnomes
<gnomes@...rguk.ukuu.org.uk> wrote:
> I think you want two things at minimum
>
> owner to seal
> root can always override

Why should root be allowed to override?

> I would query the name too. Right now your assumption is 'shmem only' but
> that might change with other future use cases or types (eg some driver
> file handles) so SHMEM_ in the fcntl might become misleading.

I'm fine with F_SET/GET_SEALS. But given you suggested requiring
MFD_ALLOW_SEALS for sealing, I don't see why we couldn't limit this
interface entirely to memfd_create().

> Whether you want some way to undo a seal without an exclusive reference as
> the file owner is another question.

No. You are never allowed to undo a seal but with an exclusive
reference. This interface was created for situations _without_ any
trust relationship. So if the owner is allowed to undo seals, the
interface doesn't make any sense. The only options I see is to not
allow un-sealing at all (which I'm fine with) or tracking users (which
is way too much overhead).

Thanks
David
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