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

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Date:	Thu, 20 Mar 2014 14:41:27 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	David Herrmann <dh.herrmann@...il.com>
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()

> My first idea was to add MFD_ALLOW_SEALING as memfd_create() flag,
> which enables the sealing-API for that file. Then I looked at POSIX

This actually seems the most sensible to me. The reason being that if I
have some existing used object there is no way on earth I can be sure who
has existing references to it, and we don't have revoke() to fix that.

So it pretty much has to be a new object in a sane programming model.

> mandatory locking and noticed that it provides similar restrictions on
> _all_ files. Mandatory locks can be more easily removed, but an

The fact someone got it past a standards body doesn't make it a good idea.

> attacker could just re-apply them in a loop, so that's not really an
> argument. Furthermore, sealing requires _write_ access so I wonder
> what kind of DoS attacks are possible with sealing that are not
> already possible with write access? And sealing is only possible if no
> writable, shared mapping exists. So even if an attacker seals a file,
> all that happens is EPERM, not SIGBUS (still a possible
> denial-of-service scenario).

I think you want two things at minimum

owner to seal
root can always 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.

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

Alan
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