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

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Message-ID: <5346EE5D.2020503@amacapital.net>
Date:	Thu, 10 Apr 2014 12:17:49 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Florian Weimer <fweimer@...hat.com>,
	David Herrmann <dh.herrmann@...il.com>,
	linux-kernel@...r.kernel.org
CC:	Hugh Dickins <hughd@...gle.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Matthew Wilcox <matthew@....cx>,
	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@...aro.org, Greg Kroah-Hartman <greg@...ah.com>,
	Tejun Heo <tj@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	dri-devel@...ts.freedesktop.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...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()

On 04/08/2014 06:00 AM, Florian Weimer wrote:
> On 03/19/2014 08:06 PM, David Herrmann wrote:
> 
>> Unlike existing techniques that provide similar protection, sealing
>> allows
>> file-sharing without any trust-relationship. This is enforced by
>> rejecting seal
>> modifications if you don't own an exclusive reference to the given
>> file. So if
>> you own a file-descriptor, you can be sure that no-one besides you can
>> modify
>> the seals on the given file. This allows mapping shared files from
>> untrusted
>> parties without the fear of the file getting truncated or modified by an
>> attacker.
> 
> How do you keep these promises on network and FUSE file systems?  Surely
> there is still some trust involved for such descriptors?
> 
> What happens if you create a loop device on a sealed descriptor?
> 
> Why does memfd_create not create a file backed by a memory region in the
> current process?  Wouldn't this be a far more generic primitive?
> Creating aliases of memory regions would be interesting for many things
> (not just libffi bypassing SELinux-enforced NX restrictions :-).

If you write a patch to prevent selinux from enforcing NX, I will ack
that patch with all my might.  I don't know how far it would get me, but
I think that selinux has no business going anywhere near execmem.

Adding a clone mode to mremap might be a better bet.  But memfd solves
that problem, too, albeit messily.

--Andy
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