linux-kernel - Re: [PATCH v3 0/7] File Sealing & memfd

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Date:	Tue, 17 Jun 2014 11:54:17 +0200
From:	Florian Weimer <fweimer@...hat.com>
To:	David Herrmann <dh.herrmann@...il.com>,
	Andy Lutomirski <luto@...capital.net>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	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@...ck.org" <linux-mm@...ck.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Greg Kroah-Hartman <greg@...ah.com>,
	John Stultz <john.stultz@...aro.org>,
	Lennart Poettering <lennart@...ttering.net>,
	Daniel Mack <zonque@...il.com>, Kay Sievers <kay@...y.org>,
	Hugh Dickins <hughd@...gle.com>,
	Tony Battersby <tonyb@...ernetics.com>
Subject: Re: [PATCH v3 0/7] File Sealing & memfd_create()

On 06/13/2014 05:33 PM, David Herrmann wrote:
> On Fri, Jun 13, 2014 at 5:17 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> Isn't the point of SEAL_SHRINK to allow servers to mmap and read
>> safely without worrying about SIGBUS?
>
> No, I don't think so.
> The point of SEAL_SHRINK is to prevent a file from shrinking. SIGBUS
> is an effect, not a cause. It's only a coincidence that "OOM during
> reads" and "reading beyond file-boundaries" has the same effect:
> SIGBUS.
> We only protect against reading beyond file-boundaries due to
> shrinking. Therefore, OOM-SIGBUS is unrelated to SEAL_SHRINK.
>
> Anyone dealing with mmap() _has_ to use mlock() to protect against
> OOM-SIGBUS. Making SEAL_SHRINK protect against OOM-SIGBUS would be
> redundant, because you can achieve the same with SEAL_SHRINK+mlock().

I don't think this is what potential users expect because mlock requires 
capabilities which are not available to them.

A couple of weeks ago, sealing was to be applied to anonymous shared 
memory.  Has this changed?  Why should *reading* it trigger OOM?

-- 
Florian Weimer / Red Hat Product Security Team
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