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

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Date:	Thu, 10 Apr 2014 13:37:26 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Theodore Ts'o" <tytso@....edu>,
	Andy Lutomirski <luto@...capital.net>,
	David Herrmann <dh.herrmann@...il.com>,
	linux-kernel <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>,
	John Stultz <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" <dri-devel@...ts.freedesktop.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-mm <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 Thu, Apr 10, 2014 at 1:32 PM, Theodore Ts'o <tytso@....edu> wrote:
> On Thu, Apr 10, 2014 at 12:14:27PM -0700, Andy Lutomirski wrote:
>>
>> This is the second time in a week that someone has asked for a way to
>> have a struct file (or struct inode or whatever) that can't be reopened
>> through /proc/pid/fd.  This should be quite easy to implement as a
>> separate feature.
>
> What I suggested on a different thread was to add the following new
> file descriptor flags, to join FD_CLOEXEC, which would be maniuplated
> using the F_GETFD and F_SETFD fcntl commands:
>
> FD_NOPROCFS     disallow being able to open the inode via /proc/<pid>/fd
>
> FD_NOPASSFD     disallow being able to pass the fd via a unix domain socket
>
> FD_LOCKFLAGS    if this bit is set, disallow any further changes of FD_CLOEXEC,
>                 FD_NOPROCFS, FD_NOPASSFD, and FD_LOCKFLAGS flags.
>
> Regardless of what else we might need to meet the use case for the
> proposed File Sealing API, I think this is a useful feature that could
> be used in many other contexts besides just the proposed
> memfd_create() use case.

It occurs to me that, before going nuts with these kinds of flags, it
may pay to just try to fix the /proc/self/fd issue for real -- we
could just make open("/proc/self/fd/3", O_RDWR) fail if fd 3 is
read-only.  That may be enough for the file sealing thing.

--Andy
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