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

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Message-ID: <CALCETrXVzLcVVSuPMFUgoRvpQgcQNy2_rW6=11CJAME0W2GyYQ@mail.gmail.com>
Date:	Thu, 10 Apr 2014 14:16:03 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	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>,
	"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 1:49 PM, David Herrmann <dh.herrmann@...il.com> wrote:
> Hi
>
> On Thu, Apr 10, 2014 at 10:37 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> 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.
>
> For the sealing API, none of this is needed. As long as the inode is
> owned by the uid who creates the memfd, you can pass it around and
> no-one besides root and you can open /proc/self/fd/$fd (assuming chmod
> 700). If you share the fd with someone with the same uid as you,
> you're screwed anyway. We don't protect users against themselves (I
> mean, they can ptrace you, or kill()..). Therefore, I'm not really
> convinced that we want this for memfd. At least no-one has provided a
> _proper_ use-case for this so far.

Hmm.  Fair enough.

Would it make sense for the initial mode on a memfd inode to be 000?
Anyone who finds this to be problematic could use fchmod to fix it.

I might even go so far as to suggest that the default uid on the inode
should be 0 (i.e. global root), since there is the odd corner case of
root setting euid != 0, creating a memfd, and setting euid back to 0.
The latter might cause resource accounting issues, though.

--Andy
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