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

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Date:	Thu, 20 Mar 2014 12:38:06 -0400
From:	tytso@....edu
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	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, Mar 20, 2014 at 04:48:30PM +0100, David Herrmann wrote:
> On Thu, Mar 20, 2014 at 4:32 PM,  <tytso@....edu> wrote:
> > Why not make sealing an attribute of the "struct file", and enforce it
> > at the VFS layer?  That way all file system objects would have access
> > to sealing interface, and for memfd_shmem, you can't get another
> > struct file pointing at the object, the security properties would be
> > identical.
> 
> Sealing as introduced here is an inode-attribute, not "struct file".
> This is intentional. For instance, a gfx-client can get a read-only FD
> via /proc/self/fd/ and pass it to the compositor so it can never
> overwrite the contents (unless the compositor has write-access to the
> inode itself, in which case it can just re-open it read-write).

Hmm, good point.  I had forgotten about the /proc/self/fd hole.
Hmm... what if we have a SEAL_PROC which forces the permissions of
/proc/self/fd to be 000?

So if it is a property of the attribute, SEAL_WRITE and SEAL_GROW is
basically equivalent to using chattr to set the immutable and
append-only attribute, except for the "you can't undo the seal unless
you have exclusive access to the inode" magic.

That does make it pretty memfd_create specific and not a very general
API, which is a little unfortunate; hence why I'm trying to explore
ways of making a bit more generic and hopefully useful for more use
cases.

Cheers,

					- Ted
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