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

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Date:	Thu, 20 Mar 2014 16:48:30 +0100
From:	David Herrmann <dh.herrmann@...il.com>
To:	tytso@....edu, 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()

Hi

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).

Furthermore, I don't see any use-case besides memfd for sealing, so I
purposely avoided changing core VFS interfaces. Protecting
page-allocation/access for SEAL_WRITE like I do in shmem.c is not that
easy to do generically. So if we moved this interface to "struct
inode", all that would change is moving "u32 seals;" from one struct
to the other. Ok, some protections might get easily implemented
generically, but I without proper insight in the underlying
implemenation, I couldn't verify all paths and possible races. Isn't
keeping the API generic enough so far? Changing the underlying
implementation can be done once we know what we want.

Thanks
David
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