linux-kernel - Re: [GIT PULL] kdbus for 4.1-rc1

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Date:	Tue, 21 Apr 2015 14:20:31 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Andy Lutomirski <luto@...capital.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Richard Weinberger <richard@....at>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Al Viro <viro@...iv.linux.org.uk>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tom Gundersen <teg@...m.no>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Daniel Mack <daniel@...que.org>,
	Djalal Harouni <tixxdz@...ndz.org>
Subject: Re: [GIT PULL] kdbus for 4.1-rc1

On Tue 21-04-15 12:17:49, David Herrmann wrote:
> Hi
> 
> On Tue, Apr 21, 2015 at 11:35 AM, One Thousand Gnomes
> <gnomes@...rguk.ukuu.org.uk> wrote:
> >> On top of that, I think that someone into resource management needs to
> >> seriously consider whether having a broadcast send do get_user_pages
> >> or the equivalent on pages supplied by untrusted recipients (plural!)
> >> is a good idea.
> >
> > Oh but its so much fun if you pass pages belonging to a device driver, or
> > pass bits of a GEM object thereby keeping entire graphics textures
> > referenced 8)
> 
> We do not use GUP, nor do we pass around pinned pages. All we use is
> __vfs_read() / __vfs_write() on shmem. Whether generic_file_write() /
> copy_from_user() internally relies on GUP or not, is an orthogonal
> issue that does not belong here.

It kind of does AFAIU. If for nothing else then the memcg reasons mentioned in
other email (http://marc.info/?l=linux-kernel&m=142953380508188). If an
untrusted user is allowed to hand over a shmem backed buffer which hasn't
been charged yet (read faulted in) and then kdbus forced to fault it in
a different user's context then you basically allow to hide memory
allocations from the memcg. That is a clear show stopper.

Or have I misunderstood the way how shmem buffers are used here?

-- 
Michal Hocko
SUSE Labs
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