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

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Date:	Tue, 21 Apr 2015 16:47:42 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Michal Hocko <mhocko@...e.cz>
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

Hi

On Tue, Apr 21, 2015 at 4:27 PM, Michal Hocko <mhocko@...e.cz> wrote:
> On Tue 21-04-15 16:01:01, David Herrmann wrote:
>> On Tue, Apr 21, 2015 at 2:20 PM, Michal Hocko <mhocko@...e.cz> wrote:
>> > 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?
>>
>> ..as you mentioned memcg, lets figure that out here. shmem buffers are
>> used as receive-buffers by kdbus peers. They are read-only to
>> user-space. All allocations are done by the kernel on message passing.
>
> OK, so the shmem buffer is allocated on the kernels behalf and under
> its control and no userspace can hand over one to kdbus. Do I get
> it right? If yes then the memcg escape I was describing above is
> not possible of course. This wasn't clear to me from the previous
> discussion. Thanks for the clarification!

Exactly. Much appreciated!

Thanks
David
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