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Date: Tue, 21 Apr 2015 16:47:42 +0200
Message-ID: <CANq1E4RmWkB04vy9QHF2oMFVN8Hcb8_Wfhg+cby2KyG3uR4pbg@mail.gmail.com>
Subject: Re: [GIT PULL] kdbus for 4.1-rc1
From: David Herrmann <dh.herrmann@gmail.com>
To: Michal Hocko <mhocko@suse.cz>
Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
        Andy Lutomirski <luto@amacapital.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Richard Weinberger <richard@nod.at>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Jiri Kosina <jkosina@suse.cz>,
        Al Viro <viro@zeniv.linux.org.uk>, Borislav Petkov <bp@alien8.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Tom Gundersen <teg@jklm.no>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Daniel Mack <daniel@zonque.org>, Djalal Harouni <tixxdz@opendz.org>
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Hi

On Tue, Apr 21, 2015 at 4:27 PM, Michal Hocko <mhocko@suse.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@suse.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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