linux-kernel - Re: kdbus: add code for buses, domains and endpoints

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Date:	Thu, 30 Oct 2014 07:58:04 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Djalal Harouni <tixxdz@...ndz.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>,
	Arnd Bergmann <arnd@...db.de>, Tejun Heo <tj@...nel.org>,
	Marcel Holtmann <marcel@...tmann.org>,
	Ryan Lortie <desrt@...rt.ca>,
	Bastien Nocera <hadess@...ess.net>,
	David Herrmann <dh.herrmann@...il.com>,
	Simon McVittie <simon.mcvittie@...labora.co.uk>,
	Daniel Mack <daniel@...que.org>,
	"alban.crequy" <alban.crequy@...labora.co.uk>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	Tom Gundersen <teg@...m.no>
Subject: Re: kdbus: add code for buses, domains and endpoints

On Thu, Oct 30, 2014 at 7:48 AM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> On Thu, Oct 30, 2014 at 05:15:04AM -0700, Eric W. Biederman wrote:
>> Djalal Harouni <tixxdz@...ndz.org> writes:
>> What others are doing makes it very hard to safely use allow those
>> ioctls in a tightly sandboxed application, as it is unpredictable
>> what the sandboxed ioctl can do with the file descriptor.
>>
>> Further an application that calls setresuid at different times during
>> it's application will behave differently.  Which makes ioctls that do
>> not have consistent behavior after open time inappropriate for use in
>> userspace libraries.
> We are consistent in our checks, you say that the application will
> behave differently when it calls setresuid() sure! If it changes its
> creds then regain of course it will behave differently! and the checks
> are here to make sure that setresuid() and alike work correctly when the
> application changes its creds and calls-in.
>

Except that it isn't consistent.

If I open a postgresql socket that wants me to be root and then I drop
privileges, I can keep talking to postresql.  This is a good thing,
because it means that I can keep talking to postgresql but I lose my
privilege to do other things.

The new kdbus model breaks this.  If I start as root and drop
privileges to UID_PRIVSEP, then my attempts to communicate over
already-open connections shouldn't consider UID_PRIVSEP.  In the, they
shouldn't tell the other endpoints that UID_PRIVSEP exists at all
unless I've explicitly asked the kernel for this behavior.

I suggest reading up on the object capability model.  Linux isn't one,
but large deviations (like kdbus') from an object capability model are
rarely a good thing.

--Andy
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