linux-kernel - Re: Issues with capability bits and meta-data in kdbus

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Date:	Wed, 22 Apr 2015 11:45:38 +0100
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>, Tom Gundersen <teg@...m.no>,
	Jiri Kosina <jkosina@...e.cz>,
	Andy Lutomirski <luto@...capital.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Daniel Mack <daniel@...que.org>,
	David Herrmann <dh.herrmann@...il.com>,
	Djalal Harouni <tixxdz@...ndz.org>
Subject: Re: Issues with capability bits and meta-data in kdbus

> > - Access to the capability bits is guarded with PTRACE_MAY_READ
> >   kdbus does not honor that and thus leaks information.
> 
> Now, this is likely not a real problem.
> 
> Yes, when you try to read other processes capabilities, you need
> PTRACE_MAY_READ to see them. HOWEVER, that's not really what a kdbus
> message would do - it doesn't "read somebody elses capabilities". When
> you do a kdbus write, you export your *own* capabilities. If you don't
> want others to know what privileges you have, then you shouldn't be
> using kdbus.

That's broken but fixable.

It should not share any capability information *unless* you pass a flag
which says "flash my security badges around".

That fails safe (descriptor passed to another process), and gives a
default behaviour which is non surprising, non leaky and useful for
general purposes. This is also mirroring AF_LOCAL/AF_UNIX where you have
to choose to wave your bits in public.

(again its showing that kdbus really should be done by adding multicast
reliable delivery to AF_LOCAL sockets)

> So I think that one is a real and serious bug. But the other
> complaints seem to be off the mark. It seems quite reasonable to me to
> say that a recipient should be able to distinguish between *root*
> sending it a dbus message to take down the system, and some random
> luser doing the same.

Agreed but there are better ways to do this including opening some
kind of capability object and passing it as proof.

Also do I need to be root when I send the message or root when you ask ...


Alan
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