linux-kernel - Re: [RFC 2/2] prctl: PR_SET_MM -- Introduce PR_SET_MM

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Date:	Thu, 24 Jul 2014 20:42:26 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Andrew Vagin <avagin@...allels.com>
Cc:	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Andrew Vagin <avagin@...nvz.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Julien Tinnes <jln@...gle.com>
Subject: Re: [RFC 2/2] prctl: PR_SET_MM -- Introduce PR_SET_MM_MAP operation

On Thu, Jul 24, 2014 at 05:48:28PM +0400, Andrew Vagin wrote:
> On Tue, Jul 22, 2014 at 01:07:51PM -0700, Kees Cook wrote:
> > >  - @exe_fd is referred from /proc/$pid/exe and when generating
> > >    coredump. We uses prctl_set_mm_exe_file_locked helper to update
> > >    this member, so exe-file link modification remains one-shot
> > >    action.
> > 
> > Controlling exe_fd without privileges may turn out to be dangerous. At
> > least things like tomoyo examine it for making policy decisions (see
> > tomoyo_manager()).
> >
> 
> We don't want to reduce security. How can we get a process with a
> target exe link, which executes our code?
> 
> We can execute the target file and attach to it with ptrace. ptrace
> allows to inject and execute any code.
> 
> So if we are sure that we are able to do a previous scenario, we can
> safely change exe-link, can't we?
> 
> prctl already has a check of permissions to execute the target file.
> If we execute a file. What can prevent us to attach to the process with ptrace?
> 
> The file can have a suid bit, so after executing it we may lose ability
> to attach to it. To check that we can check that uid and gid is zero
> in a current userns (local root).
> 
> What else do we need to check?

Good question. I suppose plain check for local root should be enough.
Guys, I'm about to send a new series for review. Please take a look
once time permit.
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