linux-kernel - Re: [PATCH] Add PR_{GET,SET}_NO_NEW_PRIVS to prevent execve from granting privs

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Date:	Sat, 14 Jan 2012 13:55:49 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Oleg Nesterov <oleg@...hat.com>,
	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	keescook@...omium.org, john.johansen@...onical.com,
	serge.hallyn@...onical.com, coreyb@...ux.vnet.ibm.com,
	pmoore@...hat.com, eparis@...hat.com, djm@...drot.org,
	segoon@...nwall.com, rostedt@...dmis.org, jmorris@...ei.org,
	scarybeasts@...il.com, avi@...hat.com, penberg@...helsinki.fi,
	viro@...iv.linux.org.uk, luto@....edu, mingo@...e.hu,
	akpm@...ux-foundation.org, khilman@...com, borislav.petkov@....com,
	amwang@...hat.com, ak@...ux.intel.com, eric.dumazet@...il.com,
	gregkh@...e.de, dhowells@...hat.com, daniel.lezcano@...e.fr,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com, corbet@....net
Subject: Re: [PATCH] Add PR_{GET,SET}_NO_NEW_PRIVS to prevent execve from
 granting privs

Andy Lutomirski wrote:
> > So I bet the google chrome people are not at all interested in
> > "running random binaries", and might want execve() very much for
> > "running some specific binaries that we ship with or install from the
> > browser".
> 
> I want to be able to run "magic_seccomp_sandbox gv somefile.ps".  And
> I want that to still work even if my distro gets fancy and uses
> selinux policy to prevent gv from accessing the network (which would
> not be an unreasonable thing to do).  The chromium project already has
> a little seccomp wrapper that can (sort of) do this.

Chrome also comes with NaCL.  They *are* "running random binaries",
but they use code analysis to check the code first.  Maybe because
seccomp wasn't good enough ;-)

> > So I really think that the *only* valid model is the "fail the execve
> > on any changes", not the "mnt-nosuid" approach of trying to run things
> > with the wrong permissions and get perhaps odd results. And I think it
> > works even - and perhaps *especially* - in models like selinux or
> > apparmor that do have a lot of "implicit MAC knowledge" on specific
> > binaries.
> 
> Is the current exec_no_trans check enough for you?  With my patch,
> selinux can already block the execve if it wants.  (The policy is the
> same as it would be if a program explicitly asked to run the new
> executable with an unchanged security context.)  I'd be happy to fail
> the exec in AppArmor, and then maybe AppArmor will change its mind
> if/when users get annoyed :)

Does SELinux block if userspace does exec entirely in userspace using
mmap() and not execve()?  If not, why is execve() allowed to be different?

-- Jamie
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