linux-kernel - Re: [PATCH PLACEHOLDER 1/3] fs/exec: "always

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Date:	Sun, 15 Jan 2012 12:59:41 -0800
From:	Andrew Lutomirski <luto@....edu>
To:	Casey Schaufler <casey@...aufler-ca.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jamie Lokier <jamie@...reable.org>,
	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, mingo@...e.hu, akpm@...ux-foundation.org,
	khilman@...com, borislav.petkov@....com, amwang@...hat.com,
	oleg@...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,
	alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH PLACEHOLDER 1/3] fs/exec: "always_unprivileged" patch

On Sun, Jan 15, 2012 at 12:16 PM, Casey Schaufler
<casey@...aufler-ca.com> wrote:
> On 1/14/2012 12:22 PM, Linus Torvalds wrote:
>>
>> And yes, I really seriously do believe that is both safer and simpler
>> than some model that says "you can drop stuff", and then you have to
>> start making up rules for what "dropping" means.
>>
>> Does "dropping" mean allowing setuid(geteuid()) for example? That *is*
>> dropping the uid in a _POSIX_SAVED_IDS environment. And I'm saying
>> that no, we should not even allow that. It's simply all too "subtle".
>
>
> I am casting my two cents worth behind Linus. Dropping
> privilege can be every bit as dangerous as granting privilege
> in the real world of atrocious user land code. Especially in
> the case of security policy enforcing user land code.

Can you think of *any* plausible attack that is possible with my patch
(i.e. no_new_privs allows setuid, setresuid, and capset) that would be
prevented or even mitigated if I blocked those syscalls?  I can't.
(The sendmail-style attack is impossible with no_new_privs.)

Also, how would you even block setuid(2) in a non-confusing manner?
The semantics and error returns are already such a disaster than it's
barely worth it for anything to check the return value.

>
> This even more important in environments that support fine
> granularity of privilege, including capabilities and SELinux.
> Under SELinux a domain transition can increase, decrease or
> completely change a process' access rights and there is really
> no way for the kernel to tell which it is because that's all
> encoded in the arbitrary SELinux policy. Smack does not try
> to maintain a notion of hierarchy of privilege, so the notion
> of any change being equivalent to any other is in line with
> the Smack philosophy.
>

My patch does not (barring bugs) allow selinux domain transitions.  I
certainly think that all security transitions that vary across
distributions should be blocked by no_new_privs.

--Andy
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