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

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Message-ID: <4F0FDCF2.6090100@canonical.com>
Date:	Fri, 13 Jan 2012 08:27:46 +0100
From:	John Johansen <john.johansen@...onical.com>
To:	Kees Cook <keescook@...omium.org>
CC:	Andy Lutomirski <luto@...capital.net>,
	Will Drewry <wad@...omium.org>, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, serge.hallyn@...onical.com,
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	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, 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,
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	mhalcrow@...gle.com, dlaor@...hat.com, corbet@....net,
	alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH] Add PR_{GET,SET}_NO_NEW_PRIVS to prevent execve from
 granting privs

On 01/13/2012 07:09 AM, Kees Cook wrote:
> On Thu, Jan 12, 2012 at 10:02 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Thu, Jan 12, 2012 at 9:58 PM, Kees Cook <keescook@...omium.org> wrote:
>>> On Thu, Jan 12, 2012 at 6:12 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>>> With this set, a lot of dangerous operations (chroot, unshare, etc)
>>>> become a lot less dangerous because there is no possibility of
>>>> subverting privileged binaries.
>>>>
>>>> This patch completely breaks apparmor.  Someone who understands (and
>>>> uses) apparmor should fix it or at least give me a hint.
>>>>
>>>> Signed-off-by: Andy Lutomirski <luto@...capital.net>
>>>> [....]
>>>> diff --git a/security/apparmor/domain.c b/security/apparmor/domain.c
>>>> index c1e18ba..7f480b7 100644
>>>> --- a/security/apparmor/domain.c
>>>> +++ b/security/apparmor/domain.c
>>>> @@ -360,6 +360,9 @@ int apparmor_bprm_set_creds(struct linux_binprm *bprm)
>>>>        if (bprm->cred_prepared)
>>>>                return 0;
>>>>
>>>> +       /* XXX: someone who understands apparmor needs to fix this. */
>>>> +       BUG_ON(bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS);
>>>> +
>>>>        cxt = bprm->cred->security;
>>>>        BUG_ON(!cxt);
>>>>
>>>
>>> Since apparmor_bprm_set_creds() calls cap_bprm_set_creds() already[1],
>>> I think AppArmor needs no changes at all, but John will know better.
>>> :)
>>
>> I think that AppArmor determines what a program is allowed to do by
>> looking at the path of the executable.  We don't want newly-executed
>> programs to gain permissions because they're a different executable
>> when we're in no_new_privs mode, so (if I'm right) something different
>> needs to happen.
> 
> I'll have to go look more closely. I thought cap_bprm_set_creds() was
> already evaluating the new privs and blocking any gained privs with
> the changes you were making.
> 
We do want to do something more.  A first pass at it would be to allow execs
that inherit the current context, and we will also want to reject apparmor's
equiv of setcon, and setexeccon, at the interface.
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