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

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Date:	Thu, 12 Jan 2012 22:09:03 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Will Drewry <wad@...omium.org>, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.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, 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] Add PR_{GET,SET}_NO_NEW_PRIVS to prevent execve from
 granting privs

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.

-Kees

-- 
Kees Cook
ChromeOS Security
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