linux-kernel - Re: [PATCH v2 2/8] exec: Move security_bprm

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Date:   Mon, 10 Jul 2017 19:07:37 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        Serge Hallyn <serge@...lyn.com>,
        John Johansen <john.johansen@...onical.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Michal Hocko <mhocko@...nel.org>,
        Ben Hutchings <ben@...adent.org.uk>,
        Hugh Dickins <hughd@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...hat.com>,
        James Morris <james.l.morris@...cle.com>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Ingo Molnar <mingo@...nel.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Paul Moore <paul@...l-moore.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Mickaël Salaün <mic@...ikod.net>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "<linux-security-module@...r.kernel.org>" 
        <linux-security-module@...r.kernel.org>,
        SE Linux <selinux@...ho.nsa.gov>
Subject: Re: [PATCH v2 2/8] exec: Move security_bprm_secureexec() earlier

On Mon, Jul 10, 2017 at 10:18 AM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Kees Cook <keescook@...omium.org> writes:
>
>> On Mon, Jul 10, 2017 at 1:57 AM, Eric W. Biederman
>> <ebiederm@...ssion.com> wrote:
>>> Kees Cook <keescook@...omium.org> writes:
>>>
>>>> There are several places where exec needs to know if a privilege-gain has
>>>> happened. These should be using the results of security_bprm_secureexec()
>>>> but it is getting (needlessly) called very late.
>>>
>>> It is hard to tell at a glance but I believe this introduces a
>>> regression.
>>>
>>> cap_bprm_set_creds is currently called before cap_bprm_secureexec and
>>> it has a number of cases such as no_new_privs and ptrace that can result
>>> in some of the precomputed credential changes not happening.
>>>
>>> Without accounting for that I believe your cap_bprm_securexec now
>>> returns a postive value too early.
>>
>> It's still before cap_bprm_secureexec. cap_brpm_set_creds() is in
>> prepare_binprm(), which is well before exec_binprm() and it's eventual
>> call to setup_new_exec().
>
> Good point.  I didn't double check and the set in the name had me
> thinking it was setting the creds on current.
>
> Is there any reason we need a second security hook?  It feels like we
> should be able to just fold the secureexec hook into the set_creds hook.
>
> The two are so interrelated I fear that having them separate only
> encourages them to diverge in trivial ways as it is easy to forget about
> some detail or other.
>
> Certainly having them called from different functions seems wrong.  If
> we know enough in prepare_binprm we know enough.

Hmmm, yes. That would let us have the secureexec-ness knowledge before
copy_strings(), in case we ever need to make that logic
secureexec-aware.

I'll dig through the LSMs to examine the set_creds hooks to see if
this could be possible.

-Kees

-- 
Kees Cook
Pixel Security