Re: [PATCH v2 2/8] exec: Move security_bprm_secureexec() earlier

[Kees Cook]

On Mon, Jul 10, 2017 at 10:18 AM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
> Kees Cook <keescook@xxxxxxxxxxxx> writes:
>
>> On Mon, Jul 10, 2017 at 1:57 AM, Eric W. Biederman
>> <ebiederm@xxxxxxxxxxxx> wrote:
>>> Kees Cook <keescook@xxxxxxxxxxxx> 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