Re: Documenting ptrace access mode checking

[Casey Schaufler <casey@schaufler-ca.com>]

On 6/24/2016 1:40 AM, Michael Kerrisk (man-pages) wrote:
> On 06/22/2016 11:11 PM, Kees Cook wrote:
>> On Wed, Jun 22, 2016 at 12:21 PM, Michael Kerrisk (man-pages)
>> <mtk.manpages@gmail.com> wrote:
>>> On 06/21/2016 10:55 PM, Jann Horn wrote:
>>>> On Tue, Jun 21, 2016 at 11:41:16AM +0200, Michael Kerrisk (man-pages)
>>>> wrote:
>>>>>        5.  The  kernel LSM security_ptrace_access_check() interface is
>>>>>            invoked to see if ptrace access is permitted.  The  results
>>>>>            depend on the LSM.  The implementation of this interface in
>>>>>            the default LSM performs the following steps:
>>>>
>>>>
>>>> For people who are unaware of how the LSM API works, it might be good to
>>>> clarify that the commoncap LSM is *always* invoked; otherwise, it might
>>>> give the impression that using another LSM would replace it.
>>>
>>>
>>> As we can see, I am one of those who are unaware of how the LSM API
>>> works :-/.
>>>
>>>> (Also, are there other documents that refer to it as "default LSM"? I
>>>> think that that term is slightly confusing.)
>>>
>>>
>>> No, that's a terminological confusion of my own making. Fixed now.
>>>
>>> I changed this text to:
>>>
>>>        Various parts of the kernel-user-space API (not just  ptrace(2)
>>>        operations), require so-called "ptrace access mode permissions"
>>>        which are gated by any enabled Linux Security Module (LSMs)—for
>>>        example,  SELinux,  Yama, or Smack—and by the the commoncap LSM
>>>        (which is always invoked).  Prior to  Linux  2.6.27,  all  such
>>>        checks  were  of a single type.  Since Linux 2.6.27, two access
>>>        mode levels are distinguished:
>>>
>>> BTW, can you point me at the piece(s) of kernel code that show that
>>> "commoncap" is always invoked in addition to any other LSM that has
>>> been installed?
>>
>> It's not entirely obvious, but the bottom of security/commoncap.c shows:
>>
>> #ifdef CONFIG_SECURITY
>>
>> struct security_hook_list capability_hooks[] = {
>>         LSM_HOOK_INIT(capable, cap_capable),
>> ...
>> };
>>
>> void __init capability_add_hooks(void)
>> {
>>         security_add_hooks(capability_hooks, ARRAY_SIZE(capability_hooks));
>> }
>>
>> #endif
>>
>> And security/security.c shows the initialization order of the LSMs:
>>
>> int __init security_init(void)
>> {
>>         pr_info("Security Framework initialized\n");
>>
>>         /*
>>          * Load minor LSMs, with the capability module always first.
>>          */
>>         capability_add_hooks();
>>         yama_add_hooks();
>>         loadpin_add_hooks();
>>
>>         /*
>>          * Load all the remaining security modules.
>>          */
>>         do_security_initcalls();
>>
>>         return 0;
>> }
>
> So, I just want to check my understanding of a couple of points:
>
> 1. The commoncap LSM is invoked first, and if it denies access,
>    then no further LSM is/needs to be called.

Yes. The LSM infrastructure is "bail on fail".

>
> 2. Is it the case that only one of the other LSMs (SELinux, Yama,
>    AppArmor, etc.) is invoked, or can more than one be invoked.
>    I thought only one is invoked, but perhaps I am out of date
>    in my understanding.

All registered modules are invoked, but only one "major"
module can be registered. The "minor" modules show up in
security_init, while the majors come in via do_security_initcalls.

I am in the process of messing that all up with patches
allowing multiple major modules. Stay tuned.

>
> Cheers,
>
> Michael
>
>