Re: Documenting ptrace access mode checking

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

On 06/24/2016 05:18 PM, Casey Schaufler wrote:
>
>
> 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.

Thanks for the info, Casey.

Cheers,

Michael



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/