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Message-ID: <91afccbb-7ad0-040c-4171-ffacf98002d1@gmail.com>
Date: Sat, 25 Jun 2016 09:21:45 +0200
From: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To: Casey Schaufler <casey@...aufler-ca.com>,
Kees Cook <keescook@...omium.org>
Cc: mtk.manpages@...il.com, Jann Horn <jann@...jh.net>,
James Morris <jmorris@...ei.org>,
linux-man <linux-man@...r.kernel.org>,
Stephen Smalley <sds@...ho.nsa.gov>,
lkml <linux-kernel@...r.kernel.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
linux-security-module <linux-security-module@...r.kernel.org>,
Linux API <linux-api@...r.kernel.org>
Subject: Re: Documenting ptrace access mode checking
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@...il.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/
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