linux-kernel - Re: Documenting ptrace access mode checking

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Message-ID: <CAGXu5j+DoWCT=NwZK-yNBmZOu9LdEn=0-gjJvtPVmNJFanWoKA@mail.gmail.com>
Date:	Wed, 22 Jun 2016 14:11:38 -0700
From:	Kees Cook <keescook@...omium.org>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:	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>,
	Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: Documenting ptrace access mode checking

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;
}


-Kees



-- 
Kees Cook
Chrome OS & Brillo Security