On Wed, Jun 22, 2016 at 12:21 PM, Michael Kerrisk (man-pages)
<mtk.manpages@xxxxxxxxx> 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
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