Re: Documenting ptrace access mode checking

[Michael Kerrisk (man-pages)]

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@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)âfor
> >        example,  SELinux,  Yama, or Smackâand 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.

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.

Cheers,

Michael


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