linux-kernel - Re: Documenting ptrace access mode checking

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Message-ID: <fb9bbc54-857a-5187-7932-add65f45c179@gmail.com>
Date:	Fri, 24 Jun 2016 11:57:43 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
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>,
	Kees Cook <keescook@...omium.org>,
	linux-security-module <linux-security-module@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: Documenting ptrace access mode checking

Hi Eric,

On 06/23/2016 09:04 PM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:
>
>> Hi Eric,
>>
>> On 06/21/2016 09:55 PM, Eric W. Biederman wrote:
>>> Hmm.
>>>
>>> When I gave this level of detail about the user namespace permission
>>> checks you gave me some flack, because it was not particularly
>>> comprehensible to the end users.  I think you deserve the same feedback.
>>>
>>> How do we say this in a way that does not describes a useful way to
>>> think about it.  I read this and I know a lot of what is going on and my
>>> mind goes numb.
>>>
>>> How about something like this:
>>>
>>>    If the callers uid and gid are the same as a processes uids and gids
>>>    and the processes is configured to allow core dumps (aka it was never
>>>    setuid or setgid) then the caller is allowed to ptrace a process.
>>>
>>>    Otherwise the caller must have CAP_SYS_PTRACE.
>>>
>>>    Linux security modules impose additional restrictions.
>>>
>>>    For consistency access to various process attributes are guarded with
>>>    the same security checks as the ptrace system call itself.  As they are
>>>    all methods to get information about a process.
>>>
>>> We certainly need something that gives a high level view so people
>>> reading the man page can know what to expect.   If you get down into the
>>> weeds we run the danger of people beginning to think they can depend
>>> upon bugs in the implementation.
>>
>> Thanks for the feedback, but I think more detail is required than you
>> suggest. (And I added all of that detail somewhat reluctantly.)
>> See my other replies for my rationale.
>
> What I saw badly missing from your description is not the level of
> detail but bring things into a form that ordinary mortals can
> understand.
>
> For an explanation to be clear I think we very much need the high level
> overview first.  Then we can expand that description with the very
> detailed view.
>
> I very much think we need to describe things in such a way that people
> understand the principles behind the permission checks, and not just
> have the documentation echo the code, so that people can know what weird
> things LSMs like yama are likely to do, and how these checks are likely
> to evolve in the future.

So, I completely agree with you, and I agree that this could be better.
At first, I understood your meaning to be that I should avoid all of the
detail, and just limit the man page to some very high level text as
you proposed. So, I think it's worth prefixing the details with some
attempt at a high-level picture. How about this as an introductory
paragraph:

        Various parts of the kernel-user-space API (not just  ptrace(2)
        operations),  require  so-called  "ptrace  access mode" checks,
        whose outcome determines whether an operation is permitted (or,
        in  a  few cases, causes a "read" operation to return sanitized
        data).  These checks are performed in cases where  one  process
        can  inspect sensitive information about, or in some cases mod‐
        ify the state of, another process.  The  checks  are  based  on
        factors  such  as  the  credentials and capabilities of the two
        processes, whether or not the "target" process is dumpable, and
        the  results  of checks performed by any enabled Linux Security
        Module (LSM)—for example, SELinux, Yama, or  Smack—and  by  the
        commoncap LSM (which is always invoked).

?

> Because one thing is clear to me.  The evolution of these details is
> clearly not done, and will continue to change in the future.

Maybe people will even write man page patches when that happens :-).

Cheers,

Michael


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