linux-kernel - Re: Documenting ptrace access mode checking

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Message-ID: <871t3nka3c.fsf@x220.int.ebiederm.org>
Date:	Thu, 23 Jun 2016 13:56:39 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Michael Kerrisk \(man-pages\)" <mtk.manpages@...il.com>
Cc:	Oleg Nesterov <oleg@...hat.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>
Subject: Re: Documenting ptrace access mode checking

"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:

> Hi Oleg,
>
> On 06/22/2016 11:51 PM, Oleg Nesterov wrote:
>> On 06/21, Eric W. Biederman wrote:
>>>
>>> Adding Oleg just because he seems to do most of the ptrace related
>>> maintenance these days.
>>
>> so I have to admit that I never even tried to actually understand
>> ptrace_may_access ;)
>>
>>> 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.
>>
>> Personally I agree. I think "man ptrace" shouldn't not tell too much
>> about kernel internals.
>
> See my other replies on this topic. Somehow, we need a way of
> describing the behavior that user-space sees. I think it's
> inevitable that that means talking about what;s going on
> "under the hood".
>
> Regarding Eric's point that "we run the danger of people beginning
> to think they can depend upon bugs in the implementation": when it
> comes to breaking the ABI, the presence or absence of documentation
> doesn't save us on that point (Linus has a few times made his position
> wrt to documentation clear).

Which are interesting in this respect as a bug in the implementation
that is a security issue can and will be changed, even if userspace
breaks.  Breaking userspace is not desirable but when there is no other
reasonable choice it will happen.

Eric