linux-kernel - Re: scope of cred_guard

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Date:   Fri, 7 Apr 2017 15:07:24 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Aleksa Sarai <asarai@...e.com>,
        Andy Lutomirski <luto@...capital.net>,
        Attila Fazekas <afazekas@...hat.com>,
        Jann Horn <jann@...jh.net>, Michal Hocko <mhocko@...nel.org>,
        Ulrich Obergfell <uobergfe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Eugene Syromiatnikov <esyr@...hat.com>
Subject: Re: scope of cred_guard_mutex.

On Thu, Apr 6, 2017 at 8:55 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> And this makes me think again that we need to restart this discusion with
> more CC's.

I'm a fan of that; I've not been able to follow this thread as it
seems to have gone far from the original deadlock problem. :) I've
seen issues with ptrace, zombies, and now exec. I'm lost. :P

>> Partly I object because your understanding and my understanding of
>> cred_guard_mutex are very different.
>>
>> As I read and understand the code the job of cred_guard_mutex is to keep
>> ptrace (and other threads of the proccess) from interferring in
>> exec and to ensure old resources are accessed with permission checks
>> using our original credentials and that new and modified resources are
>> accessed with permission checks using our new credentials.
>
> Yes, this is clear.

Maybe stupid idea: can we get a patch that just adds this kind of
documentation somewhere in the source? If we can agree on the purpose
of cred_guard_mutex, and get it into the code, that seems like a good
step in discussion...

-Kees

-- 
Kees Cook
Pixel Security