Sender: Tejun Heo <htejun@gmail.com>
Date:   Wed, 8 Dec 2021 09:49:17 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Linus Torvalds <torvalds@linuxfoundation.org>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>,
        Kees Cook <keescook@chromium.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jim Newsome <jnewsome@torproject.org>,
        Alexey Gladkov <legion@kernel.org>,
        Security Officers <security@kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH] exit: Retain nsproxy for exit_task_work() work entries
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Hello,

On Wed, Dec 08, 2021 at 11:39:43AM -0800, Linus Torvalds wrote:
>  (b) alternatively, go ahead and do the permission check at IO time,
> but do it using "file->f_cred" (ie the open-time permission), not the
> current process ones.
> 
> In the above, (a) and (b) are basically the same: it uses f_cred for
> permission checking. The only difference is that in (a) you may be
> using some function that _technically_ uses the implicit "current
> credentials" (there are many of them), and you just separately make
> sure that those current credentials are identical to what they were at
> open time.
> 
> Obviously (b) is hugely preferred, but sometimes the code organization
> (ie "file or f_cred just isn't passed down deep enough") means that
> (a) might be the only realistic option.
> 
> IOW, it's not *wrong* to do permission checking at IO time, but it
> absolutely needs to be done using the open-time credentials.

Yeah, (b) sounds good to me. Will look into it.

Thanks.

-- 
tejun