Re: [PATCH] exit: Retain nsproxy for exit_task_work() work entries

From: Tejun Heo <tj@kernel.org>
To: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
	"Michal Koutný" <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
Date: Wed, 8 Dec 2021 09:49:17 -1000	[thread overview]
Message-ID: <YbEMPal0sKkk0+Tl@slm.duckdns.org> (raw)
In-Reply-To: <CAHk-=wjcWEYSEVKvowUA0yEeDM279Zg-ptM_SsCMxmRSPJHjAw@mail.gmail.com>

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