linux-kernel - Re: [PATCH] exit: Retain nsproxy for exit_task

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Message-ID: <YbEMPal0sKkk0+Tl@slm.duckdns.org>
Date:   Wed, 8 Dec 2021 09:49:17 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Linus Torvalds <torvalds@...uxfoundation.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Michal Koutný <mkoutny@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Kees Cook <keescook@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jim Newsome <jnewsome@...project.org>,
        Alexey Gladkov <legion@...nel.org>,
        Security Officers <security@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] exit: Retain nsproxy for exit_task_work() work entries

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