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

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Date:   Thu, 9 Dec 2021 15:08:26 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Tejun Heo <tj@...nel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.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

On Thu, Dec 09, 2021 at 02:44:19PM +0100, Michal Koutný wrote:
> On Wed, Dec 08, 2021 at 01:07:54PM -1000, Tejun Heo <tj@...nel.org> wrote:
> 
> > +	saved_cred = override_creds(of->file->f_cred);
> >  	ret = cgroup_attach_permissions(src_cgrp, dst_cgrp,
> > -					of->file->f_path.dentry->d_sb, threadgroup);
> > +					of->file->f_path.dentry->d_sb,
> > +					threadgroup, ctx->ns);
> > +	revert_creds(saved_cred);
> 
> I wonder now whether such a wrap shouldn't also be around cgroup_kill()
> too (+ replacement of send_sig() with group_send_sig_info() [1])?

send_sig() isn't used that was changed in response to a review. I'm
confused. 

> 
> This shouldn't break the use case of passing cgroup kill fd to a less
> privileged task for (auto)destruction purposes but on the other hand it
> would prevent subverting the fd to a more privileged confused task to
> kill otherwise disallowed processes.

Kill and freeze only do time permission checking at open. Why would you
introduce another write time check?