Date:   Thu, 9 Dec 2021 15:47:00 +0100
From:   Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     Tejun Heo <tj@kernel.org>,
        Linus Torvalds <torvalds@linuxfoundation.org>,
        "Eric W. Biederman" <ebiederm@xmission.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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On Thu, Dec 09, 2021 at 03:08:26PM +0100, Christian Brauner <christian.brauner@ubuntu.com> wrote:
> send_sig() isn't used that was changed in response to a review. I'm
> confused. 

Sorry for ambiguity, I meant this instance [1].

> Kill and freeze only do time permission checking at open. Why would you
> introduce another write time check?
 
Let's have a cgroup G with tasks t1,...,tn (run by user u) and some
monitoring tasks m1,...,mk belonging to a different user v != u.

Currently u can kill also the tasks of v -- I'm not sure if that's
intentional. My argument would apply if it wasn't -- it'd be suscebtible
to similar abuse, i.e. passing the opened fd to a more privileged
process to kill also v's tasks. (But if the intention is to be able to
kill anyone in the cgroup, then it likely doesn't matter.)


Michal

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/cgroup/cgroup.c?h=v5.16-rc4#n3762