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

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Date:   Mon, 13 Dec 2021 09:24:55 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Michal Koutný <mkoutny@...e.com>
Cc:     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>, Tejun Heo <tj@...nel.org>,
        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

Michal Koutný <mkoutny@...e.com> writes:

> On Wed, Dec 08, 2021 at 12:45:54PM -0600, "Eric W. Biederman" <ebiederm@...ssion.com> wrote:
>> TL;DR the cgroup file system is checking permissions at write time.
>
> Thank you for bringing that up (handled in a separate thread now).
>
>> I think I follow your reasoning and I think it will even fix the issue
>> but no.
>
> FTR, part of Tejun's series [1] ensures that cgroup_ns is accessed
> directly without nsproxy and a reference to it is kept while the file
> is opened. I.e. that'd properly fix this particular crash reported by
> syzbot.
>
>> Please don't apply this patch.
>> 
>> exit_task_work running after exit_task_namespaces is the messenger
>> that just told us about something ugly.
>
> In (my) theory some other task_work callbacks could (transitively) rely
> on the current->nsproxy which could still be cleared by
> exit_task_namespaces().
> Is there another reason why to have exit_task_namespaces() before
> exit_task_work()?

We already have the principle that things are going to be cleaned up
before exit_task_work is called and exit_files depends upon that.

So I think the burden is to find a good reason why exit_task_work should
move not to defend it.

If we don't want things cleaned up before exit_task_work it should come
at the start of do_exit and exit_files and others need to stop depending
upon it.  Which seems like challenging change to make.

Eric