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

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Message-ID: <YbPe4+K+RlDgccIh@blackbook>
Date:   Sat, 11 Dec 2021 00:12:35 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.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

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()?

Thanks,
Michal

[1] https://lore.kernel.org/r/20211209214707.805617-4-tj@kernel.org/