Date:   Sat, 11 Dec 2021 00:12:35 +0100
From:   Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     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>, Tejun Heo <tj@kernel.org>,
        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
Message-ID: <YbPe4+K+RlDgccIh@blackbook>
References: <20211208180501.11969-1-mkoutny@suse.com>
 <87sfv3540t.fsf@email.froward.int.ebiederm.org>
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On Wed, Dec 08, 2021 at 12:45:54PM -0600, "Eric W. Biederman" <ebiederm@xmission.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/