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Date:   Fri, 21 Feb 2020 17:50:37 +0100
From:   Oleg Nesterov <oleg@redhat.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        LKML <linux-kernel@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux API <linux-api@vger.kernel.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Linux Security Module <linux-security-module@vger.kernel.org>,
        Akinobu Mita <akinobu.mita@gmail.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Daniel Micay <danielmicay@gmail.com>,
        Djalal Harouni <tixxdz@gmail.com>,
        "Dmitry V . Levin" <ldv@altlinux.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        "J . Bruce Fields" <bfields@fieldses.org>,
        Jeff Layton <jlayton@poochiereds.net>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Solar Designer <solar@openwall.com>
Subject: Re: [PATCH 7/7] proc: Ensure we see the exit of each process tid
 exactly once
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On 02/20, Eric W. Biederman wrote:
>
> +void exchange_tids(struct task_struct *ntask, struct task_struct *otask)
> +{
> +	/* pid_links[PIDTYPE_PID].next is always NULL */
> +	struct pid *npid = READ_ONCE(ntask->thread_pid);
> +	struct pid *opid = READ_ONCE(otask->thread_pid);
> +
> +	rcu_assign_pointer(opid->tasks[PIDTYPE_PID].first, &ntask->pid_links[PIDTYPE_PID]);
> +	rcu_assign_pointer(npid->tasks[PIDTYPE_PID].first, &otask->pid_links[PIDTYPE_PID]);
> +	rcu_assign_pointer(ntask->thread_pid, opid);
> +	rcu_assign_pointer(otask->thread_pid, npid);

this breaks has_group_leader_pid()...

proc_pid_readdir() can miss a process doing mt-exec but this looks fixable,
just we need to update ntask->thread_pid before updating ->first.

The more problematic case is __exit_signal() which does
		
		if (unlikely(has_group_leader_pid(tsk)))
			posix_cpu_timers_exit_group(tsk);

Oleg.