linux-kernel - Re: [PATCH] Convert struct pid count to refcount

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Message-ID: <20190328142619.GA19441@redhat.com>
Date:   Thu, 28 Mar 2019 15:26:19 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Jann Horn <jannh@...gle.com>, Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>
Subject: Re: [PATCH] Convert struct pid count to refcount_t

On 03/27, Joel Fernandes wrote:
>
> Also, based on Kees comment, I think it appears to me that get_pid and
> put_pid can race in this way in the original code right?
>
> get_pid			put_pid
>
> 			atomic_dec_and_test returns 1
> atomic_inc
> 			kfree
>
> deref pid /* boom */
> -------------------------------------------------
>
> I think get_pid needs to call atomic_inc_not_zero()

No.

get_pid() should only be used if you already have a reference or you do
something like

	rcu_read_lock();
	pid = find_vpid();
	get_pid();
	rcu_read_lock();

in this case we rely on call_rcu(delayed_put_pid) which drops the initial
reference.

If put_pid() sees pid->count == 1, then a) nobody else has a reference and
b) nobody else can find this pid on rcu-protected lists, so it is safe to
free it.

Oleg.