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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.
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