linux-kernel - Re: [PATCH] sys_getppid: add missing rcu

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Message-ID: <CAGXu5j+Epkypk-AKi0QWrKewW9BX8k8xaW0bkPb82mHnYVECtw@mail.gmail.com>
Date:	Wed, 7 Dec 2011 15:03:24 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Mandeep Singh Baines <msb@...omium.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Pavel Emelyanov <xemul@...nvz.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	John Johansen <john.johansen@...onical.com>
Subject: Re: [PATCH] sys_getppid: add missing rcu_dereference

On Tue, Dec 6, 2011 at 6:45 PM, Mandeep Singh Baines <msb@...omium.org> wrote:
> In order to safely dereference current->real_parent inside an
> rcu_read_lock, we need an rcu_dereference.
>
> Signed-off-by: Mandeep Singh Baines <msb@...omium.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Pavel Emelyanov <xemul@...nvz.org>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Kees Cook <keescook@...omium.org>
> ---
>  kernel/timer.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/timer.c b/kernel/timer.c
> index dbaa624..9c3c62b 100644
> --- a/kernel/timer.c
> +++ b/kernel/timer.c
> @@ -1368,7 +1368,7 @@ SYSCALL_DEFINE0(getppid)
>        int pid;
>
>        rcu_read_lock();
> -       pid = task_tgid_vnr(current->real_parent);
> +       pid = task_tgid_vnr(rcu_dereference(current->real_parent));
>        rcu_read_unlock();
>
>        return pid;

Should parent and real_parent also be marked in sched.h with __rcu so
sparse can find other missing rcu_dereference()s? And if not, why?
(tasklist lock?)

I think I see at least are few other users (security/apparmor/audit.c,
security/tomoyo/common.h, kernel/sched.c) that need rcu_dereference()
when accessing real_parent, there are probably more.

-Kees

-- 
Kees Cook
ChromeOS Security
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