linux-kernel - Re: [patch 3/3] futex: Prevent attaching to kernel threads

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Date:	Mon, 12 May 2014 14:59:19 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>, Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Darren Hart <darren@...art.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Clark Williams <williams@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Roland McGrath <roland@...k.frob.com>,
	Carlos ODonell <carlos@...hat.com>,
	Jakub Jelinek <jakub@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [patch 3/3] futex: Prevent attaching to kernel threads

On Mon, 2014-05-12 at 20:45 +0000, Thomas Gleixner wrote:
> We happily allow userspace to declare a random kernel thread to be the
> owner of a user space PI futex.
> 
> Found while analysing the fallout of Dave Jones syscall fuzzer.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: stable@...r.kernel.org
> ---
>  kernel/futex.c |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> Index: linux-2.6/kernel/futex.c
> ===================================================================
> --- linux-2.6.orig/kernel/futex.c
> +++ linux-2.6/kernel/futex.c
> @@ -682,6 +682,8 @@ static struct task_struct * futex_find_g
>         p = find_task_by_vpid(pid);
>         if (p)
>                 get_task_struct(p);
> +       else
> +               p = NULL;
>  
>         rcu_read_unlock();
>  
> @@ -814,6 +816,11 @@ lookup_pi_state(u32 uval, struct futex_h
>         if (!p)
>                 return -ESRCH;
>  
> +       if (!p->mm) {
> +               put_task_struct(p);
> +               return -EPERM;
> +       }

It might make sense to check the mm inside futex_find_get_task(), and
simply never take the refcount if NULL.

Thanks,
Davidlohr

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