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

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Date:	Tue, 13 May 2014 00:18:10 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davidlohr Bueso <davidlohr@...com>
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, 12 May 2014, Davidlohr Bueso wrote:
> 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;

Duh, that hunk is actually left over from the attempt to do what you
suggest below :(

> >  
> >         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.

Right, but for some reason it was a more ugly patch than the one I
posted (w/o the stupid hunk above) and I tried to minimize the impact
for stable backporting.

Thanks,

	tglx




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