linux-kernel - Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand unprotected when accessed by /proc)

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Date:	Mon, 9 Jun 2014 20:15:53 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Clark Williams <williams@...hat.com>
Subject: Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand
	unprotected when accessed by /proc)

On 06/09, Paul E. McKenney wrote:
>
> On Sun, Jun 08, 2014 at 03:07:18PM +0200, Oleg Nesterov wrote:
> >
> > I only meant that afaics rcu_read_unlock_special() equally depends on the
> > fact that rt_mutex_unlock() does nothing with "struct rt_mutex" after it
> > makes another rt_mutex_lock() + rt_mutex_unlock() possible, otherwise this
> > code is wrong (and unlock_task_sighand() would be wrong too).
> >
> > Just to simplify the discussion... suppose we add "atomic_t nr_slow_unlock"
> > into "struct rt_mutex" and change rt_mutex_slowunlock() to do
> > atomic_inc(&lock->nr_slow_unlock) after it drops ->wait_lock. Of course this
> > would be ugly, just for illustration.
>
> That would indeed be a bad thing, as it could potentially lead to
> use-after-free bugs.  Though one could argue that any code that resulted
> in use-after-free would be quite aggressive.  But still...

And once again, note that the normal mutex is already unsafe (unless I missed
something).

> > So _perhaps_ we should not rely on this property of rt_mutex "too much".
>
> Well, I could easily move the rt_mutex from rcu_boost()'s stack to the
> rcu_node structure, if that would help.  That said, I still have my
> use-after-free concern above.

Or we can document that rt_mutex is special and rt_mutex_unlock() should be
"atomic" and safe as spin_unlock() or complete().

Oleg.

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