Re: [PATCH 14/15] futex: convert futex_pi_state.refcount to refcount_t

From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Elena Reshetova <elena.reshetova@intel.com>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	gregkh@linuxfoundation.org, viro@zeniv.linux.org.uk,
	tj@kernel.org, mingo@redhat.com, hannes@cmpxchg.org,
	lizefan@huawei.com, acme@kernel.org,
	alexander.shishkin@linux.intel.com, eparis@redhat.com,
	akpm@linux-foundation.org, arnd@arndb.de, luto@kernel.org,
	keescook@chromium.org, dvhart@infradead.org,
	ebiederm@xmission.com
Subject: Re: [PATCH 14/15] futex: convert futex_pi_state.refcount to refcount_t
Date: Fri, 1 Sep 2017 11:38:52 +0200	[thread overview]
Message-ID: <20170901093852.it4d4bxoy2lmojrk@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <alpine.DEB.2.20.1709010939360.1857@nanos>

On Fri, Sep 01, 2017 at 09:39:50AM +0200, Thomas Gleixner wrote:
> On Wed, 30 Aug 2017, Elena Reshetova wrote:
> > atomic_t variables are currently used to implement reference
> > counters with the following properties:
> >  - counter is initialized to 1 using atomic_set()
> >  - a resource is freed upon counter reaching zero
> >  - once counter reaches zero, its further
> >    increments aren't allowed
> >  - counter schema uses basic atomic operations
> >    (set, inc, inc_not_zero, dec_and_test, etc.)
> > 
> > Such atomic variables should be converted to a newly provided
> > refcount_t type and API that prevents accidental counter overflows
> > and underflows. This is important since overflows and underflows
> > can lead to use-after-free situation and be exploitable.
> > 
> > The variable futex_pi_state.refcount is used as pure
> > reference counter. Convert it to refcount_t and fix up
> > the operations.
> > 
> > Suggested-by: Kees Cook <keescook@chromium.org>
> > Reviewed-by: David Windsor <dwindsor@gmail.com>
> > Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
> > Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
> 
> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

So the thing to be careful with for things like futex and some of the
other core kernel code is the memory ordering.

atomic_dec_and_test() provides a full smp_mb() before and after,
refcount_dec_and_test() only provides release semantics.

This is typically sufficient, and I would argue that if we rely on more
than that, there _should_ be a comment, however reality isn't always as
nice.

That said, I think this conversion is OK, pi_state->refcount isn't
relied upon to provide additional memory ordering above and beyond what
refcounting requires.