linux-kernel - Re: [PATCH 14/15] futex: convert futex_pi_state.refcount to refcount

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Date:   Fri, 1 Sep 2017 11:38:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Elena Reshetova <elena.reshetova@...el.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        gregkh@...uxfoundation.org, viro@...iv.linux.org.uk, tj@...nel.org,
        mingo@...hat.com, hannes@...xchg.org, lizefan@...wei.com,
        acme@...nel.org, alexander.shishkin@...ux.intel.com,
        eparis@...hat.com, akpm@...ux-foundation.org, arnd@...db.de,
        luto@...nel.org, keescook@...omium.org, dvhart@...radead.org,
        ebiederm@...ssion.com
Subject: Re: [PATCH 14/15] futex: convert futex_pi_state.refcount to
 refcount_t

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@...omium.org>
> > Reviewed-by: David Windsor <dwindsor@...il.com>
> > Reviewed-by: Hans Liljestrand <ishkamiel@...il.com>
> > Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
> 
> Reviewed-by: Thomas Gleixner <tglx@...utronix.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.