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

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Date:   Fri, 1 Sep 2017 09:39:50 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Elena Reshetova <elena.reshetova@...el.com>
cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        peterz@...radead.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 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>