linux-kernel - Re: [PATCH] sparc64: convert mdesc_handle.refcnt from atomic_t to refcount

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Message-Id: <20171115.144215.1875016289797792026.davem@davemloft.net>
Date:   Wed, 15 Nov 2017 14:42:15 +0900 (KST)
From:   David Miller <davem@...emloft.net>
To:     elena.reshetova@...el.com
Cc:     linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
        shannon.nelson@...cle.com, jag.raman@...cle.com,
        peterz@...radead.org, keescook@...omium.org
Subject: Re: [PATCH] sparc64: convert mdesc_handle.refcnt from atomic_t to
 refcount_t

From: Elena Reshetova <elena.reshetova@...el.com>
Date: Fri, 20 Oct 2017 10:57:57 +0300

> 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 mdesc_handle.refcnt 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>

Applied.