linux-kernel - Re: [PATCH 1/4] bcache: convert cached_dev.count from atomic_t to refcount

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Date:   Fri, 20 Oct 2017 11:39:42 -0700
From:   Michael Lyle <mlyle@...e.org>
To:     Elena Reshetova <elena.reshetova@...el.com>, dm-devel@...hat.com
Cc:     linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-raid@...r.kernel.org, kent.overstreet@...il.com,
        koverstreet@...gle.com, ejt@...hat.comg, snitzer@...hat.com,
        shli@...nel.org, agk@...hat.com, peterz@...radead.org,
        keescook@...omium.org
Subject: Re: [PATCH 1/4] bcache: convert cached_dev.count from atomic_t to
 refcount_t

On 10/20/2017 12:37 AM, 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 cached_dev.count 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: Michael Lyle <mlyle@...e.org>

Thanks for this-- I'm including it in my tree for possible inclusion in
4.15 (I've already sent my main chunk of changes upwards).

Mike