linux-kernel - Re: [PATCH] fs, cifs: convert tcon_link.tl_count from atomic_t to refcount

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Date:   Mon, 30 Oct 2017 16:03:02 +0100
From:   Aurélien Aptel <aaptel@...e.com>
To:     Elena Reshetova <elena.reshetova@...el.com>,
        linux-cifs@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, sfrench@...ba.org,
        samba-technical@...ts.samba.org, peterz@...radead.org,
        keescook@...omium.org, Elena Reshetova <elena.reshetova@...el.com>
Subject: Re: [PATCH] fs, cifs: convert tcon_link.tl_count from atomic_t to refcount_t

Elena Reshetova <elena.reshetova@...el.com> writes:
> 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 tcon_link.tl_count is used as pure reference counter.
> Convert it to refcount_t and fix up the operations.

<snip>

> -		cifs_get_tlink(tlink);
> +		refcount_set(&tlink->tl_count, 1);

Since cifs_get_tlink() basically only increments the reference count and
given that this is a new tlink I also think this replacement is ok.

Looks good to me and passes 'make C=1'.

Reviewed-by: Aurelien Aptel <aaptel@...e.com>

-- 
Aurélien Aptel / SUSE Labs Samba Team
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