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

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Message-ID: <CAH2r5mtWf5mxKjPNsBs5TRAaPJMaPKwnWTMYz6o49_+K_zGk_g@mail.gmail.com>
Date:   Wed, 1 Nov 2017 12:56:29 -0500
From:   Steve French <smfrench@...il.com>
To:     Aurélien Aptel <aaptel@...e.com>
Cc:     Elena Reshetova <elena.reshetova@...el.com>,
        "linux-cifs@...r.kernel.org" <linux-cifs@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steve French <sfrench@...ba.org>,
        samba-technical <samba-technical@...ts.samba.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] fs, cifs: convert tcon_link.tl_count from atomic_t to refcount_t

merged into cifs-2.6.git for-next

On Mon, Oct 30, 2017 at 10:03 AM, Aurélien Aptel <aaptel@...e.com> wrote:
> 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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-- 
Thanks,

Steve