linux-kernel - Re: [PATCH 1/4] fs, afs: convert afs_cell.usage from atomic_t to refcount

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Message-ID: <CAGXu5jLVv+_RLth0A=4YGyOWcUv4JsMTpeOWrO3389dTRf-S8g@mail.gmail.com>
Date:   Wed, 22 Feb 2017 09:39:38 -0800
From:   Kees Cook <keescook@...omium.org>
To:     David Howells <dhowells@...hat.com>
Cc:     "Reshetova, Elena" <elena.reshetova@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-afs@...ts.infradead.org" <linux-afs@...ts.infradead.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>
Subject: Re: [PATCH 1/4] fs, afs: convert afs_cell.usage from atomic_t to refcount_t

On Wed, Feb 22, 2017 at 9:29 AM, David Howells <dhowells@...hat.com> wrote:
> Reshetova, Elena <elena.reshetova@...el.com> wrote:
>
>> Thank you very much David for testing the patches!
>> I guess for this one and other two patches it means that if we want to do the atomic_t --> refcount_t conversions,
>> we need to do +1 on the whole counting scheme to avoid issues around reaching zero.
>> Do you see this approach reasonable? I can give it a try, if it makes sense in your opinion.
>
> Or you could create a refcount_inc_may_resurrect() function that does allow
> increment from 0.  Make it take a lock-check like the rcu functions do.

We can't allow the increment from 0 since it violates the intended
use-after-free protections. If "0" means "still valid" then this
sounds like it needs a global +1, as Elena suggested in her reply.

-Kees

-- 
Kees Cook
Pixel Security