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

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Message-ID: <32460.1487946105@warthog.procyon.org.uk>
Date:   Fri, 24 Feb 2017 14:21:45 +0000
From:   David Howells <dhowells@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     dhowells@...hat.com,
        "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

Kees Cook <keescook@...omium.org> wrote:

> We can't allow the increment from 0 since it violates the intended
> use-after-free protections.

I would have thought that the protections would've been against the carry flag
getting set.

> If "0" means "still valid" then this
> sounds like it needs a global +1, as Elena suggested in her reply.

This makes it sound like refcount_t is then unsuitable for this.

Since I want to overhaul the code to use more RCU and eliminate some of the
locking, it might be worth waiting on the patches.

David