Re: [PATCH 1/4] fs, afs: convert afs_cell.usage from atomic_t to refcount_t

[Kees Cook]

On Wed, Feb 22, 2017 at 9:29 AM, David Howells <dhowells@xxxxxxxxxx> wrote:
> Reshetova, Elena <elena.reshetova@xxxxxxxxx> 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