Re: [PATCH 0/5] block subsystem refcounter conversions

[Peter Zijlstra]

On Mon, Feb 20, 2017 at 07:41:01AM -0800, James Bottomley wrote:
> On Mon, 2017-02-20 at 08:15 -0700, Jens Axboe wrote:
> > On 02/20/2017 04:16 AM, Elena Reshetova wrote:
> > > Now when new refcount_t type and API are finally merged
> > > (see include/linux/refcount.h), the following
> > > patches convert various refcounters in the block susystem from 
> > > atomic_t to refcount_t. By doing this we prevent intentional or 
> > > accidental underflows or overflows that can led to use-after-free
> > > vulnerabilities.
> 
> This description isn't right ... nothing is prevented; we get warnings
> on saturation and use after free with this.

The thing that is prevented is overflow and then a use-after-free by
making it a leak.

Modular stuff, you put and free at: (n+1) mod n, by saturating at n-1
we'll never get there.

So you loose use-after-free, you gain a resource leak. The general idea
being that use-after-free is a nice trampoline for exploits, leaks are
'only' a DoS.