Re: [kernel-hardening] Re: Add overflow protection to kref

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Feb 17, 2012 at 05:44:57PM -0800, Roland Dreier wrote:
> On Fri, Feb 17, 2012 at 3:39 PM, Djalal Harouni <tixxdz@opendz.org> wrote:
> >> 2) what to do with architectures-loosers?
> > There is lib/atomic64.c but with a static hashed array of raw_spinlocks.
> 
> Even leaving aside performance impact of atomic64_t (and probably
> in most cases the performance of kref is not important at all), it is
> unfortunate to bloat the size from 4 bytes to 8 bytes.
> 
> It seems much better to have some out-of-line code for overflow
> checking rather than increasing the size of every data structure
> that embeds a kref.

Please realize that kref is an in-line structure now.

> Greg, I'm not sure why you're opposed to adding this checking...
> it's pretty clear that buggy error paths that forget to do a put are
> pretty common and will continue to be common in new code, and
> making them harder to exploit seems pretty sane to me.
> 
> What's the downside?

The downside is that there has not even been a patch sent for any of
this.  Combine that with a lack of understanding about reference
counting and atomic_t usages in the kernel, and the whole thing is ripe
for misunderstanding and confusion.

greg k-h