Re: [PATCH v2] fork: Unconditionally clear stack on fork

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 21 Feb 2018 11:29:33 +0100 Michal Hocko <mhocko@kernel.org> wrote:

> On Tue 20-02-18 18:16:59, Kees Cook wrote:
> > One of the classes of kernel stack content leaks[1] is exposing the
> > contents of prior heap or stack contents when a new process stack is
> > allocated. Normally, those stacks are not zeroed, and the old contents
> > remain in place. In the face of stack content exposure flaws, those
> > contents can leak to userspace.
> > 
> > Fixing this will make the kernel no longer vulnerable to these flaws,
> > as the stack will be wiped each time a stack is assigned to a new
> > process. There's not a meaningful change in runtime performance; it
> > almost looks like it provides a benefit.
> > 
> > Performing back-to-back kernel builds before:
> > 	Run times: 157.86 157.09 158.90 160.94 160.80
> > 	Mean: 159.12
> > 	Std Dev: 1.54
> > 
> > and after:
> > 	Run times: 159.31 157.34 156.71 158.15 160.81
> > 	Mean: 158.46
> > 	Std Dev: 1.46
> 
> /bin/true or similar would be more representative for the worst case
> but it is good to see that this doesn't have any visible effect on
> a more real usecase.

Yes, that's a pretty large memset.  And while it will populate the CPU
cache with the stack contents, doing so will evict other things.

So some quite careful quantitative testing is needed here, methinks.