Re: [DISCUSSION] kstack offset randomization: bugs and performance

[Will Deacon <will@kernel.org>]

On Mon, Nov 24, 2025 at 09:11:23AM -0800, Kees Cook wrote:
> On November 24, 2025 6:36:25 AM PST, Will Deacon <will@kernel.org> wrote:
> >On Mon, Nov 17, 2025 at 11:31:22AM +0000, Ryan Roberts wrote:
> >> On 17/11/2025 11:30, Ryan Roberts wrote:
> >> > Could this give us a middle ground between strong-crng and
> >> > weak-timestamp-counter? Perhaps the main issue is that we need to store the
> >> > secret key for a long period?
> >> > 
> >> > 
> >> > Anyway, I plan to work up a series with the bugfixes and performance
> >> > improvements. I'll add the siphash approach as an experimental addition and get
> >> > some more detailed numbers for all the options. But wanted to raise it all here
> >> > first to get any early feedback.
> >
> >FWIW, I share Mark's concerns about using a counter for this. Given that
> >the feature currently appears to be both slow _and_ broken I'd vote for
> >either removing it or switching over to per-thread offsets as a first
> >step.
> 
> That it has potential weaknesses doesn't mean it should be entirely
> removed.

Well, we can always bring it back when it does something useful :)

> > We already have a per-task stack canary with
> >CONFIG_STACKPROTECTOR_PER_TASK so I don't understand the reluctance to
> >do something similar here.
> 
> That's not a reasonable comparison: the stack canary cannot change
> arbitrarily for a task or it would immediately crash on a function return.
> :)

Fair enough, but I was thinking more about concerns relating to the
size of task struct. I don't think that's a huge concern in this case
and we already have tonnes of junk in thread_struct if you want to put
it there instead. Certainly, persevering with per-cpu data just feels
like the wrong approach to me based on Ryan's report.

> >Speeding up the crypto feels like something that could happen separately.
> 
> Sure. But let's see what Ryan's patches look like. The suggested changes
> sound good to me.

I guess we'll have to wait and see but some of the ideas in this thread
(e.g. using the counter and interrupt timing) seem pretty flawed to me
so I was trying to avoid Ryan wasting his time.

Will