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

[Kees Cook <kees@kernel.org>]

On Mon, Nov 24, 2025 at 05:50:14PM +0000, Ryan Roberts wrote:
> On 24/11/2025 17:11, 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.
> > 
> >> 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. :)
> > 
> >> 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.
> 
> Just to say I haven't forgotten about this; I ended up having to switch to
> something more urgent. Hoping to get back to it later this week. I don't think
> this is an urgent issue, so hopefully folks are ok waiting.
> 
> I propose to post whatever I end up with then we can all disscuss from there.
> But the rough shape so far:
> 
> Fixes:
>  - Remove choose_random_kstack_offset()
>  - arch passes random into add_random_kstack_offset() (fixes migration bypass)
>  - Move add_random_kstack_offset() to el0_svc()/el0_svc_compat() (before
>    enabling interrupts) to fix non-preemption requirement (arm64)

I thought we'd keep choose_random_kstack_offset() and just move
everything into a per-task location? (And for arm64 only)

-- 
Kees Cook