[DISCUSSION] kstack offset randomization: bugs and performance

[Ryan Roberts]

On 24/11/2025 20:51, Kees Cook wrote:
> 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 at 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)

Err... I thought you were the one arguing against per-task state? I'm not really
keen on having arm64 do a completely different thing to everyone else; It seems
reasonable to me that we only need to (continue to) abstract the random source
per-arch and the rest should remain common?

Per my previous mails, I'm not really sure what choose_random_kstack_offset() is
giving us in practice. Why not simplify?