Re: [PATCH v3 0/3] Fix bugs and performance of kstack offset randomisation

[David Laight <david.laight.linux@gmail.com>]

On Mon, 19 Jan 2026 10:52:59 +0000
Mark Rutland <mark.rutland@arm.com> wrote:

> On Fri, Jan 02, 2026 at 01:11:51PM +0000, Ryan Roberts wrote:
> > Hi All,  
> 
> Hi Ryan,
> 
> > As I reported at [1], kstack offset randomisation suffers from a couple of bugs
> > and, on arm64 at least, the performance is poor. This series attempts to fix
> > both; patch 1 provides back-portable fixes for the functional bugs. Patches 2-3
> > propose a performance improvement approach.
> > 
> > I've looked at a few different options but ultimately decided that Jeremy's
> > original prng approach is the fastest. I made the argument that this approach is
> > secure "enough" in the RFC [2] and the responses indicated agreement.  
> 
> FWIW, the series all looks good to me. I understand you're likely to
> spin a v4 with a couple of minor tweaks (fixing typos and adding an
> out-of-line wrapper for a prandom function), but I don't think there's
> anything material that needs to change.
> 
> I've given my Ack on all three patches. I've given the series a quick
> boot test (atop v6.19-rc4) with a bunch of debug options enabled, and
> all looks well.
> 
> Kees, do you have any comments? It would be nice if we could queue this
> up soon.

I don't want to stop this being queued up in its current form.
But I don't see an obvious need for multiple per-cpu prng
(there are a couple of others lurking), surely one will do.

How much overhead does the get_cpu_var() add?
I think it has to disable pre-emption (or interrupts) which might
be more expensive on non-x86 (which can just do 'inc %gs:address').

I'm sure I remember a version that used a per-task prng.
That just needs 'current' - which might be known and/or be cheaper
to get.
(Although I also remember a reference some system where it was slow...)

The other option is just to play 'fast and loose' with the prng data.
Using the state from the 'wrong cpu' (if the code is pre-empted) won't
really matter.
You might get a RrwW (or even RrwrwW) sequence, but the prng won't be used
for anything 'really important' so it shouldn't matter.

	David