Re: [RFC PATCH v1 2/2] randomize_kstack: Unify random source across arches

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Dec 02, 2025 at 09:39:40AM +0000, Ryan Roberts wrote:
> On 02/12/2025 09:35, Mark Rutland wrote:
> > On Tue, Dec 02, 2025 at 10:15:22AM +0100, Ard Biesheuvel wrote:
> >> On Mon, 1 Dec 2025 at 19:20, Ryan Roberts <ryan.roberts@arm.com> wrote:
> >>> On 28/11/2025 11:01, Ard Biesheuvel wrote:
> >>>> On Thu, 27 Nov 2025 at 12:00, Ryan Roberts <ryan.roberts@arm.com> wrote:
> >>>>> diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
> >>>>> index 089b1432f7e6..83c7e6710f6d 100644
> >>>>> --- a/include/linux/randomize_kstack.h
> >>>>> +++ b/include/linux/randomize_kstack.h
> >>>>> @@ -6,6 +6,7 @@
> >>>>>  #include <linux/kernel.h>
> >>>>>  #include <linux/jump_label.h>
> >>>>>  #include <linux/percpu-defs.h>
> >>>>> +#include <linux/prandom.h>
> >>>>>
> >>>>>  DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
> >>>>>                          randomize_kstack_offset);
> >>>>> @@ -45,9 +46,13 @@ DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
> >>>>>  #define KSTACK_OFFSET_MAX(x)   ((x) & 0b1111111100)
> >>>>>  #endif
> >>>>>
> >>>>> +static __always_inline u32 get_update_kstack_offset(void)
> >>>>> +{
> >>>>> +       return prandom_u32_state(&current->kstack_rnd_state);
> >>>
> >>> I've got bot warnings because this is being called from noinstr code. I guess
> >>> the best option is to just move add_random_kstack_offset() to after
> >>> instrumentation is enabled for the affected arches.
> >>
> >> Just put instrumentation_begin()/instrumentation_end() around the call
> >> to prandom_u32_state() - that seems to be the common approach for
> >> punching holes into the 'noinstr' validation.
> > 
> > That silences the warning, but isn't necessarily safe, so please DO NOT
> > do that blindly. The instrumentation_{begin,end}() annotations are only
> > supposed to be used when we know by construction that instrumentation is
> > safe.
> > 
> > Generally, if you can move this to after instrumentation is already
> > enabled, that should be safe, and so that'd be the better approach.
> > 
> > Ryan, can you share those warnings (e.g. link to those reports)?
> 
> https://lore.kernel.org/oe-kbuild-all/202511302137.u4iIA2kf-lkp@intel.com/
> https://lore.kernel.org/oe-kbuild-all/202511302317.BGFIlAYX-lkp@intel.com/

Cool, so those are __do_fast_syscall_32() and do_syscall_64(), which I'd
expect (and those are legitimate warnings).

From a quick scan of v6.18, it looks like arm64, loongarch, powerpc, and
riscv perform all the necessary entry work before calling
add_random_kstack_offset(), and we'll need to change:

* s390's __do_syscall()
* x86's do_syscall_64()
* x86's do_int80_syscall_32()
* x86's __do_fast_syscall_32()

Mark.