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

[Ryan Roberts]

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 at arm.com> wrote:
>>> On 28/11/2025 11:01, Ard Biesheuvel wrote:
>>>> On Thu, 27 Nov 2025 at 12:00, Ryan Roberts <ryan.roberts at 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/

> 
> IIUC only x86 has noinstr validation, and from a quick scan, I expect
> you see warnings from:
> 
> * do_syscall_64()
> * do_int80_syscall_32()
> * __do_fast_syscall_32()
> 
> For all of these, it is not safe to call instrumentable code before the
> calls to {syscall_,}enter_from_user_mode{,_prepare}(). You'll need to
> move the stack rnadomization after the existing instrumentation_begin()
> calls.
> 
> We'll need to go check the other architectures similarly.

OK understood. I'll do it this way then.

> 
> Mark.