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 <[email protected]> wrote: >>> On 28/11/2025 11:01, Ard Biesheuvel wrote: >>>> On Thu, 27 Nov 2025 at 12:00, Ryan Roberts <[email protected]> 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(¤t->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/[email protected]/ https://lore.kernel.org/oe-kbuild-all/[email protected]/ > > 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.
