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

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Message-ID: <CAMj1kXG=HDVso1xc6TGgSrBpV3Sq2YQLdPKg2dw5z9qdrbdWBw@mail.gmail.com>
Date: Tue, 2 Dec 2025 10:47:04 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Ryan Roberts <ryan.roberts@....com>, Kees Cook <kees@...nel.org>, 
	Ard Biesheuvel <ardb+git@...gle.com>, Will Deacon <will@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	Jeremy Linton <jeremy.linton@....com>, Catalin Marinas <Catalin.Marinas@....com>, 
	"Jason A . Donenfeld" <Jason@...c4.com>, linux-hardening@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 2/2] randomize_kstack: Unify random source across arches

On Tue, 2 Dec 2025 at 10:35, Mark Rutland <mark.rutland@....com> 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@....com> wrote:
> > > On 28/11/2025 11:01, Ard Biesheuvel wrote:
> > > > On Thu, 27 Nov 2025 at 12:00, Ryan Roberts <ryan.roberts@....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.

Oops - sorry about that.

> 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)?
>
> 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.
>

Given that prandom_u32_state() does a fairly straight-forward mangle
of 4 32-bit words, might it be better to make that __always_inline
itself?