linux-kernel - Re: [PATCH v2 3/3] randomize_kstack: Unify random source across arches

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Message-ID: <CAMj1kXHaqfEqDpvC-iC1fUn+JpTj=oqfM2sh+sbaC8_PBVHBUQ@mail.gmail.com>
Date: Tue, 16 Dec 2025 09:30:46 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Kees Cook <kees@...nel.org>
Cc: Ryan Roberts <ryan.roberts@....com>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, Huacai Chen <chenhuacai@...nel.org>, 
	Madhavan Srinivasan <maddy@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>, Paul Walmsley <pjw@...nel.org>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, 
	Alexander Gordeev <agordeev@...ux.ibm.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "Gustavo A. R. Silva" <gustavoars@...nel.org>, 
	Arnd Bergmann <arnd@...db.de>, Mark Rutland <mark.rutland@....com>, 
	"Jason A. Donenfeld" <Jason@...c4.com>, Jeremy Linton <jeremy.linton@....com>, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev, 
	linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org, 
	linux-s390@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 3/3] randomize_kstack: Unify random source across arches

On Tue, 16 Dec 2025 at 09:27, Kees Cook <kees@...nel.org> wrote:
>
> On Mon, Dec 15, 2025 at 04:35:17PM +0000, Ryan Roberts wrote:
> > [...]
> > @@ -45,9 +46,22 @@ DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
> >  #define KSTACK_OFFSET_MAX(x) ((x) & 0b1111111100)
> >  #endif
> >
> > +DECLARE_PER_CPU(struct rnd_state, kstack_rnd_state);
> > +
> > +static __always_inline u32 get_kstack_offset(void)
> > +{
> > +     struct rnd_state *state;
> > +     u32 rnd;
> > +
> > +     state = &get_cpu_var(kstack_rnd_state);
> > +     rnd = prandom_u32_state(state);
> > +     put_cpu_var(kstack_rnd_state);
> > +
> > +     return rnd;
> > +}
> > [...]
> > -static inline void random_kstack_task_init(struct task_struct *tsk)
> > +static int random_kstack_init(void)
> >  {
> > -     tsk->kstack_offset = 0;
> > +     prandom_seed_full_state(&kstack_rnd_state);
> > +     return 0;
> >  }
> > +
> > +late_initcall(random_kstack_init);
>
> Doesn't this need to be run for every CPU? (And how does hotplug work
> for such things?) And doesn't it need a get_cpu_var?
>


 prandom_seed_full_state() takes a 'struct rnd_state __percpu
*pcpu_state', and performs the initialization for all possible CPUs.