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

From: Kees Cook <kees@kernel.org>
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Huacai Chen <chenhuacai@kernel.org>,
	Madhavan Srinivasan <maddy@linux.ibm.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Paul Walmsley <pjw@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Mark Rutland <mark.rutland@arm.com>,
	"Jason A. Donenfeld" <Jason@zx2c4.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Jeremy Linton <jeremy.linton@arm.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev,
	linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
	linux-s390@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH v2 3/3] randomize_kstack: Unify random source across arches
Date: Tue, 16 Dec 2025 00:27:38 -0800	[thread overview]
Message-ID: <202512160024.B688A8D0@keescook> (raw)
In-Reply-To: <20251215163520.1144179-4-ryan.roberts@arm.com>

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?

-- 
Kees Cook