linux-kernel - Re: [PATCH] randomize_kstack: Improve entropy diffusion

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Message-ID: <87seya452v.fsf@>
Date: Wed, 22 May 2024 10:35:20 +0200
From: Nicolai Stange <nstange@...e.de>
To: Kees Cook <keescook@...omium.org>
Cc: Jeremy Linton <jeremy.linton@....com>,  Arnd Bergmann <arnd@...db.de>,
  "Gustavo A. R. Silva" <gustavoars@...nel.org>,
  linux-hardening@...r.kernel.org,  Elena Reshetova
 <elena.reshetova@...el.com>,  Thomas Gleixner <tglx@...utronix.de>,
  linux-kernel@...r.kernel.org
Subject: Re: [PATCH] randomize_kstack: Improve entropy diffusion

Kees Cook <keescook@...omium.org> writes:

>
> diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
> index 5d868505a94e..6d92b68efbf6 100644
> --- a/include/linux/randomize_kstack.h
> +++ b/include/linux/randomize_kstack.h
> @@ -80,7 +80,7 @@ DECLARE_PER_CPU(u32, kstack_offset);
>  	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
>  				&randomize_kstack_offset)) {		\
>  		u32 offset = raw_cpu_read(kstack_offset);		\
> -		offset ^= (rand);					\
> +		offset = ror32(offset, 5) ^ (rand);			\

Hi Kees,

I'm wondering whether this renders the per-arch mask applied to 'rand'
at the respective choose_random_kstack_offset() invocations ineffective?

Like e.g. on x86 there is

  choose_random_kstack_offset(rdtsc() & 0xFF);

I would argue that while before the patch kstack_offset had been
guaranteed to stay within the bounds of 0xFF, it's now effectively
unlimited (well, <= (u32)-1) and only capped to 0x3ff when subsequently
applying the KSTACK_OFFSET_MAX().

Or am I simply missing something?

Thanks!

Nicolai

>  		raw_cpu_write(kstack_offset, offset);			\
>  	}								\
>  } while (0)

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