linux-kernel - Re: [PATCH 1/1] arm64: syscall: Direct PRNG kstack randomization

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Message-ID: <202403071105.C3B038C@keescook>
Date: Thu, 7 Mar 2024 11:10:47 -0800
From: Kees Cook <keescook@...omium.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: Jeremy Linton <jeremy.linton@....com>,
	linux-arm-kernel@...ts.infradead.org,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	"Jason A . Donenfeld" <Jason@...c4.com>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mark Brown <broonie@...nel.org>, Guo Hui <guohui@...ontech.com>,
	Manoj.Iyer@....com, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org, James Yang <james.yang@....com>,
	Shiyou Huang <shiyou.huang@....com>
Subject: Re: [PATCH 1/1] arm64: syscall: Direct PRNG kstack randomization

On Thu, Mar 07, 2024 at 12:10:34PM +0100, Arnd Bergmann wrote:
> For the strength, we have at least four options:
> 
> - strong rng, most expensive
> - your new prng, less strong but somewhat cheaper and/or more
>   predictable overhead
> - cycle counter, cheap but probably even less strong,
>   needs architecture code.

Are the low bits of a cycler counter really less safe than a
deterministic pRNG?

> - no rng, no overhead and no protection.

For the pRNG, why not just add a reseed timer or something that'll
happen outside the syscall window, if that's the concern about reseeding
delay? (In which case, why not continue to use the strong rng?)

-- 
Kees Cook