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

From: Kees Cook <keescook@chromium.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Jeremy Linton <jeremy.linton@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	"Jason A . Donenfeld" <Jason@zx2c4.com>,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mark Brown <broonie@kernel.org>, Guo Hui <guohui@uniontech.com>,
	Manoj.Iyer@arm.com, linux-kernel@vger.kernel.org,
	linux-hardening@vger.kernel.org, James Yang <james.yang@arm.com>,
	Shiyou Huang <shiyou.huang@arm.com>
Subject: Re: [PATCH 1/1] arm64: syscall: Direct PRNG kstack randomization
Date: Thu, 7 Mar 2024 11:10:47 -0800	[thread overview]
Message-ID: <202403071105.C3B038C@keescook> (raw)
In-Reply-To: <f1dd15ce-69af-4315-8d7c-b7a480e541aa@app.fastmail.com>

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