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 11:15:23 -0800	[thread overview]
Message-ID: <202403071112.01B4579@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:
> There is not even any attempt to use the most random bits of
> the cycle counter, as both the high 22 to 24 bits get masked
> out (to keep the wasted stack space small) and the low 3 or 4
> bits get ignored because of stack alignment. If there was
> any desire to make it more random, a trivial improvement
> would be:
> 
> +++ 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);                     \

Shouldn't this stay ^ instead of & ?

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

But yeah, we should likely make this change regardless.

> My impression is that is is already bordering on becoming
> a "bespoke rng" implementation that Jason was objecting to,
> so the current version is intentionally left weak in order
> to not even give the appearance of being a security relevant
> feature.

I don't think it's bad to make a trivial improvement to entropy diffusion.

-- 
Kees Cook