linux-kernel - Re: x86/mm: Randomize per-cpu entry area

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Message-ID: <Y0A7ZAjtDcq2PJaX@zx2c4.com>
Date:   Fri, 7 Oct 2022 08:44:52 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, Jann Horn <jannh@...gle.com>,
        Natalie Silvanovich <natashenka@...gle.com>,
        Seth Jenkins <sethjenkins@...gle.com>,
        Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: x86/mm: Randomize per-cpu entry area

On Fri, Oct 07, 2022 at 10:42:36AM +0200, Peter Zijlstra wrote:
> +	max_cea = (CPU_ENTRY_AREA_MAP_SIZE - PAGE_SIZE) / CPU_ENTRY_AREA_SIZE;
> +
> +	/* O(sodding terrible) */
> +	for_each_possible_cpu(i) {
> +		unsigned int cea;
> +
> +again:
> +		cea = prandom_u32_max(max_cea);

Just FYI, max_cea is 2274876 (at least in my test rig), which means the
values returned from prandom_u32_max() won't be uniformly distributed.
Right now the kernel doesn't have a function that does rejection
sampling to get uniform distribution, but I could add one if you need.
Alternatively, maybe you don't actually *need* this to be perfectly
distributed -- which seems to be a common perspective -- and so this is
good enough. Your call, but just wanted to make you aware.

Jason