linux-kernel - Re: [PATCH] mm: Add the ram_latent

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Date:	Thu, 11 Aug 2016 08:39:47 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Emese Revfy <re.emese@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	Vlastimil Babka <vbabka@...e.cz>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Michal Hocko <mhocko@...e.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Taku Izumi <izumi.taku@...fujitsu.com>,
	linux-doc@...r.kernel.org, linux-mm@...ck.org,
	kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] mm: Add the ram_latent_entropy kernel parameter

On 08/10/2016 03:28 PM, Kees Cook wrote:
> +	if (ram_latent_entropy && !PageHighMem(page) &&
> +		page_to_pfn(page) < 0x100000) {
> +		u64 hash = 0;
> +		size_t index, end = PAGE_SIZE * nr_pages / sizeof(hash);
> +		const u64 *data = lowmem_page_address(page);
> +
> +		for (index = 0; index < end; index++)
> +			hash ^= hash + data[index];
> +		add_device_randomness((const void *)&hash, sizeof(hash));
> +	}

When I was first reading this, I thought it was using the _addresses_ of
the freed memory for entropy.  But it's actually using the _contents_.
The description could probably use a wee bit of sprucing up.

It might also be nice to say in the patch description (and the
Documentation/) what you expect to be in this memory.  It will obviously
be zeros for the vast majority of the space, but I do wonder what else
ends up in there in practice.

Why is it limited to 4GB?  Just so it doesn't go and try to XOR the
contents of a multi-TB system if it got turned on there? :)