Re: [PATCH] mm: Add the ram_latent_entropy kernel parameter

[Dave Hansen]

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? :)