linux-kernel - Re: [tip:x86/kaslr] x86, kaslr: Provide randomness functions

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Date:	Mon, 11 Nov 2013 11:27:14 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	keescook@...omium.org, tglx@...utronix.de, hpa@...ux.intel.com
CC:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/kaslr] x86, kaslr: Provide randomness functions

On 11/11/2013 10:20 AM, Ingo Molnar wrote:
> 
> Why aren't the 3 sources of entropy XOR-ed together?
> 

Note we don't want to poke i8254 if we have any other sources, as the
i8254 may not be present on newer systems and in some other cases not
safe to touch.  We can XOR in the TSC with RDRAND although it seems a
bit odd.

HOWEVER...

+		/* Only use the low bits of rdtsc. */
+		random = raw & 0xffff;

Why?!  All that can do is throw away randomness...

+	/* Extend timer bits poorly... */
+	random |= (random << 16);
+#ifdef CONFIG_X86_64
+	random |= (random << 32);
+#endif

For smearing out bits, a better way is usually to multiply with a large
prime number (preferably a circular multiply.)

> Also, we talked about also adding system dependent entropy sources, such 
> as memory layout or the DMI table - none of that seems to have happened.
> 
> It's not like this function should be performance critical, it's run once 
> per bootup, right? There's just no excuse for not maximizing available 
> entropy in such a situation ...

	-hpa

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