x86, kaslr: Add a circular multiply for better bit diffusion

If we don't have RDRAND (in which case nothing else *should* matter),
most sources have a highly biased entropy distribution.  Use a
circular multiply to diffuse the entropic bits.  A circular multiply
is a good operation for this: it is cheap on standard hardware and
because it is symmetric (unlike an ordinary multiply) it doesn't
introduce its own bias.

Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/r/20131111222839.GA28616@www.outflux.net

arch/x86/boot/compressed/aslr.c

@@ -64,6 +64,11 @@ static unsigned long get_random_boot(void)
 
 static unsigned long get_random_long(void)
 {
+#ifdef CONFIG_X86_64
+	const unsigned long mix_const = 0x5d6008cbf3848dd3UL;
+#else
+	const unsigned long mix_const = 0x3f39e593UL;
+#endif
 	unsigned long raw, random = get_random_boot();
 	bool use_i8254 = true;
 
@@ -90,6 +95,12 @@ static unsigned long get_random_long(void)
 		random ^= i8254();
 	}
 
+	/* Circular multiply for better bit diffusion */
+	asm("mul %3"
+	    : "=a" (random), "=d" (raw)
+	    : "a" (random), "rm" (mix_const));
+	random += raw;
+
 	debug_putstr("...\n");
 
 	return random;