Commit e8236c4d authored Nov 11, 2013 by H. Peter Anvin

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

parent a653f356

arch/x86/boot/compressed/aslr.c

+11 −0

Original line number	Diff line number	Diff line
		@@ -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;