Commit-ID: e8236c4d9338d52d0f2fcecc0b792ac0542e4ee9 Gitweb: http://git.kernel.org/tip/e8236c4d9338d52d0f2fcecc0b792ac0542e4ee9 Author: H. Peter Anvin <h...@zytor.com> AuthorDate: Mon, 11 Nov 2013 22:45:20 -0800 Committer: H. Peter Anvin <h...@zytor.com> CommitDate: Mon, 11 Nov 2013 23:05:49 -0800
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 <keesc...@chromium.org> Signed-off-by: H. Peter Anvin <h...@zytor.com> Link: http://lkml.kernel.org/r/20131111222839.ga28...@www.outflux.net --- arch/x86/boot/compressed/aslr.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/arch/x86/boot/compressed/aslr.c b/arch/x86/boot/compressed/aslr.c index 8746487..38a07cc 100644 --- a/arch/x86/boot/compressed/aslr.c +++ b/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; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
