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author | H. Peter Anvin <hpa@zytor.com> | 2013-11-12 07:45:20 +0100 |
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committer | H. Peter Anvin <hpa@zytor.com> | 2013-11-12 08:05:49 +0100 |
commit | e8236c4d9338d52d0f2fcecc0b792ac0542e4ee9 (patch) | |
tree | bf3b408a92ba327ce264a47ee1bb30417feaf5d5 | |
parent | x86, kaslr: Mix entropy sources together as needed (diff) | |
download | linux-e8236c4d9338d52d0f2fcecc0b792ac0542e4ee9.tar.xz linux-e8236c4d9338d52d0f2fcecc0b792ac0542e4ee9.zip |
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
-rw-r--r-- | arch/x86/boot/compressed/aslr.c | 11 |
1 files changed, 11 insertions, 0 deletions
diff --git a/arch/x86/boot/compressed/aslr.c b/arch/x86/boot/compressed/aslr.c index 8746487fa916..38a07cc4fbac 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; |