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Message-ID: <tip-e8236c4d9338d52d0f2fcecc0b792ac0542e4ee9@git.kernel.org>
Date: Mon, 11 Nov 2013 23:09:44 -0800
From: "tip-bot for H. Peter Anvin" <tipbot@...or.com>
To: linux-tip-commits@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
keescook@...omium.org, tglx@...utronix.de
Subject: [tip:x86/kaslr] x86, kaslr:
Add a circular multiply for better bit diffusion
Commit-ID: e8236c4d9338d52d0f2fcecc0b792ac0542e4ee9
Gitweb: http://git.kernel.org/tip/e8236c4d9338d52d0f2fcecc0b792ac0542e4ee9
Author: H. Peter Anvin <hpa@...or.com>
AuthorDate: Mon, 11 Nov 2013 22:45:20 -0800
Committer: H. Peter Anvin <hpa@...or.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 <keescook@...omium.org>
Signed-off-by: H. Peter Anvin <hpa@...or.com>
Link: http://lkml.kernel.org/r/20131111222839.GA28616@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;
--
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