From: "H. Peter Anvin" <hpa@zytor.com>
To: Ingo Molnar <mingo@kernel.org>,
linux-kernel@vger.kernel.org, keescook@chromium.org,
tglx@linutronix.de, hpa@linux.intel.com
Cc: linux-tip-commits@vger.kernel.org
Subject: Re: [tip:x86/kaslr] x86, kaslr: Provide randomness functions
Date: Mon, 11 Nov 2013 11:27:14 -0800 [thread overview]
Message-ID: <52812F92.1020006@zytor.com> (raw)
In-Reply-To: <20131111182046.GA14961@gmail.com>
On 11/11/2013 10:20 AM, Ingo Molnar wrote:
>
> Why aren't the 3 sources of entropy XOR-ed together?
>
Note we don't want to poke i8254 if we have any other sources, as the
i8254 may not be present on newer systems and in some other cases not
safe to touch. We can XOR in the TSC with RDRAND although it seems a
bit odd.
HOWEVER...
+ /* Only use the low bits of rdtsc. */
+ random = raw & 0xffff;
Why?! All that can do is throw away randomness...
+ /* Extend timer bits poorly... */
+ random |= (random << 16);
+#ifdef CONFIG_X86_64
+ random |= (random << 32);
+#endif
For smearing out bits, a better way is usually to multiply with a large
prime number (preferably a circular multiply.)
> Also, we talked about also adding system dependent entropy sources, such
> as memory layout or the DMI table - none of that seems to have happened.
>
> It's not like this function should be performance critical, it's run once
> per bootup, right? There's just no excuse for not maximizing available
> entropy in such a situation ...
-hpa
next prev parent reply other threads:[~2013-11-11 19:27 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-11 0:18 [PATCH v8 0/6] Kernel base address randomization Kees Cook
2013-10-11 0:18 ` [kernel-hardening] " Kees Cook
2013-10-11 0:18 ` [PATCH v8 1/6] x86, boot: move CPU flags out of cpucheck Kees Cook
2013-10-11 0:18 ` [kernel-hardening] " Kees Cook
2013-10-13 12:01 ` [tip:x86/kaslr] x86, boot: Move " tip-bot for Kees Cook
2013-10-13 12:02 ` [tip:x86/kaslr] x86, boot: Rename get_flags() and check_flags() to *_cpuflags() tip-bot for H. Peter Anvin
2013-10-11 0:18 ` [PATCH v8 2/6] x86, kaslr: return location from decompress_kernel Kees Cook
2013-10-11 0:18 ` [kernel-hardening] " Kees Cook
2013-10-13 12:01 ` [tip:x86/kaslr] x86, kaslr: Return " tip-bot for Kees Cook
2013-10-11 0:18 ` [PATCH v8 3/6] x86, kaslr: provide randomness functions Kees Cook
2013-10-11 0:18 ` [kernel-hardening] " Kees Cook
2013-10-13 12:01 ` [tip:x86/kaslr] x86, kaslr: Provide " tip-bot for Kees Cook
2013-11-11 18:20 ` Ingo Molnar
2013-11-11 18:31 ` Ingo Molnar
2013-11-11 19:32 ` Kees Cook
2013-11-11 19:37 ` H. Peter Anvin
2013-11-11 20:07 ` Ingo Molnar
2013-11-11 20:11 ` Kees Cook
2013-11-11 20:16 ` H. Peter Anvin
2013-11-11 19:27 ` H. Peter Anvin [this message]
2013-11-11 19:37 ` Kees Cook
2013-11-11 19:42 ` H. Peter Anvin
2013-11-11 19:58 ` Ingo Molnar
2013-11-11 20:04 ` H. Peter Anvin
2013-11-11 20:09 ` Ingo Molnar
2013-11-13 18:16 ` Pavel Machek
2013-11-13 18:40 ` H. Peter Anvin
2013-11-13 23:23 ` Pavel Machek
2013-11-13 23:25 ` H. Peter Anvin
2013-10-11 0:18 ` [PATCH v8 4/6] x86, kaslr: select random position from e820 maps Kees Cook
2013-10-11 0:18 ` [kernel-hardening] " Kees Cook
2013-10-13 12:01 ` [tip:x86/kaslr] x86, kaslr: Select " tip-bot for Kees Cook
2013-10-11 0:18 ` [PATCH v8 5/6] x86, kaslr: report kernel offset on panic Kees Cook
2013-10-11 0:18 ` [kernel-hardening] " Kees Cook
2013-10-13 12:01 ` [tip:x86/kaslr] x86, kaslr: Report " tip-bot for Kees Cook
2013-10-11 0:18 ` [PATCH v8 6/6] x86, kaslr: raise max positions to 1GiB on x86_64 Kees Cook
2013-10-11 0:18 ` [kernel-hardening] " Kees Cook
2013-10-13 12:02 ` [tip:x86/kaslr] x86, kaslr: Raise the maximum virtual address to -1 GiB " tip-bot for Kees Cook
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