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Message-ID: <517AF6A5.7070507@zytor.com>
Date: Fri, 26 Apr 2013 14:50:29 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Kees Cook <keescook@...omium.org>
CC: linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
x86@...nel.org, Jarkko Sakkinen <jarkko.sakkinen@...el.com>,
Matthew Garrett <mjg@...hat.com>,
Matt Fleming <matt.fleming@...el.com>,
Eric Northup <digitaleric@...gle.com>,
Dan Rosenberg <drosenberg@...curity.com>,
Julien Tinnes <jln@...gle.com>, Will Drewry <wad@...omium.org>
Subject: Re: [PATCH 4/6] x86: kaslr: select random base offset
On 04/26/2013 12:03 PM, Kees Cook wrote:
> +
> +static unsigned long get_random_long(void)
> +{
> + if (has_cpuflag(X86_FEATURE_RDRAND)) {
> + unsigned long random;
> +
> + debug_putstr("KASLR using RDRAND...\n");
> + if (rdrand(&random))
> + return random;
> + }
> +
> + if (has_cpuflag(X86_FEATURE_TSC)) {
> + uint32_t raw;
> + unsigned long timer;
> +
> + debug_putstr("KASLR using RDTSC...\n");
> + rdtscl(raw);
> +
> + /* Repeat the low bits of rdtsc. */
> + timer = raw & 0xffff;
> + timer |= (timer << 16);
> +#ifdef CONFIG_X86_64
> + timer |= (timer << 32) | (timer << 48);
> +#endif
> +
This seems like a very odd thing to do. If you want to scramble bits,
it would make more sense to do a multiply -- or much better, a
*circular* multiply -- with a large constant.
> + return timer;
> + }
> +
> + debug_putstr("KASLR found no entropy source...\n");
> + return 0;
> +}
>
It might be safe to assume that anything old enough to lack RDTSC
(basically a 486) will have an 8254, and reading back the 8254 counter
register.
-hpa
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