Re: [BUG] perf: bogus correlation of kernel symbols

[Ingo Molnar]

* Valdis.Kletnieks@xxxxxx <Valdis.Kletnieks@xxxxxx> wrote:

> On Mon, 23 May 2011 12:49:02 +0200, Ingo Molnar said:
> > Well, since entropy does not get reduced on addition of independent variables 
> > the right sequence is (pseudocode):
> >
> > 	rnd  = entropy_cycles();
> > 	rnd += entropy_rdrand();
> > 	rnd += entropy_RTC();
> > 	rnd += entropy_system();
> 
> I'm having trouble convincing myself that RTC and cycles are truly independent
> variables.... ;)

Generally the RTC stores absolute time in seconds (it stores the date), while 
cycles start new when the CPU is reset.

So they are independent.

The question i think you are asking is whether the fact that we can observe 
current values of them after bootup can be used to figure out their value:

> Consider the case of a fixed-frequency CPU - if you know the time since boot, 
> and the current RTC, and the current cycle count, you can work backwards to 
> find the RTC and cycle count at boot. [...]

Yes, you are correct, if you are local then the guessing the RTC to the second 
is probably possible.

Guessing the cycle counter's value will be hard: see the natural noise it has 
at a fixed instruction after bootup in the same-bzImage test i performed - with 
no IRQs having executed at all yet ...

The RTC is still reasonably noisy to external attackers though.

> [...] I'm not sure that a variable clockspeed helps all that much - an 
> attacker can perhaps find a way to force the highest/ lowest CPU speed - or 
> the system may even helpfully do it for the attacker - I've seen plenty of 
> misconfigured laptops that force lowest supported CPU clockspeed on battery 
> rather than race-to-idle.

The tests i performed were on a fixed frequency system - the cycle counter was 
still largely random during early bootup.

Others should try it too - i've attached a simple patch. Maybe my system has 
more bootup noise than others.

> Having said that, the 13 bootup rdtsc values you list *seem* to have on the 
> order of 24-28 bits of entropy, and only the lowest-order bit seems to be 
> non-random (the low-order byte of the 13 values are 28, b6, 44, 54, dc, 78, 
> 2c, 38, 02, 58, 76, 16, and be).  So rdtsc appears to be good enough for what 
> we want here...

Yeah. And for cases that the rdtsc might be predictable for some weird reason 
(say it would be 0 on an old system with no RDTSC), the RTC would give some 
minimal fallback seed to make the canary at least not remotely guessable.

Thanks,

	Ingo

---
 init/main.c |    6 ++++++
 1 file changed, 6 insertions(+)

Index: linux/init/main.c
===================================================================
--- linux.orig/init/main.c
+++ linux/init/main.c
@@ -472,6 +472,12 @@ asmlinkage void __init start_kernel(void
 	 */
 	boot_init_stack_canary();
 
+	{
+		u64 cycles = get_cycles();
+
+		printk("RDTSC: %Ld / %08Lx\n", cycles, cycles);
+	}
+
 	cgroup_init_early();
 
 	local_irq_disable();
--
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