[PATCH,RFC] random: collect cpu randomness

[PATCH,RFC] random: collect cpu randomness

[Jörn Engel]

Collects entropy from random behaviour all modern cpus exhibit.  The
scheduler and slab allocator are instrumented for this purpose.  How
much randomness can be gathered is clearly hardware-dependent and hard
to estimate.  Therefore the entropy estimate is zero, but random bits
still get mixed into the pools.

Performance overhead seems low.  I did 20 kernel compiles with
allnoconfig, everything precompiled and warm caches.  Difference was in
the noise and on average performance was actually better when collecting
randomness.

To judge the benefits I ran this in kvm with instrumentation to print
out the various input values.  Entropy was estimated by booting ten
times, collecting the output for each boot, doing a diff between any two
and counting the diff hunks.  Numbers like "46 .. 215" means the two
most similar runs had 46 hunks, the two most dissimilar ones had 215.

Running kvm with -smp 4 gave the following results:
                 slab+sched     slab            sched
input[0]         46 .. 215      202 .. 342        0 ..   0
input[1]         76 .. 180        0 ..   4        0 ..   0
input[2]        147 .. 388        4 ..  44      286 .. 464
input[3]         56 .. 185        0 ..   2        9 ..  40
jiffies           8 ..  67       10 ..  28       19 ..  50
caller          114 .. 306       25 .. 178      219 .. 422
val              54 .. 254       15 .. 106      175 .. 321
&input           50 .. 246        0 ..  59      178 .. 387

First column collected entropy from both the slab allocator and the
scheduler, second and third only collected from one source.  I only used
the first 512 values per cpu.  Therefore the combined numbers can be
lower than the individual ones - there was more entropy collected, it
was just swamped by non-entropy.

Lots of entropy gets collected and about half of it stems from the
uninitialized valued of input[] on the stack.

Rerunning only the first column with -smp 1 was more sobering:
                slab+sched
input[0]          0 ..  13
input[1]          0 ..  13
input[2]          0 ..  14
input[3]          0 ..  11
jiffies           2 ..  22
caller            0 ..  19
val               0 ..  16
&input            0 ..   4

Every single input value contains entropy in some runs, but the only one
that was an entropy guarantee was jiffies.  In other words, a
low-precision clock.  This clearly shows how important it is to have a
high-precision clock, which would gather significantly more entropy.

Measuring the randomness from random_get_entropy() with above approach
failed because there was so much randomness.  All numbers in all runs
were different.  Taking the delta between the numbers, again almost all
numbers were different with at most 1 identical delta per 1000.
Compared to a high-precision clock, no other input comes within two
orders of magnitude.

An interesting aside is that the cpu actually functions as a
high-precision clock.  This partially explains why the -smp 4 number are
so much better - any drift between the four cpu-clocks gets turned into
randomness.  The other explanation is that kvm is actually collecting
randomness from the host system.  I tried to minimize that effect by not
touching my notebook while running these tests, but a hardware test box
would clearly yield more meaningful results.

I invite others to also test whether the patch collects useful
randomness or causes a measureable performance loss on any hardware or
workload.  Preferrably not using my methods, in order to avoid
systematic errors.

Signed-off-by: Joern Engel <joern@logfs.org>
---
 drivers/char/random.c  | 61 ++++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/random.h |  2 ++
 kernel/sched/core.c    |  1 +
 mm/slab.c              |  1 +
 4 files changed, 65 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 429b75bb60e8..693dea730a3e 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -587,6 +587,30 @@ static void fast_mix(struct fast_pool *f, __u32 input[4])
 }
 
 /*
+ * An even faster variant of fast_mix, albeit with worse mixing.
+ */
+static void weak_mix(struct fast_pool *f, __u32 input[4])
+{
+	__u32 acc, carry = f->pool[3] >> 25;
+
+	acc = (f->pool[0] << 7) ^ input[0] ^ carry;
+	carry = f->pool[0] >> 25;
+	f->pool[0] = acc;
+
+	acc = (f->pool[1] << 7) ^ input[1] ^ carry;
+	carry = f->pool[1] >> 25;
+	f->pool[1] = acc;
+
+	acc = (f->pool[2] << 7) ^ input[2] ^ carry;
+	carry = f->pool[2] >> 25;
+	f->pool[2] = acc;
+
+	acc = (f->pool[3] << 7) ^ input[3] ^ carry;
+	//carry = f->pool[3] >> 25;
+	f->pool[3] = acc;
+}
+
+/*
  * Credit (or debit) the entropy store with n bits of entropy.
  * Use credit_entropy_bits_safe() if the value comes from userspace
  * or otherwise should be checked for extreme values.
@@ -833,6 +857,43 @@ void add_input_randomness(unsigned int type, unsigned int code,
 }
 EXPORT_SYMBOL_GPL(add_input_randomness);
 
+static DEFINE_PER_CPU(struct fast_pool, cpu_randomness);
+
+void __add_cpu_randomness(void *caller, void *val)
+{
+	struct entropy_store	*r;
+	struct fast_pool	*fast_pool = &__get_cpu_var(cpu_randomness);
+	unsigned long		now = jiffies;
+	__u32			input[4], cycles = random_get_entropy();
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wuninitialized"
+	input[0] ^= cycles ^ jiffies;
+	input[1] ^= (unsigned long)caller;
+	input[2] ^= (unsigned long)val;
+	input[3] ^= (unsigned long)&input;
+#pragma GCC diagnostic pop
+
+	weak_mix(fast_pool, input);
+
+	if ((fast_pool->count++ & 1023) &&
+	    !time_after(now, fast_pool->last + HZ))
+		return;
+
+	fast_pool->last = now;
+	fast_pool->count = 1;
+
+	r = nonblocking_pool.initialized ? &input_pool : &nonblocking_pool;
+	__mix_pool_bytes(r, &fast_pool->pool, sizeof(fast_pool->pool), NULL);
+}
+
+void add_cpu_randomness(void *caller, void *val)
+{
+	preempt_disable();
+	__add_cpu_randomness(caller, val);
+	preempt_enable();
+}
+
 static DEFINE_PER_CPU(struct fast_pool, irq_randomness);
 
 void add_interrupt_randomness(int irq, int irq_flags)
diff --git a/include/linux/random.h b/include/linux/random.h
index 4002b3df4c85..ce0ccdcd1d63 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -12,6 +12,8 @@
 extern void add_device_randomness(const void *, unsigned int);
 extern void add_input_randomness(unsigned int type, unsigned int code,
 				 unsigned int value);
+extern void __add_cpu_randomness(void *caller, void *val);
+extern void add_cpu_randomness(void *caller, void *val);
 extern void add_interrupt_randomness(int irq, int irq_flags);
 
 extern void get_random_bytes(void *buf, int nbytes);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a88f4a485c5e..7af6389f9b9e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2511,6 +2511,7 @@ need_resched:
 	rq = cpu_rq(cpu);
 	rcu_note_context_switch(cpu);
 	prev = rq->curr;
+	__add_cpu_randomness(__builtin_return_address(1), prev);
 
 	schedule_debug(prev);
 
diff --git a/mm/slab.c b/mm/slab.c
index eb043bf05f4c..ea5a30d44ad1 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -3587,6 +3587,7 @@ static __always_inline void *__do_kmalloc(size_t size, gfp_t flags,
 	trace_kmalloc(caller, ret,
 		      size, cachep->size, flags);
 
+	add_cpu_randomness(__builtin_return_address(2), ret);
 	return ret;
 }
 
-- 
1.8.5.2

Re: [PATCH,RFC] random: collect cpu randomness

[Stephan Mueller]

Am Sonntag, 2. Februar 2014, 15:36:17 schrieb Jörn Engel:

Hi Jörn,

> Collects entropy from random behaviour all modern cpus exhibit.  The
> scheduler and slab allocator are instrumented for this purpose.  How
> much randomness can be gathered is clearly hardware-dependent and hard
> to estimate.  Therefore the entropy estimate is zero, but random bits
> still get mixed into the pools.

May I ask what the purpose of the patches is when no entropy is implied? I see 
that the pool is stirred more. But is that really a problem that needs 
addressing?

Please, do not get me wrong with the presented critisism here -- the approach 
in general looks interesting.

However, the following patches makes me wonder big time.

>  extern void get_random_bytes(void *buf, int nbytes);
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a88f4a485c5e..7af6389f9b9e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2511,6 +2511,7 @@ need_resched:
>  	rq = cpu_rq(cpu);
>  	rcu_note_context_switch(cpu);
>  	prev = rq->curr;
> +	__add_cpu_randomness(__builtin_return_address(1), prev);
> 
>  	schedule_debug(prev);
> 
> diff --git a/mm/slab.c b/mm/slab.c
> index eb043bf05f4c..ea5a30d44ad1 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3587,6 +3587,7 @@ static __always_inline void *__do_kmalloc(size_t size,
> gfp_t flags, trace_kmalloc(caller, ret,
>  		      size, cachep->size, flags);
> 
> +	add_cpu_randomness(__builtin_return_address(2), ret);
>  	return ret;
>  }

First, the noise source you add is constantly triggered throughout the 
execution of the kernel. Entropy is very important, we (who are interested in 
crypto) know that. But how often is entropy needed? Other folks wonder about 
the speed of the kernel. And with these two patches, every kmalloc and every 
scheduling invocation now dives into the random.c code to do something. I 
would think this is a bit expensive, especially to stir the pool without 
increasing the entropy estimator. I think entropy collection should be 
performed when it is needed and not throughout the lifetime of the system.

Second, when I offered my initial patch which independently collects some 
entropy on the CPU execution timing, I got shot down with one concern raised 
by Ted, and that was about whether a user can influence the entropy collection 
process. When I am trying to measure CPU execution timing in the RNG, the 
concern was raised that the measured timing variations was due to CPU states 
that were influenced by users. Your patch here clearly hooks into code paths 
which are definitely affected by user actions. So, this patch therefore would 
be subject to the same concerns. I personally think that this is not so much 
an issue, yet it was raised previously.

It seems I have a bad timing, because just two days ago I released a new 
attempt on the CPU jitter RNG [1] with a new noise source, and I was just 
about to prepare a release email. With that attempt, both issues raised above 
are addressed, including a theoretical foundation of the noise source.

[1] http://www.chronox.de/

Ciao
Stephan
-- 
| Cui bono? |

Re: [PATCH,RFC] random: collect cpu randomness

[Jörn Engel]

On Sun, 2 February 2014 22:25:31 +0100, Stephan Mueller wrote:
> Am Sonntag, 2. Februar 2014, 15:36:17 schrieb Jörn Engel:
> 
> > Collects entropy from random behaviour all modern cpus exhibit.  The
> > scheduler and slab allocator are instrumented for this purpose.  How
> > much randomness can be gathered is clearly hardware-dependent and hard
> > to estimate.  Therefore the entropy estimate is zero, but random bits
> > still get mixed into the pools.
> 
> May I ask what the purpose of the patches is when no entropy is implied? I see 
> that the pool is stirred more. But is that really a problem that needs 
> addressing?

For my part, I think the whole business of estimating entropy is
bordering on the esoteric.  If the hash on the output side is any
good, you have a completely unpredictable prng once the entropy pool
is unpredictable.  Additional random bits are nice, but not all that
useful.  Blocking /dev/random based on entropy estimates is likewise
not all that useful.

Key phrase is "once the entropy pool is unpredictable".  So early in
bootup it may make sense to estimate the entropy.  But here the
problem is that you cannot measure entropy, at least not within a
single system and a reasonable amount of time.  That leaves you with a
heuristic that, like all heuristics, is wrong.

I personally care more about generating high-quality randomness as
soon as possible and with low cost to the system.  Feel free to
disagree or set your priorities differently.

> Please, do not get me wrong with the presented critisism here -- the approach 
> in general looks interesting.
> 
> However, the following patches makes me wonder big time.
> 
> >  extern void get_random_bytes(void *buf, int nbytes);
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index a88f4a485c5e..7af6389f9b9e 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -2511,6 +2511,7 @@ need_resched:
> >  	rq = cpu_rq(cpu);
> >  	rcu_note_context_switch(cpu);
> >  	prev = rq->curr;
> > +	__add_cpu_randomness(__builtin_return_address(1), prev);
> > 
> >  	schedule_debug(prev);
> > 
> > diff --git a/mm/slab.c b/mm/slab.c
> > index eb043bf05f4c..ea5a30d44ad1 100644
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -3587,6 +3587,7 @@ static __always_inline void *__do_kmalloc(size_t size,
> > gfp_t flags, trace_kmalloc(caller, ret,
> >  		      size, cachep->size, flags);
> > 
> > +	add_cpu_randomness(__builtin_return_address(2), ret);
> >  	return ret;
> >  }
> 
> First, the noise source you add is constantly triggered throughout the 
> execution of the kernel. Entropy is very important, we (who are interested in 
> crypto) know that. But how often is entropy needed? Other folks wonder about 
> the speed of the kernel. And with these two patches, every kmalloc and every 
> scheduling invocation now dives into the random.c code to do something. I 
> would think this is a bit expensive, especially to stir the pool without 
> increasing the entropy estimator. I think entropy collection should be 
> performed when it is needed and not throughout the lifetime of the system.

Please measure how expensive it really is.  My measurement gave me a
"doesn't matter" result, surprising as it may seem.

If the cost actually matters, we can either disable or rate-limit the
randomness collection at some point after boot.  But that would bring
us back into the estimation business.

> Second, when I offered my initial patch which independently collects some 
> entropy on the CPU execution timing, I got shot down with one concern raised 
> by Ted, and that was about whether a user can influence the entropy collection 
> process. When I am trying to measure CPU execution timing in the RNG, the 
> concern was raised that the measured timing variations was due to CPU states 
> that were influenced by users. Your patch here clearly hooks into code paths 
> which are definitely affected by user actions. So, this patch therefore would 
> be subject to the same concerns. I personally think that this is not so much 
> an issue, yet it was raised previously.

The nice thing about the random pool is that mixing any amount of
deterministic data into it does not diminish the randomness already in
it.  Given that attribute, I don't understand the concern.

> It seems I have a bad timing, because just two days ago I released a new 
> attempt on the CPU jitter RNG [1] with a new noise source, and I was just 
> about to prepare a release email. With that attempt, both issues raised above 
> are addressed, including a theoretical foundation of the noise source.
> 
> [1] http://www.chronox.de/

I am not married to my patch.  If the approach makes sense, let's
merge it.  If the approach does not make sense or there is a better
alternative, drop it on the floor.

The problem I see with your approach is this:
"The only prerequisite is the availability of a high-resolution timer
that is available in modern CPUs."

Given a modern CPU with a high-resolution timer, you will almost
certainly collect enough randomness for good random numbers.  Problem
solved and additional improvements are useless.

But on embedded systems with less modern CPUs, few interrupt sources,
no user interface, etc. you may have trouble collecting enough
randomness or doing it soon enough.  That is the problem worth fixing.
It is also a hard problem to fix and I am not entirely convinced I
found a good approach.

Jörn

--
It's just what we asked for, but not what we want!
-- anonymous

Re: [PATCH,RFC] random: collect cpu randomness

[H. Peter Anvin]

On 02/02/2014 05:24 PM, Jörn Engel wrote:
> 
> For my part, I think the whole business of estimating entropy is
> bordering on the esoteric.  If the hash on the output side is any
> good, you have a completely unpredictable prng once the entropy pool
> is unpredictable.  Additional random bits are nice, but not all that
> useful.  Blocking /dev/random based on entropy estimates is likewise
> not all that useful.
> 
> Key phrase is "once the entropy pool is unpredictable".  So early in
> bootup it may make sense to estimate the entropy.  But here the
> problem is that you cannot measure entropy, at least not within a
> single system and a reasonable amount of time.  That leaves you with a
> heuristic that, like all heuristics, is wrong.
> 

The entropy bound needs to be a conservative lower bound.  Its main use
is to provide backpressure (should we spend more CPU time producing
entropy) although the forward pressure on /dev/random is potentially
useful for high security applications.

This does NOT mean that zero-credit entropy generation is useless, far
from it.  It just means that we are doing it on an "it can't hurt"
basis, rather than "I know for sure that this is valuable."

	-hpa

Re: [PATCH,RFC] random: collect cpu randomness

[Stephan Mueller]

Am Sonntag, 2. Februar 2014, 20:24:21 schrieb Jörn Engel:

Hi Jörn,

>On Sun, 2 February 2014 22:25:31 +0100, Stephan Mueller wrote:
>> Am Sonntag, 2. Februar 2014, 15:36:17 schrieb Jörn Engel:
>> > Collects entropy from random behaviour all modern cpus exhibit. 
>> > The
>> > scheduler and slab allocator are instrumented for this purpose. 
>> > How
>> > much randomness can be gathered is clearly hardware-dependent and
>> > hard
>> > to estimate.  Therefore the entropy estimate is zero, but random
>> > bits
>> > still get mixed into the pools.
>> 
>> May I ask what the purpose of the patches is when no entropy is
>> implied? I see that the pool is stirred more. But is that really a
>> problem that needs addressing?
>
>For my part, I think the whole business of estimating entropy is
>bordering on the esoteric.  If the hash on the output side is any
>good, you have a completely unpredictable prng once the entropy pool
>is unpredictable.  Additional random bits are nice, but not all that
>useful.  Blocking /dev/random based on entropy estimates is likewise
>not all that useful.

I really like that statement, because for the most part I concur :-)

However, there are a number of cryptographers out there, which insist on 
such entropy assessments and even the blocking behavior. For example, I 
work with cryptographers from the German BSI. We created a quantitative 
assessment of /dev/random for them (see [1]). During the discussion, I 
learned that the key reason they like /dev/random and dislike 
/dev/urandom is the fact that /dev/random ensures that any output of 
data is always backed by hardware entropy. In order to ensure that you 
always have hardware entropy that backs your output, you somehow must 
quantify that hardware entropy. Thus, a dropping of the entropy 
estimation would be catastrophic for them.

When you look at NIST and the base discussions in SP800-90A, you see 
that for deterministic RNGs, NIST is not that strict as BSI. Yet, they 
require a DRNG to be reseeded with entropy after a (large) number of 
generated bits. For that reseeding process, some entropy estimation is 
needed. But when looking at SP800-90B, things get hairy again where some 
strict entropy estimations are needed.

Even if you subscribe to the notion that an RNG only needs some X bits 
of entropy for starters and then can spin indefinitely on this entropy, 
there is still a need on estimating entropy, at least at the beginning.
>
>Key phrase is "once the entropy pool is unpredictable".  So early in
>bootup it may make sense to estimate the entropy.  But here the

I am fully in agreement here.

>problem is that you cannot measure entropy, at least not within a
>single system and a reasonable amount of time.  That leaves you with a
>heuristic that, like all heuristics, is wrong.

No argument here :-)

(side note: The interesting thing is that the /dev/random heuristic on 
entropy seems to underestimate the entropy present in events where the 
heuristic assumes low entropy, but way overestimates entropy where the 
heuristic entropy assumes high entropy)
>
>I personally care more about generating high-quality randomness as
>soon as possible and with low cost to the system.  Feel free to
>disagree or set your priorities differently.

Fully in agreement

[..]
>> First, the noise source you add is constantly triggered throughout
>> the
>> execution of the kernel. Entropy is very important, we (who are
>> interested in crypto) know that. But how often is entropy needed?
>> Other folks wonder about the speed of the kernel. And with these two
>> patches, every kmalloc and every scheduling invocation now dives
>> into the random.c code to do something. I would think this is a bit
>> expensive, especially to stir the pool without increasing the
>> entropy estimator. I think entropy collection should be performed
>> when it is needed and not throughout the lifetime of the system.
>Please measure how expensive it really is.  My measurement gave me a
>"doesn't matter" result, surprising as it may seem.

That sounds really good.
>
>If the cost actually matters, we can either disable or rate-limit the
>randomness collection at some point after boot.  But that would bring
>us back into the estimation business.
>
>> It seems I have a bad timing, because just two days ago I released a
>> new attempt on the CPU jitter RNG [1] with a new noise source, and I
>> was just about to prepare a release email. With that attempt, both
>> issues raised above are addressed, including a theoretical
>> foundation of the noise source.
>> 
>> [1] http://www.chronox.de/
>
>I am not married to my patch.  If the approach makes sense, let's
>merge it.  If the approach does not make sense or there is a better
>alternative, drop it on the floor.
>
>The problem I see with your approach is this:
>"The only prerequisite is the availability of a high-resolution timer
>that is available in modern CPUs."

Right, and with the absence of a high-resolution counter, my RNG breaks 
down. Though, I have more goals than to just run my RNG inside the Linux 
kernel and thus the reliance on only the timer.

However, during my testing on also embedded systems, a significant 
number have them -- at least a counter that is integrated with the 
clock_source framework.

Allow me to explain my RNG and the new developments in a separate email 
to not hijack your thread here.

I think both have merits, considering that based on your statement, the 
integration into schedule/kmalloc code paths is not that expensive.
>
>Given a modern CPU with a high-resolution timer, you will almost
>certainly collect enough randomness for good random numbers.  Problem
>solved and additional improvements are useless.
>
>But on embedded systems with less modern CPUs, few interrupt sources,
>no user interface, etc. you may have trouble collecting enough
>randomness or doing it soon enough.  That is the problem worth fixing.
>It is also a hard problem to fix and I am not entirely convinced I
>found a good approach.

I do not think that there can be a right approach given the variety of 
systems Linux can run on. Though, the current random.c definitely is 
challenged in embedded systems, but also on "normal" headless systems 
with SSDs. Thus, any proposal of using new entropy sources is good.

[1] 
https://www.bsi.bund.de/DE/Publikationen/Studien/LinuxRNG/index_htm.html

Ciao
Stephan

Re: [PATCH,RFC] random: collect cpu randomness

[Theodore Ts'o]

On Sun, Feb 02, 2014 at 10:25:31PM +0100, Stephan Mueller wrote:
> Second, when I offered my initial patch which independently collects some 
> entropy on the CPU execution timing, I got shot down with one concern raised 
> by Ted, and that was about whether a user can influence the entropy collection 
> process.

Um, that wasn't my concern.  After all, when we sample keyboard timing
while trying to generate a GPG key, of course the user can and does
influence the entropy collection process.

The question is whether an attacker who has deep knowledge of the how
the CPU works internally, perhaps made worse with quantization effects
(i.e., it doesn't matter if analog-generated settling time is measured
in microseconds if the output is being clocked out in milliseconds),
such that it is predictable.

I really like Jörn's tests doing repeated boot testing and observing
on a SMP system, the slab allocation pattern is quite deterministic.
So even though the numbers might *look* random, an attacker with deep
knowledge of how the kernel was compiled and what memory allocations
get done during the boot sequence would be able to quite successfuly
measure it.

I'm guessing that indeed, on a 4-CPU KVM system, what you're measuring
is the when the host OS happens to be scheduling the KVM threads, with
some variability caused by external networking interrupts, etc.  It
would definitely be a good idea to retry that experiment on a real
4-CPU system to see what sort of results you might get.  It might very
well be that the attacker who knows the relative ordering of the
slab/thread activations but for which it's not entirely clear whether
one cpu will be ahead of another, that there is *some* entropy, but
perhaps only a handful bits.  It's the fact that we can't be sure how
much uncertainty there might be with an attacker with very deep
knowledge the CPU which is why Jörn's conservatism of not crediting
the entropy counter is quite understandable.

Of course, this doesn't help someone who is trying to speed up the
time it takes GPG to generate a new key pair.  But in terms of
improving /dev/urandom as it is used by many crypto applications, it
certainly can't hurt.

The real question is how much overhead does it add, and is it worth
it.  Jörn, I take it that was the reason for creating an even faster,
but weaker mixing function?  Was the existing "fast mix" causing a
measurable overhead, or was this your just being really paranoid about
not adding anything to the various kernel fastpaths?

    	   	       	   	   	  - Ted

Re: [PATCH,RFC] random: collect cpu randomness

[Jörn Engel]

On Sun, 2 February 2014 20:39:22 -0500, Theodore Ts'o wrote:
> 
> The real question is how much overhead does it add, and is it worth
> it.  Jörn, I take it that was the reason for creating an even faster,
> but weaker mixing function?  Was the existing "fast mix" causing a
> measurable overhead, or was this your just being really paranoid about
> not adding anything to the various kernel fastpaths?

It was paranoia.  And I am still somewhat paranoid and don't trust my
benchmark results yet.  Maybe on an 1024-CPU Altix with a 100k-thread
workload the overhead is too much.  Just because I couldn't measure a
difference on my wimpy notebook does not mean much.

Jörn

--
One of the painful things about our time is that those who feel certainty
are stupid, and those with any imagination and understanding are filled
with doubt and indecision.
-- Bertrand Russell

Re: [PATCH,RFC] random: collect cpu randomness

[Thorsten Glaser]

Theodore Ts'o dixit:

>I really like Jvrn's tests doing repeated boot testing and observing
>on a SMP system, the slab allocation pattern is quite deterministic.
>So even though the numbers might *look* random, an attacker with deep
>knowledge of how the kernel was compiled and what memory allocations

For this reason, we pre-initialise (one of) the RNG during kernel
compile time, using host randomness, in MirBSD. We’re also considering
pushing some additional seed using the bootloader, that can be mixed
in very very early. As an added benefit, everything in the kernel can
call arc4random(), arc4random_buf() and arc4random_uniform() without
worrying about initialisation, at all times. (This is mostly for the
places where it replaced random(), or which explicitly waited for the
regular RNG initialisation to be done, or which reseeded after that.)

In GNU/Linux, this could work like, GRUB will offer the contents of
/boot/grub/randseed.bin to the Linux kernel, which will add it into
the pool using the normal mechanisms, and (very?) early userspace
will then recreate that file (to prevent seed reuse, just like the
normal /var/db/host.random or /var/lib/urandom/random-seed processing
is done). Downside: write access to the boot medium needed. (No GRUB
modification needed, this could be passed as “faux kernel module”.)
Upside: this is way earlier than anything user space can do, and
e.g. early enough to affect things like kernel memory management.

bye,
//mirabilos
-- 
<Natureshadow> Ach, mach doch was du willst, du hast doch eh immer Recht!
<mirabilos> jupp ~/.etc/sig………
<Natureshadow> unfaßbar…
<Natureshadow> Mit Eszett sogar, unfaßbar!

Re: [PATCH,RFC] random: collect cpu randomness

[Stephan Mueller]

Am Sonntag, 2. Februar 2014, 20:39:22 schrieb Theodore Ts'o:

Hi Theodore,

>On Sun, Feb 02, 2014 at 10:25:31PM +0100, Stephan Mueller wrote:
>> Second, when I offered my initial patch which independently collects
>> some entropy on the CPU execution timing, I got shot down with one
>> concern raised by Ted, and that was about whether a user can
>> influence the entropy collection process.
>
>Um, that wasn't my concern.  After all, when we sample keyboard timing
>while trying to generate a GPG key, of course the user can and does
>influence the entropy collection process.

Thank you for clarifying and sorry that I misunderstood you.

>I really like Jörn's tests doing repeated boot testing and observing
>on a SMP system, the slab allocation pattern is quite deterministic.
>So even though the numbers might *look* random, an attacker with deep
>knowledge of how the kernel was compiled and what memory allocations
>get done during the boot sequence would be able to quite successfuly
>measure it.
>
>I'm guessing that indeed, on a 4-CPU KVM system, what you're measuring

Please let me point out that I am not testing on KVM, but on Linux 
running natively on the hardware. All major CPUs were tested. Even tests 
are executed on bare metal, i.e. without any OS and interrupts disabled. 
But I will explain that in a separate email and do not want to hijack 
this thread.

Ciao
Stephan

Re: [PATCH,RFC] random: collect cpu randomness

[Jörn Engel]

On Sun, 2 February 2014 15:36:17 -0500, Jörn Engel wrote:
> 
> Measuring the randomness from random_get_entropy() with above approach
> failed because there was so much randomness.  All numbers in all runs
> were different.  Taking the delta between the numbers, again almost all
> numbers were different with at most 1 identical delta per 1000.
> Compared to a high-precision clock, no other input comes within two
> orders of magnitude.

I think this is a key result from my tests.  The best source is a
timer (counter) that is both
a) high-resolution and
b) asynchronous to the measurement event.

If the measurement event is an interrupt and the CPU has a
cycle-counter, you are set.  On interesting systems lacking a
cycle-counter, we still have a high-resolution counter or sorts that
is the CPU itself.

Instruction pointer and stack pointer for both kernel and userland are
one way to read out the "counter".  Main problem here are tight loops
where your "counter" is not high-resolution at all.  But something
within the CPU is constantly changing.  And that something tends to be
contained in the registers.

How about taking the saved registers from the interrupted CPU, xor'ing
them all and calling the result random_get_entropy() on systems
lacking a good cycles-counter?

Jörn

--
I can say that I spend most of my time fixing bugs even if I have lots
of new features to implement in mind, but I give bugs more priority.
-- Andrea Arcangeli, 2000

Re: [PATCH,RFC] random: collect cpu randomness

[Theodore Ts'o]

On Mon, Feb 03, 2014 at 10:50:42AM -0500, Jörn Engel wrote:
> If the measurement event is an interrupt and the CPU has a
> cycle-counter, you are set.  On interesting systems lacking a
> cycle-counter, we still have a high-resolution counter or sorts that
> is the CPU itself.
> 
> Instruction pointer and stack pointer for both kernel and userland are
> one way to read out the "counter".  Main problem here are tight loops
> where your "counter" is not high-resolution at all.  But something
> within the CPU is constantly changing.  And that something tends to be
> contained in the registers.
> 
> How about taking the saved registers from the interrupted CPU, xor'ing
> them all and calling the result random_get_entropy() on systems
> lacking a good cycles-counter?

So we could take the struct pt_regs which we get from get_irq_regs(),
XOR them together and use them to feed into input[2] amd input[3] in
add_interrupt_randomness().  Or some other way of distributing the
values of all of the irq registers into the __u32 input[4] array.

That would probably be a good and useful thing to do.  Was that
basically what you were suggesting?

						- Ted

Re: [PATCH,RFC] random: collect cpu randomness

[Jörn Engel]

On Mon, 3 February 2014 11:37:29 -0500, Theodore Ts'o wrote:
> 
> So we could take the struct pt_regs which we get from get_irq_regs(),
> XOR them together and use them to feed into input[2] amd input[3] in
> add_interrupt_randomness().  Or some other way of distributing the
> values of all of the irq registers into the __u32 input[4] array.
> 
> That would probably be a good and useful thing to do.  Was that
> basically what you were suggesting?

Yes.

Jörn

--
When you copy some code, you are supposed to read it.  If nothing else,
there's a chance to spot and fix an obvious bug instead of sharing it...
-- Al Viro

[PATCH] random: mix all saved registers into entropy pool

[Jörn Engel]

On Mon, 3 February 2014 13:48:57 -0500, Jörn Engel wrote:
> On Mon, 3 February 2014 11:37:29 -0500, Theodore Ts'o wrote:
> > 
> > So we could take the struct pt_regs which we get from get_irq_regs(),
> > XOR them together and use them to feed into input[2] amd input[3] in
> > add_interrupt_randomness().  Or some other way of distributing the
> > values of all of the irq registers into the __u32 input[4] array.
> > 
> > That would probably be a good and useful thing to do.  Was that
> > basically what you were suggesting?
> 
> Yes.

And here is a patch to do just that.  I am extremely unhappy with it
because it plain Does Not Help(tm).  At least it does not when using
kvm without VGA output.  Booting kvm 1000 times gave me the exact same
values for every single boot.  Quite sobering.

This patch didn't collect a single bit of entropy!

Surprisingly I find this failure quite valuable.  I managed to prove
the futility of my method - at least on automated virtual machines
without network interactions.  Afaiu none of the other entropy sources
have proven to be of any value under these circumstances either.  It
is entirely possible that zero bits of entropy is the final verdict
for such virtual machines.  Not sure about everyone else, but that is
quite a surprise to me.

Next step will be to test this patch on some entropy-challenged
hardware.

Jörn

--
The object-oriented version of 'Spaghetti code' is, of course, 'Lasagna code'.
(Too many layers).
-- Roberto Waltman.


The single biggest entropy source is a high-resolution timer running
asynchronous to the triggering event.  That leaves systems without a
useful get_cycles() implementation with significantly less entropy.
Significant means orders of magnitude, not a few percent.

An alternate high-resolution timer is the register content at the time
of an interrupt.  While not monotonic, it is guaranteed to change every
few clock cycles and very unlikely to repeat the same pattern.  Not
useful for interpretation as time, but we only care about some bits
of the "clock" to flip in an unpredictable fashion.

Signed-off-by: Joern Engel <joern@logfs.org>
---
 drivers/char/random.c | 85 ++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 81 insertions(+), 4 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 693dea730a3e..4a0633973aa7 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -257,6 +257,7 @@
 #include <linux/kmemcheck.h>
 #include <linux/workqueue.h>
 #include <linux/irq.h>
+#include <linux/crc32.h>
 
 #include <asm/processor.h>
 #include <asm/uaccess.h>
@@ -553,6 +554,8 @@ static void mix_pool_bytes(struct entropy_store *r, const void *in,
 
 struct fast_pool {
 	__u32		pool[4];
+	u32		last_shift;
+	u32		regs_count;
 	unsigned long	last;
 	unsigned short	count;
 	unsigned char	rotate;
@@ -896,6 +899,79 @@ void add_cpu_randomness(void *caller, void *val)
 
 static DEFINE_PER_CPU(struct fast_pool, irq_randomness);
 
+/*
+ * Ratelimit to a steady state of about once per jiffy.  A naïve approach
+ * would be to return 1 every time jiffies changes.  But we want to avoid
+ * being closely coupled to the timer interrupt.  So instead we increment
+ * a counter on every call and shift it right every time jiffies changes.
+ * If the counter is a power of two, return false;
+ *
+ * Effect is that some time after a jiffies change and cutting the counter
+ * in half we reach another power of two and return false.  But the
+ * likelihood of this happening is about the same at any time within a
+ * jiffies interval.
+ */
+static inline int ratelimited(struct fast_pool *p)
+{
+	int ret = !(p->regs_count == 0 || is_power_of_2(p->regs_count));
+
+	p->regs_count++;
+	if (p->last_shift != (u32)jiffies) {
+		p->regs_count >>= min(31u, (u32)jiffies - p->last_shift);
+		p->last_shift = (u32)jiffies;
+	}
+	return ret;
+}
+
+#define BOOT_IRQS 1024
+
+#if 1 /* XXX: create config option, as it is a bit spammy */
+/*
+ * We have a few conflicting requirements.  In order to judge the quality
+ * of randomness we want to print out inputs on the console.  Clearly we
+ * also want to keep the inputs secret.  Tradeoff is to print a hash of
+ * the inputs a couple of times during boot.
+ */
+static void trace_boot_regs(struct pt_regs *regs, int boot_count)
+{
+	static u32 log[BOOT_IRQS];
+	int i;
+	log[BOOT_IRQS - boot_count] = crc32(0, regs, sizeof(*regs));
+	if (boot_count == 1) {
+		for (i = 0; i < BOOT_IRQS; i++)
+			pr_info("irq regs(%d): %x\n", i, log[i]);
+	}
+}
+#endif
+
+/*
+ * The single biggest entropy source is a high-resolution timer running
+ * asynchronous to the triggering event.  That leaves systems without a
+ * useful get_cycles() implementation with significantly less entropy.
+ * Significant means orders of magnitude, not a few percent.
+ *
+ * An alternate high-resolution timer is the register content at the time
+ * of an interrupt.  While not monotonic, it is guaranteed to change every
+ * few clock cycles and very unlikely to repeat the same pattern.  Not
+ * useful for interpretation as time, but we only care about some bits
+ * of the "clock" to flip in an unpredictable fashion.
+ */
+static void mix_regs(struct pt_regs *regs, struct fast_pool *fast_pool)
+{
+	/* Two variables avoid decrementing by two without using atomics */
+	static int boot_count = BOOT_IRQS;
+	int in_boot = boot_count;
+
+	if (in_boot) {
+		trace_boot_regs(regs, boot_count);
+		boot_count = in_boot - 1;
+	} else if (ratelimited(fast_pool))
+		return;
+
+	mix_pool_bytes(&input_pool, regs, sizeof(*regs), NULL);
+	mix_pool_bytes(&nonblocking_pool, regs, sizeof(*regs), NULL);
+}
+
 void add_interrupt_randomness(int irq, int irq_flags)
 {
 	struct entropy_store	*r;
@@ -906,13 +982,14 @@ void add_interrupt_randomness(int irq, int irq_flags)
 	__u32			input[4], c_high, j_high;
 	__u64			ip;
 
+	mix_regs(regs, fast_pool);
 	c_high = (sizeof(cycles) > 4) ? cycles >> 32 : 0;
 	j_high = (sizeof(now) > 4) ? now >> 32 : 0;
-	input[0] = cycles ^ j_high ^ irq;
-	input[1] = now ^ c_high;
+	input[0] ^= cycles ^ j_high ^ irq;
+	input[1] ^= now ^ c_high;
 	ip = regs ? instruction_pointer(regs) : _RET_IP_;
-	input[2] = ip;
-	input[3] = ip >> 32;
+	input[2] ^= ip;
+	input[3] ^= ip >> 32;
 
 	fast_mix(fast_pool, input);
 
-- 
1.8.5.3

Re: [PATCH,RFC] random: collect cpu randomness

[Maciej W. Rozycki]

On Mon, 3 Feb 2014, Theodore Ts'o wrote:

> > How about taking the saved registers from the interrupted CPU, xor'ing
> > them all and calling the result random_get_entropy() on systems
> > lacking a good cycles-counter?
> 
> So we could take the struct pt_regs which we get from get_irq_regs(),
> XOR them together and use them to feed into input[2] amd input[3] in
> add_interrupt_randomness().  Or some other way of distributing the
> values of all of the irq registers into the __u32 input[4] array.

 Can we be sure we don't leak information this way?  Just being 
paranoid...

  Maciej

Re: [PATCH,RFC] random: collect cpu randomness

[Theodore Ts'o]

On Mon, Feb 03, 2014 at 09:54:22PM +0000, Maciej W. Rozycki wrote:
> 
>  Can we be sure we don't leak information this way?  Just being 
> paranoid...

The register information will be mixed pretty thoroughly by the time
it gets to the entropy pool, and then we don't ever expose the entropy
pool to userspace.  So no, I don't think we need to worry about
leaking information.

					- Ted

Re: [PATCH,RFC] random: collect cpu randomness

[Kees Cook]

Hi Jörn,

On Sun, Feb 02, 2014 at 03:36:17PM -0500, Jörn Engel wrote:
> Collects entropy from random behaviour all modern cpus exhibit.  The
> scheduler and slab allocator are instrumented for this purpose.  How
> much randomness can be gathered is clearly hardware-dependent and hard
> to estimate.  Therefore the entropy estimate is zero, but random bits
> still get mixed into the pools.

Have you seen this work from PaX Team?

http://grsecurity.net/pipermail/grsecurity/2012-July/001093.html

See http://grsecurity.net/test/grsecurity-3.0-3.13.1-201402052349.patch
and search for PAX_LATENT_ENTROPY.

-Kees

-- 
Kees Cook                                            @outflux.net

Re: [PATCH,RFC] random: collect cpu randomness

[Dave Taht]

On Thu, Feb 6, 2014 at 5:20 PM, Kees Cook <kees@outflux.net> wrote:
> Hi Jörn,
>
> On Sun, Feb 02, 2014 at 03:36:17PM -0500, Jörn Engel wrote:
>> Collects entropy from random behaviour all modern cpus exhibit.  The
>> scheduler and slab allocator are instrumented for this purpose.  How
>> much randomness can be gathered is clearly hardware-dependent and hard
>> to estimate.  Therefore the entropy estimate is zero, but random bits
>> still get mixed into the pools.
>
> Have you seen this work from PaX Team?
>
> http://grsecurity.net/pipermail/grsecurity/2012-July/001093.html
>
> See http://grsecurity.net/test/grsecurity-3.0-3.13.1-201402052349.patch
> and search for PAX_LATENT_ENTROPY.

The hardware rng world just got easier with the "hashlet".

https://plus.google.com/u/0/107942175615993706558/posts/4iq6W524SxL

Kernel driver wanted...

> -Kees
>
> --
> Kees Cook                                            @outflux.net



-- 
Dave Täht

Fixing bufferbloat with cerowrt: http://www.teklibre.com/cerowrt/subscribe.html

Re: [PATCH,RFC] random: collect cpu randomness

[Jörn Engel]

On Thu, 6 February 2014 14:20:02 -0800, Kees Cook wrote:
> On Sun, Feb 02, 2014 at 03:36:17PM -0500, Jörn Engel wrote:
> > Collects entropy from random behaviour all modern cpus exhibit.  The
> > scheduler and slab allocator are instrumented for this purpose.  How
> > much randomness can be gathered is clearly hardware-dependent and hard
> > to estimate.  Therefore the entropy estimate is zero, but random bits
> > still get mixed into the pools.
> 
> Have you seen this work from PaX Team?
> 
> http://grsecurity.net/pipermail/grsecurity/2012-July/001093.html

Interesting.

> See http://grsecurity.net/test/grsecurity-3.0-3.13.1-201402052349.patch
> and search for PAX_LATENT_ENTROPY.

Server gives me an error.  Archive.org doesn't have a copy either,
thanks to robots.txt.

Can you send me a copy via mail?

Jörn

--
Functionality is an asset, but code is a liability.
--Ted Dziuba

Re: [PATCH,RFC] random: collect cpu randomness

[Paolo Bonzini]

Il 02/02/2014 21:36, Jörn Engel ha scritto:
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wuninitialized"
> +	input[0] ^= cycles ^ jiffies;
> +	input[1] ^= (unsigned long)caller;
> +	input[2] ^= (unsigned long)val;
> +	input[3] ^= (unsigned long)&input;
> +#pragma GCC diagnostic pop

Your tests demonstrate that this works, and presumably you have checked 
the assembly too.  Still, this is invoking undefined behavior and the 
compiler could justifiably change those "^=" to "=".

An "asm" would be a safer way to convince the compiler that input[] is 
now initialized:

     asm volatile ("" :
         "=m" (input[0]), "=m" (input[1]),
         "=m" (input[2]), "=m" (input[3]));

and *really* XOR the values into the contents of the stack.

Of course the compiler could still have a "feature" where it 
pre-initializes the whole stack frame with some kind of canary, but that 
would be a problem even with your version of the code.

Paolo

[PATCH] random: mix all saved registers into entropy pool

[Jörn Engel]

The single biggest entropy source is a high-resolution timer running
asynchronous to the triggering event.  That leaves systems without a
useful get_cycles() implementation with significantly less entropy.
Significant means orders of magnitude, not a few percent.

An alternate high-resolution timer is the register content at the time
of an interrupt.  While not monotonic, it is guaranteed to change every
few clock cycles and very unlikely to repeat the same pattern.  Not
useful for interpretation as time, but we only care about some bits
of the "clock" flipping in an unpredictable fashion.

Experimentation show this to be an excellent entropy source.  Doing 1000
boottests with kvm and dumping a hash of the registers for the first
1024 interrupts each, >40% of all hashes were unique and >80% of all
hashes occurred less than once 1% of the time.

Even assuming that only unique register hashes yield any entropy at all
and only one bit each, we would gather >400 bits of entropy early in
boot and another 40 bits/s from the timer interrupt during runtime.  I
would feel confident with this amount of entropy in the absence of any
other source.

Repeating the test on real hardware, albeit with fewer repetitions
yields roughly the same results, slightly above 40% unique hashes.

Signed-off-by: Joern Engel <joern@logfs.org>
---
 drivers/char/random.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 62 insertions(+), 4 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 429b75bb60e8..b4a8968fe28d 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -553,6 +553,8 @@ static void mix_pool_bytes(struct entropy_store *r, const void *in,
 
 struct fast_pool {
 	__u32		pool[4];
+	u32		last_shift;
+	u32		regs_count;
 	unsigned long	last;
 	unsigned short	count;
 	unsigned char	rotate;
@@ -835,6 +837,61 @@ EXPORT_SYMBOL_GPL(add_input_randomness);
 
 static DEFINE_PER_CPU(struct fast_pool, irq_randomness);
 
+/*
+ * Ratelimit to a steady state of about once per jiffy.  A naïve approach
+ * would be to return 1 every time jiffies changes.  But we want to avoid
+ * being closely coupled to the timer interrupt.  So instead we increment
+ * a counter on every call and shift it right every time jiffies changes.
+ * If the counter is a power of two, return false;
+ *
+ * Effect is that some time after a jiffies change and cutting the counter
+ * in half we reach another power of two and return false.  But the
+ * likelihood of this happening is about the same at any time within a
+ * jiffies interval.
+ */
+static inline int ratelimited(struct fast_pool *p)
+{
+	int ret = !(p->regs_count == 0 || is_power_of_2(p->regs_count));
+
+	p->regs_count++;
+	if (p->last_shift != (u32)jiffies) {
+		p->regs_count >>= min(31u, (u32)jiffies - p->last_shift);
+		p->last_shift = (u32)jiffies;
+	}
+	return ret;
+}
+
+#define BOOT_IRQS 1024
+
+/*
+ * The single biggest entropy source is a high-resolution timer running
+ * asynchronous to the triggering event.  That leaves systems without a
+ * useful get_cycles() implementation with significantly less entropy.
+ * Significant means orders of magnitude, not a few percent.
+ *
+ * An alternate high-resolution timer is the register content at the time
+ * of an interrupt.  While not monotonic, it is guaranteed to change every
+ * few clock cycles and very unlikely to repeat the same pattern.  Not
+ * useful for interpretation as time, but we only care about some bits
+ * of the "clock" flipping in an unpredictable fashion.
+ */
+static void mix_regs(struct pt_regs *regs, struct fast_pool *fast_pool)
+{
+	struct entropy_store	*r;
+	/* Two variables avoid decrementing by two without using atomics */
+	static int boot_count = BOOT_IRQS;
+	int in_boot = boot_count;
+
+	if (in_boot) {
+		boot_count = in_boot - 1;
+	} else if (ratelimited(fast_pool))
+		return;
+
+	/* During bootup we alternately feed both pools */
+	r = (in_boot & 1) ? &nonblocking_pool : &input_pool;
+	__mix_pool_bytes(r, regs, sizeof(*regs), NULL);
+}
+
 void add_interrupt_randomness(int irq, int irq_flags)
 {
 	struct entropy_store	*r;
@@ -845,13 +902,14 @@ void add_interrupt_randomness(int irq, int irq_flags)
 	__u32			input[4], c_high, j_high;
 	__u64			ip;
 
+	mix_regs(regs, fast_pool);
 	c_high = (sizeof(cycles) > 4) ? cycles >> 32 : 0;
 	j_high = (sizeof(now) > 4) ? now >> 32 : 0;
-	input[0] = cycles ^ j_high ^ irq;
-	input[1] = now ^ c_high;
+	input[0] ^= cycles ^ j_high ^ irq;
+	input[1] ^= now ^ c_high;
 	ip = regs ? instruction_pointer(regs) : _RET_IP_;
-	input[2] = ip;
-	input[3] = ip >> 32;
+	input[2] ^= ip;
+	input[3] ^= ip >> 32;
 
 	fast_mix(fast_pool, input);
 
-- 
2.0.0.rc0.1.g7b2ba98

Re: [PATCH] random: mix all saved registers into entropy pool

[Jörn Engel]

On Mon, 19 May 2014 17:17:19 -0400, Jörn Engel wrote:
> 
> Experimentation show this to be an excellent entropy source.  Doing 1000
> boottests with kvm and dumping a hash of the registers for the first
> 1024 interrupts each, >40% of all hashes were unique and >80% of all
> hashes occurred less than once 1% of the time.

And since I previously claimed the opposite, the negative result was
caused by a kvm oddity.  When starting kvm in the background, it will
run just fine.  But when starting kvm with "-nographic" in the
background, the process gets stopped.  No output is generated and the
output file is not even truncated before kvm is stopped.  Therefore
every single run will have identical kernel output - that of the
previous run.

With that embarrassment out of the way, I find this approach hugely
valuable.  Even if you disagree with some detail of this patch, we
should definitely merge something roughly related.

Jörn

--
A defeated army first battles and then seeks victory.
-- Sun Tzu

Re: [PATCH] random: mix all saved registers into entropy pool

[H. Peter Anvin]

On 05/19/2014 02:23 PM, Jörn Engel wrote:
> On Mon, 19 May 2014 17:17:19 -0400, Jörn Engel wrote:
>>
>> Experimentation show this to be an excellent entropy source.  Doing 1000
>> boottests with kvm and dumping a hash of the registers for the first
>> 1024 interrupts each, >40% of all hashes were unique and >80% of all
>> hashes occurred less than once 1% of the time.
> 
> And since I previously claimed the opposite, the negative result was
> caused by a kvm oddity.  When starting kvm in the background, it will
> run just fine.  But when starting kvm with "-nographic" in the
> background, the process gets stopped.  No output is generated and the
> output file is not even truncated before kvm is stopped.  Therefore
> every single run will have identical kernel output - that of the
> previous run.
> 
> With that embarrassment out of the way, I find this approach hugely
> valuable.  Even if you disagree with some detail of this patch, we
> should definitely merge something roughly related.
> 
> Jörn
> 

I think this is likely to be particularly valuable during boot.  I can
see it becoming substantially less valuable after that point, but during
boot is when the most critical.

What I do see as an issue is that the value is probably impossible to
quantify, and so I feel more than a bit queasy about giving it
/dev/random credit.  However, making sure the pool is well stirred
during boot is really way more important.

	-hpa

Re: [PATCH] random: mix all saved registers into entropy pool

[Jörn Engel]

On Mon, 19 May 2014 15:18:01 -0700, H. Peter Anvin wrote:
> 
> I think this is likely to be particularly valuable during boot.  I can
> see it becoming substantially less valuable after that point, but during
> boot is when the most critical.
> 
> What I do see as an issue is that the value is probably impossible to
> quantify, and so I feel more than a bit queasy about giving it
> /dev/random credit.  However, making sure the pool is well stirred
> during boot is really way more important.

I would feel fairly confident giving this .25 bits of entropy per
event.  With 40% unique hashes and assuming at most 1 bit of entropy
for a unique hash, that is a fairly conservative underestimate.

But as you can see from the patch, I feel less confident doing any
kind of entropy-guessing in general and have skipped that step.  By
the .25 bits metric, my patch should feed 128 bits of entropy into
both the non-blocking pool and the input pool within the first 5s or
so of boot time.  In reality it is likely more.

The other problem to solve is VM clones sharing the same state for the
various pools.  Assuming HZ=100 no external interrupts and .25 bits, we
would gather 128 bits in just above 5s.  That is not great, but okish.

What worries me is that much of those 5s may be spent inside the idle
loop and have rather predictable register content and we may run
tickless.  I have no great solution for that case.

We could change the ratelimiting to, say, 4 interrupts per jiffie.
But that would still not help much in the worst case scenario above,
so the benefits don't seem convincing.

Jörn

--
When you admit that some people are too important to prosecute, it's
just a few short steps to the obvious corollary – that everybody else
is unimportant enough to jail.
-- Matt Taibbi

Re: [PATCH] random: mix all saved registers into entropy pool

[H. Peter Anvin]

On 05/19/2014 03:39 PM, Jörn Engel wrote:
> On Mon, 19 May 2014 15:18:01 -0700, H. Peter Anvin wrote:
>>
>> I think this is likely to be particularly valuable during boot.  I can
>> see it becoming substantially less valuable after that point, but during
>> boot is when the most critical.
>>
>> What I do see as an issue is that the value is probably impossible to
>> quantify, and so I feel more than a bit queasy about giving it
>> /dev/random credit.  However, making sure the pool is well stirred
>> during boot is really way more important.
> 
> I would feel fairly confident giving this .25 bits of entropy per
> event.  With 40% unique hashes and assuming at most 1 bit of entropy
> for a unique hash, that is a fairly conservative underestimate.
> 

Sure, but that is for a specific workload.

> What worries me is that much of those 5s may be spent inside the idle
> loop and have rather predictable register content and we may run
> tickless.  I have no great solution for that case.
>
> We could change the ratelimiting to, say, 4 interrupts per jiffie.
> But that would still not help much in the worst case scenario above,
> so the benefits don't seem convincing.

Exactly.

	-hpa

Re: [PATCH] random: mix all saved registers into entropy pool

[Jörn Engel]

On Mon, 19 May 2014 16:05:06 -0700, H. Peter Anvin wrote:
> On 05/19/2014 03:39 PM, Jörn Engel wrote:
> > 
> > I would feel fairly confident giving this .25 bits of entropy per
> > event.  With 40% unique hashes and assuming at most 1 bit of entropy
> > for a unique hash, that is a fairly conservative underestimate.
> 
> Sure, but that is for a specific workload.

The workload is called boot.  That is one of the two critical ones -
the other being a cloned VM.  If we can gather enough entropy during
boot and somehow avoid the cloned VM problem, we have won.  No more
need to feed in entropy from the previous run, which on many embedded
systems won't happen anyway.

If you complained about the limited selection of hardware I tested on,
you would be right.  So far it has been x86 kvm and an x86 notebook.
It would be great to get numbers for some embedded system with arm or
mips as well.

Jörn

--
When in doubt, punt.  When somebody actually complains, go back and fix it...
The 90% solution is a good thing.
-- Rob Landley

Re: [PATCH] random: mix all saved registers into entropy pool

[Andi Kleen]

Jörn Engel <joern@logfs.org> writes:
>
> An alternate high-resolution timer is the register content at the time
> of an interrupt. 

So if you interrupt a cryptographic function you may hash in parts
of the key?

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only

Re: [PATCH] random: mix all saved registers into entropy pool

[Jörn Engel]

On Tue, 20 May 2014 05:12:07 -0700, Andi Kleen wrote:
> Jörn Engel <joern@logfs.org> writes:
> >
> > An alternate high-resolution timer is the register content at the time
> > of an interrupt. 
> 
> So if you interrupt a cryptographic function you may hash in parts
> of the key?

Yes.  And if there was an efficient way to deduce random generator
inputs, that would be a new side channel attack.  An efficient way to
deduce random generator inputs would allow many other attacks as well.
I don't know of such an attack nor can I conceive it being possible
under normal circumstances.

There are of course two exceptions.  If the attacker can read
arbitrary kernel memory - and therefore could read the private key
directly.  And if there is so little entropy that an attacker can
enumerate all possible states of the random generator and read enough
random numbers to exclude most of those states.

The second case also allows for many more interesting attacks and is
exactly the sort of hole I want to plug with this patch.

I think leaking of private keys or similar information is not a
concern.  But please prove me wrong.  Better you now than someone else
later.

Jörn

--
When in doubt, punt.  When somebody actually complains, go back and fix it...
The 90% solution is a good thing.
-- Rob Landley

Re: [PATCH] random: mix all saved registers into entropy pool

[Andi Kleen]

> 
> I think leaking of private keys or similar information is not a
> concern.  But please prove me wrong.  Better you now than someone else
> later.

While I don't have a concrete exploit it seems seems dangerous to me. 
The LibreSSL people just removed a similar behavior from OpenSSL.

-Andi

Re: [PATCH] random: mix all saved registers into entropy pool

[Jörn Engel]

On Wed, 21 May 2014 21:39:05 +0200, Andi Kleen wrote:
> 
> > I think leaking of private keys or similar information is not a
> > concern.  But please prove me wrong.  Better you now than someone else
> > later.
> 
> While I don't have a concrete exploit it seems seems dangerous to me. 
> The LibreSSL people just removed a similar behavior from OpenSSL.

Similar, yes.  But there is a difference:
Unconditionally mixing your private key into the entropy pool yields
zero entropy.  While your private is hopefully unpredictable to an
attacker, it never changes.  OpenSSL mixed the private keys for no
benefit.

The reason why I want this patch in is to avoid a different
non-theoretical dangerous situation.  get_cycles() returns 0 on
several architectures and Ted's recent "improvements" are thus far
only noise.  Architectures could define random_get_entropy() to
something else, but no architecture does it.  Therefore we have
millions of embedded systems with interrupts as their sole entropy
source and jiffies as the only timestamp component.

To top it off, those same systems usually lack a read-write filesystem
and will not initialize /dev/random from state read out before the
last reboot.  Now try to estimate how much time has to pass since
reboot until those systems have accumulated 128 bits of entropy.  I
have not heard an estimate from anyone yet.

If you have an alternative approach to fix this very real problem
without potentially leaking private keys under conditions that also
allow more efficient attacks, I would love to know about it.  So far
this is my best attempt and it beats all alternatives I know about.
Which just shows you how horrible the alternatives are.

Also see:
http://blog.fefe.de/?ts=ada960dc
https://factorable.net/weakkeys12.extended.pdf

Jörn

--
The art of propaganda is not telling lies, but rather selecting
the truth you require and giving it mixed up with some truths
the audience wants to hear.
-- Richard Crossman

Re: [PATCH] random: mix all saved registers into entropy pool

[Jörn Engel]

On Wed, 21 May 2014 21:39:05 +0200, Andi Kleen wrote:
> 
> > I think leaking of private keys or similar information is not a
> > concern.  But please prove me wrong.  Better you now than someone else
> > later.
> 
> While I don't have a concrete exploit it seems seems dangerous to me. 
> The LibreSSL people just removed a similar behavior from OpenSSL.

Btw, if your concern were justified, that would also speak volumes
about the quality of our random generator - with or without my patch.
Either you are wrong or we have a real problem on our hands already.

Jörn

--
This above all: to thine own self be true.
-- Shakespeare

Re: [PATCH] random: mix all saved registers into entropy pool

[Jörn Engel]

Ping.

Jörn

--
Computer system analysis is like child-rearing; you can do grievous damage,
but you cannot ensure success."
-- Tom DeMarco

Re: [PATCH] random: mix all saved registers into entropy pool

[Theodore Ts'o]

On Mon, May 19, 2014 at 05:17:19PM -0400, Jörn Engel wrote:
> +/*
> + * Ratelimit to a steady state of about once per jiffy.  A naïve approach
> + * would be to return 1 every time jiffies changes.  But we want to avoid
> + * being closely coupled to the timer interrupt.  So instead we increment
> + * a counter on every call and shift it right every time jiffies changes.
> + * If the counter is a power of two, return false;
> + *
> + * Effect is that some time after a jiffies change and cutting the counter
> + * in half we reach another power of two and return false.  But the
> + * likelihood of this happening is about the same at any time within a
> + * jiffies interval.
> + */
> +static inline int ratelimited(struct fast_pool *p)
> +{
> +	int ret = !(p->regs_count == 0 || is_power_of_2(p->regs_count));
> +
> +	p->regs_count++;
> +	if (p->last_shift != (u32)jiffies) {
> +		p->regs_count >>= min(31u, (u32)jiffies - p->last_shift);
> +		p->last_shift = (u32)jiffies;
> +	}
> +	return ret;
> +}

I wasn't convinced this would do the right thing, so I wrote a quick
test program where the main loop was basically this:

	for (i=0; i < 1024; i++) {
		jiffies = i >> 2;
		r = ratelimited(jiffies);
		printf("%5u %5u %5u %d\n", i, jiffies, regs_count, r);
	}

... which basically simulated a very simple scheme where there were
four interrupts for each clock tick.  In the steady state ratelimited
returns true 75% of the time.  If this was as documented, we would
expect it to return true 25% of the time.  So I don't think this is
working quite right:

   20     5     3 1
   21     5     4 1
   22     5     5 0
   23     5     6 1
   24     6     3 1
   25     6     4 1
   26     6     5 0
   27     6     6 1
   28     7     3 1
   29     7     4 1
   30     7     5 0
   31     7     6 1
   32     8     3 1


> +static void mix_regs(struct pt_regs *regs, struct fast_pool *fast_pool)
> +{
> +	struct entropy_store	*r;
> +	/* Two variables avoid decrementing by two without using atomics */
> +	static int boot_count = BOOT_IRQS;
> +	int in_boot = boot_count;
> +
> +	if (in_boot) {
> +		boot_count = in_boot - 1;
> +	} else if (ratelimited(fast_pool))
> +		return;
> +
> +	/* During bootup we alternately feed both pools */
> +	r = (in_boot & 1) ? &nonblocking_pool : &input_pool;
> +	__mix_pool_bytes(r, regs, sizeof(*regs), NULL);
> +}

I'm also concerned about how much overhead this might eat up.  I've
already had someone who was benchmarking a high performance storage
array where the increased interrupt latency before adding something
like this was something he noticed, and kvetched to me about.  The
pt_regs structure is going to be larger than the fast_pool (which is
only 16 bytes), and we're going to be doing it once every jiffy, which
means between 100 and 1000 times per second.

If we do this only during boot up, I'd be much less concerned, but
doing it all the time is making me a bit nervous about the overhead.

If we knew which parts of the pt_regs were actually the "best" in
terms of entropy, we could xor that into the input[] array in
add_interrupt_randomness(), perhaps?  


> -	input[0] = cycles ^ j_high ^ irq;
> -	input[1] = now ^ c_high;
> +	input[0] ^= cycles ^ j_high ^ irq;
> +	input[1] ^= now ^ c_high;

This is an unrelated change, so if you want to do this, let's discuss
this on a separate commit.  We used to have something like this, but
it causes static code checkers to complain about our using stack
garbage, and people argued convincingly that it really did add as much
unpredictability, especially when correlated with the ip field.

(And yes, I know that instruction_pointer(regs) doesn't work on all
platforms, and _RET_IP_ becomes largely static --- but this is why I'd
much rather have each architecture tell me which parts of regs were
actually the best, and use that as the initialized portions of the
input[] array.)

Cheers,

					- Ted

Re: [PATCH] random: mix all saved registers into entropy pool

[Jörn Engel]

On Tue, 10 June 2014 12:14:01 -0400, Theodore Ts'o wrote:
> On Mon, May 19, 2014 at 05:17:19PM -0400, Jörn Engel wrote:
> > +/*
> > + * Ratelimit to a steady state of about once per jiffy.  A naïve approach
> > + * would be to return 1 every time jiffies changes.  But we want to avoid
> > + * being closely coupled to the timer interrupt.  So instead we increment
> > + * a counter on every call and shift it right every time jiffies changes.
> > + * If the counter is a power of two, return false;
> > + *
> > + * Effect is that some time after a jiffies change and cutting the counter
> > + * in half we reach another power of two and return false.  But the
> > + * likelihood of this happening is about the same at any time within a
> > + * jiffies interval.
> > + */
> > +static inline int ratelimited(struct fast_pool *p)
> > +{
> > +	int ret = !(p->regs_count == 0 || is_power_of_2(p->regs_count));
> > +
> > +	p->regs_count++;
> > +	if (p->last_shift != (u32)jiffies) {
> > +		p->regs_count >>= min(31u, (u32)jiffies - p->last_shift);
> > +		p->last_shift = (u32)jiffies;
> > +	}
> > +	return ret;
> > +}
> 
> I wasn't convinced this would do the right thing, so I wrote a quick
> test program where the main loop was basically this:
> 
> 	for (i=0; i < 1024; i++) {
> 		jiffies = i >> 2;
> 		r = ratelimited(jiffies);
> 		printf("%5u %5u %5u %d\n", i, jiffies, regs_count, r);
> 	}
> 
> ... which basically simulated a very simple scheme where there were
> four interrupts for each clock tick.  In the steady state ratelimited
> returns true 75% of the time.  If this was as documented, we would
> expect it to return true 25% of the time.  So I don't think this is
> working quite right:
> 
>    20     5     3 1
>    21     5     4 1
>    22     5     5 0
>    23     5     6 1
>    24     6     3 1
>    25     6     4 1
>    26     6     5 0
>    27     6     6 1
>    28     7     3 1
>    29     7     4 1
>    30     7     5 0
>    31     7     6 1
>    32     8     3 1

Doh!  Removing the "!" from "!(p->regs_count..." will fix that.  Shows
I spent 100x more time testing the bootup entropy collection.

> > +static void mix_regs(struct pt_regs *regs, struct fast_pool *fast_pool)
> > +{
> > +	struct entropy_store	*r;
> > +	/* Two variables avoid decrementing by two without using atomics */
> > +	static int boot_count = BOOT_IRQS;
> > +	int in_boot = boot_count;
> > +
> > +	if (in_boot) {
> > +		boot_count = in_boot - 1;
> > +	} else if (ratelimited(fast_pool))
> > +		return;
> > +
> > +	/* During bootup we alternately feed both pools */
> > +	r = (in_boot & 1) ? &nonblocking_pool : &input_pool;
> > +	__mix_pool_bytes(r, regs, sizeof(*regs), NULL);
> > +}
> 
> I'm also concerned about how much overhead this might eat up.  I've
> already had someone who was benchmarking a high performance storage
> array where the increased interrupt latency before adding something
> like this was something he noticed, and kvetched to me about.  The
> pt_regs structure is going to be larger than the fast_pool (which is
> only 16 bytes), and we're going to be doing it once every jiffy, which
> means between 100 and 1000 times per second.

Would that someone be me? ;)

The reason I prefer doing it once every jiffy is that it doesn't
change with interrupt load.  You get 10k interrupts per second?
You pay once per jiffy.  100k interrupts per second?  Pay once per
jiffy.

I dislike the fact that HZ is anywhere between 100 and 1000.  We could
sample every time jiffies has advanced by HZ/50, which gives a stable
sample rate of 50Hz for every currently possible config.

> If we do this only during boot up, I'd be much less concerned, but
> doing it all the time is making me a bit nervous about the overhead.
> 
> If we knew which parts of the pt_regs were actually the "best" in
> terms of entropy, we could xor that into the input[] array in
> add_interrupt_randomness(), perhaps?  
> 
> 
> > -	input[0] = cycles ^ j_high ^ irq;
> > -	input[1] = now ^ c_high;
> > +	input[0] ^= cycles ^ j_high ^ irq;
> > +	input[1] ^= now ^ c_high;
> 
> This is an unrelated change, so if you want to do this, let's discuss
> this on a separate commit.  We used to have something like this, but
> it causes static code checkers to complain about our using stack
> garbage, and people argued convincingly that it really did add as much
> unpredictability, especially when correlated with the ip field.

Will remove.

> (And yes, I know that instruction_pointer(regs) doesn't work on all
> platforms, and _RET_IP_ becomes largely static --- but this is why I'd
> much rather have each architecture tell me which parts of regs were
> actually the best, and use that as the initialized portions of the
> input[] array.)

What I like about my approach is quite the opposite.  The only thing
an architecture maintainer can mess up is not saving registers on
interrupts.  And if they mess that one up, they are very likely to
notice.

Jörn

--
Victory in war is not repetitious.
-- Sun Tzu

Re: [PATCH] random: mix all saved registers into entropy pool

[Theodore Ts'o]

On Tue, Jun 10, 2014 at 08:10:09PM -0400, Jörn Engel wrote:
> > I'm also concerned about how much overhead this might eat up.  I've
> > already had someone who was benchmarking a high performance storage
> > array where the increased interrupt latency before adding something
> > like this was something he noticed, and kvetched to me about.  The
> > pt_regs structure is going to be larger than the fast_pool (which is
> > only 16 bytes), and we're going to be doing it once every jiffy, which
> > means between 100 and 1000 times per second.
> 
> Would that someone be me? ;)
> 
> The reason I prefer doing it once every jiffy is that it doesn't
> change with interrupt load.  You get 10k interrupts per second?
> You pay once per jiffy.  100k interrupts per second?  Pay once per
> jiffy.

No, actually, it was someone who was worried about tail latency.  So
even if the average overhead was small, if once a jiffy we had a much
larger time spent in the interrupt handler, that's something that
would a big concern for someone who was worried about big storage
array performance.

I'd be happier if we used fast_mix() and mixed the registers into the
fast pool.  That's much lighter weight than using mix_pool_bytes(),
which involves many more memory accesses, MUCH higher probably of
cache line bouncing between CPU's, etc.

And there has been some proposals that I've been looking at to make
fast_mix to be use even less overhead, so if we use fast_mix, any
changes we make to improve that will help here too.

> What I like about my approach is quite the opposite.  The only thing
> an architecture maintainer can mess up is not saving registers on
> interrupts.  And if they mess that one up, they are very likely to
> notice.

What probably makes sense is to make the defaults use pt_regs, but
make it be overrideable by the architecture maintainer.  For example,
if the CPU has RDRAND or RDSEED, it probably doesn't make sense to
worry about mixing in the registers.  So we could make the default be
to mix in the entire set of registers, and if someone complains about
the overhead on their system, and they have a better way of harvesting
high quality entropy, then they can use that instead of trying to rely
on the registers.

It's unlikely that someone is going to be attaching a multi-million
dollar RAID array or a PCIe attached flash device on a MIPS or m68k
platform, after all.  :-)

							- Ted

Re: [PATCH] random: mix all saved registers into entropy pool

[Jörn Engel]

On Wed, 11 June 2014 11:27:42 -0400, Theodore Ts'o wrote:
> On Tue, Jun 10, 2014 at 08:10:09PM -0400, Jörn Engel wrote:
> > > I'm also concerned about how much overhead this might eat up.  I've
> > > already had someone who was benchmarking a high performance storage
> > > array where the increased interrupt latency before adding something
> > > like this was something he noticed, and kvetched to me about.  The
> > > pt_regs structure is going to be larger than the fast_pool (which is
> > > only 16 bytes), and we're going to be doing it once every jiffy, which
> > > means between 100 and 1000 times per second.
> > 
> > Would that someone be me? ;)
> > 
> > The reason I prefer doing it once every jiffy is that it doesn't
> > change with interrupt load.  You get 10k interrupts per second?
> > You pay once per jiffy.  100k interrupts per second?  Pay once per
> > jiffy.
> 
> No, actually, it was someone who was worried about tail latency.  So
> even if the average overhead was small, if once a jiffy we had a much
> larger time spent in the interrupt handler, that's something that
> would a big concern for someone who was worried about big storage
> array performance.
> 
> I'd be happier if we used fast_mix() and mixed the registers into the
> fast pool.  That's much lighter weight than using mix_pool_bytes(),
> which involves many more memory accesses, MUCH higher probably of
> cache line bouncing between CPU's, etc.
> 
> And there has been some proposals that I've been looking at to make
> fast_mix to be use even less overhead, so if we use fast_mix, any
> changes we make to improve that will help here too.

That makes sense to me.  It would require replacing current fast_mix()
with somethat doesn't doesn't assume __u32 input[4] as the only
possible parameter.  In a previous incarnation of my patch I was using
a single round of siphash to condense the registers and added that to
our fast_pool.

While siphash is fairly fast, doing that for every interrupt seemed
too much for me, hence the current ratelimit approach.  But if people
care more about maximum latency than amortized cost, we can combine
siphash (or something similar) with the ratelimit.

At any rate, here is a slightly updated patch that is independent of
CONFIG_HZ.

Jörn

--
"Error protection by error detection and correction."
-- from a university class

>From 867d62e0165723f9d8dc568d0cb9d8898948ddaa Mon Sep 17 00:00:00 2001
From: Joern Engel <joern@logfs.org>
Date: Sat, 15 Feb 2014 16:29:28 -0800
Subject: [PATCH] random: mix all saved registers into entropy pool

The single biggest entropy source is a high-resolution timer running
asynchronous to the triggering event.  That leaves systems without a
useful get_cycles() implementation with significantly less entropy.
Significant means orders of magnitude, not a few percent.

An alternate high-resolution timer is the register content at the time
of an interrupt.  While not monotonic, it is guaranteed to change every
few clock cycles and very unlikely to repeat the same pattern.  Not
useful for interpretation as time, but we only care about some bits
of the "clock" flipping in an unpredictable fashion.

Experimentation show this to be an excellent entropy source.  Doing 1000
boottests with kvm and dumping a hash of the registers for the first
1024 interrupts each, >40% of all hashes were unique and >80% of all
hashes occurred less than once 1% of the time.

Even assuming that only unique register hashes yield any entropy at all
and only one bit each, we would gather >400 bits of entropy early in
boot and another 40 bits/s from the timer interrupt during runtime.  I
would feel confident with this amount of entropy in the absence of any
other source.

Repeating the test on real hardware, albeit with fewer repetitions
yields roughly the same results, slightly above 40% unique hashes.

Signed-off-by: Joern Engel <joern@logfs.org>
---
 drivers/char/random.c | 69 ++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 65 insertions(+), 4 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 429b75bb60e8..8e9f6274cf76 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -553,6 +553,8 @@ static void mix_pool_bytes(struct entropy_store *r, const void *in,
 
 struct fast_pool {
 	__u32		pool[4];
+	u32		last_shift;
+	u32		regs_count;
 	unsigned long	last;
 	unsigned short	count;
 	unsigned char	rotate;
@@ -835,6 +837,64 @@ EXPORT_SYMBOL_GPL(add_input_randomness);
 
 static DEFINE_PER_CPU(struct fast_pool, irq_randomness);
 
+/*
+ * Ratelimit to a steady state of about 50Hz.  A naïve approach would be
+ * to return 1 every time jiffies changes.  But we want to avoid being
+ * closely coupled to the timer interrupt.  So instead we increment a
+ * counter on every call and shift it right every time jiffies increments
+ * by 20ms.
+ * 20ms or 50Hz is chosen to work well for all options of CONFIG_HZ.
+ * If the counter is a power of two, return false.
+ *
+ * Effect is that some time after a jiffies change and cutting the counter
+ * in half we reach another power of two and return false.  But the
+ * likelihood of this happening is about the same at any time within a
+ * jiffies interval.
+ */
+static inline int ratelimited(struct fast_pool *p)
+{
+	int ret = !(p->regs_count == 0 || is_power_of_2(p->regs_count));
+	u32 jiffies_low = (u32)jiffies;
+
+	p->regs_count++;
+	if (p->last_shift > jiffies_low || p->last_shift + HZ/50 <= jiffies_low) {
+		p->regs_count >>= 1;
+		p->last_shift = jiffies_low;
+	}
+	return ret;
+}
+
+#define BOOT_IRQS 1024
+
+/*
+ * The single biggest entropy source is a high-resolution timer running
+ * asynchronous to the triggering event.  That leaves systems without a
+ * useful get_cycles() implementation with significantly less entropy.
+ * Significant means orders of magnitude, not a few percent.
+ *
+ * An alternate high-resolution timer is the register content at the time
+ * of an interrupt.  While not monotonic, it is guaranteed to change every
+ * few clock cycles and very unlikely to repeat the same pattern.  Not
+ * useful for interpretation as time, but we only care about some bits
+ * of the "clock" flipping in an unpredictable fashion.
+ */
+static void mix_regs(struct pt_regs *regs, struct fast_pool *fast_pool)
+{
+	struct entropy_store	*r;
+	/* Two variables avoid decrementing by two without using atomics */
+	static int boot_count = BOOT_IRQS;
+	int in_boot = boot_count;
+
+	if (in_boot) {
+		boot_count = in_boot - 1;
+	} else if (ratelimited(fast_pool))
+		return;
+
+	/* During bootup we alternately feed both pools */
+	r = (in_boot & 1) ? &nonblocking_pool : &input_pool;
+	__mix_pool_bytes(r, regs, sizeof(*regs), NULL);
+}
+
 void add_interrupt_randomness(int irq, int irq_flags)
 {
 	struct entropy_store	*r;
@@ -845,13 +905,14 @@ void add_interrupt_randomness(int irq, int irq_flags)
 	__u32			input[4], c_high, j_high;
 	__u64			ip;
 
+	mix_regs(regs, fast_pool);
 	c_high = (sizeof(cycles) > 4) ? cycles >> 32 : 0;
 	j_high = (sizeof(now) > 4) ? now >> 32 : 0;
-	input[0] = cycles ^ j_high ^ irq;
-	input[1] = now ^ c_high;
+	input[0] ^= cycles ^ j_high ^ irq;
+	input[1] ^= now ^ c_high;
 	ip = regs ? instruction_pointer(regs) : _RET_IP_;
-	input[2] = ip;
-	input[3] = ip >> 32;
+	input[2] ^= ip;
+	input[3] ^= ip >> 32;
 
 	fast_mix(fast_pool, input);
 
-- 
2.0.0.rc0.1.g7b2ba98

Re: [PATCH] random: mix all saved registers into entropy pool

[Jörn Engel]

On Tue, 10 June 2014 20:10:09 -0400, Jörn Engel wrote:
> On Tue, 10 June 2014 12:14:01 -0400, Theodore Ts'o wrote:
> > On Mon, May 19, 2014 at 05:17:19PM -0400, Jörn Engel wrote:
> > > +/*
> > > + * Ratelimit to a steady state of about once per jiffy.  A naïve approach
> > > + * would be to return 1 every time jiffies changes.  But we want to avoid
> > > + * being closely coupled to the timer interrupt.  So instead we increment
> > > + * a counter on every call and shift it right every time jiffies changes.
> > > + * If the counter is a power of two, return false;
> > > + *
> > > + * Effect is that some time after a jiffies change and cutting the counter
> > > + * in half we reach another power of two and return false.  But the
> > > + * likelihood of this happening is about the same at any time within a
> > > + * jiffies interval.
> > > + */
> > > +static inline int ratelimited(struct fast_pool *p)
> > > +{
> > > +	int ret = !(p->regs_count == 0 || is_power_of_2(p->regs_count));
> > > +
> > > +	p->regs_count++;
> > > +	if (p->last_shift != (u32)jiffies) {
> > > +		p->regs_count >>= min(31u, (u32)jiffies - p->last_shift);
> > > +		p->last_shift = (u32)jiffies;
> > > +	}
> > > +	return ret;
> > > +}
> > 
> > I wasn't convinced this would do the right thing, so I wrote a quick
> > test program where the main loop was basically this:
> > 
> > 	for (i=0; i < 1024; i++) {
> > 		jiffies = i >> 2;
> > 		r = ratelimited(jiffies);
> > 		printf("%5u %5u %5u %d\n", i, jiffies, regs_count, r);
> > 	}
> > 
> > ... which basically simulated a very simple scheme where there were
> > four interrupts for each clock tick.  In the steady state ratelimited
> > returns true 75% of the time.  If this was as documented, we would
> > expect it to return true 25% of the time.  So I don't think this is
> > working quite right:
> > 
> >    20     5     3 1
> >    21     5     4 1
> >    22     5     5 0
> >    23     5     6 1
> >    24     6     3 1
> >    25     6     4 1
> >    26     6     5 0
> >    27     6     6 1
> >    28     7     3 1
> >    29     7     4 1
> >    30     7     5 0
> >    31     7     6 1
> >    32     8     3 1
> 
> Doh!  Removing the "!" from "!(p->regs_count..." will fix that.  Shows
> I spent 100x more time testing the bootup entropy collection.

On second look, the function was correct.  If ratelimited() returns
true, the interrupt is ratelimited, i.e. ignored.  So correct
behaviour is to return false 25% of the time, just like it does.

You confused me for a moment!

Jörn

--
There are two ways of constructing a software design: one way is to make
it so simple that there are obviously no deficiencies, and the other is
to make it so complicated that there are no obvious deficiencies.
-- C. A. R. Hoare