linux-kernel - Re: x86/random: Speculation to the rescue

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Date:   Tue, 1 Oct 2019 19:18:05 +0200
From:   "Ahmed S. Darwish" <darwish.07@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>, a.darwish@...utronix.de,
        LKML <linux-kernel@...r.kernel.org>,
        Theodore Ts'o <tytso@....edu>,
        Nicholas Mc Guire <hofrat@...ntech.at>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: x86/random: Speculation to the rescue

On Tue, Oct 01, 2019 at 09:37:39AM -0700, Kees Cook wrote:
> On Tue, Oct 01, 2019 at 06:15:02PM +0200, Ahmed S. Darwish wrote:
> > On Sat, Sep 28, 2019 at 04:53:52PM -0700, Linus Torvalds wrote:
> > > Ahmed - would you be willing to test this on your problem case (with
> > > the ext4 optimization re-enabled, of course)?
> > >
> > 
> > So I pulled the patch and the revert of the ext4 revert as they're all
> > now merged in master. It of course made the problem go away...
> > 
> > To test the quality of the new jitter code, I added a small patch on
> > top to disable all other sources of randomness except the new jitter
> > entropy code, [1] and made quick tests on the quality of getrandom(0).
> > 
> > Using the "ent" tool, [2] also used to test randomness in the Stephen
> > Müller LRNG paper, on a 500000-byte file, produced the following
> > results:
> > 
> >     $ ent rand-file
> > 
> >     Entropy = 7.999625 bits per byte.
> > 
> >     Optimum compression would reduce the size of this 500000 byte file
> >     by 0 percent.
> > 
> >     Chi square distribution for 500000 samples is 259.43, and randomly
> >     would exceed this value 41.11 percent of the times.
> > 
> >     Arithmetic mean value of data bytes is 127.4085 (127.5 = random).
> > 
> >     Monte Carlo value for Pi is 3.148476594 (error 0.22 percent).
> > 
> >     Serial correlation coefficient is 0.001740 (totally uncorrelated = 0.0).
> > 
> > As can be seen above, everything looks random, and almost all of the
> > statistical randomness tests matched the same kernel without the
> > "jitter + schedule()" patch added (after getting it un-stuck).
> 
> Can you post the patch for [1]?
>

Yup, it was the quick&dirty patch below:

(discussion continues after the patch)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index c2f7de9dc543..26d0d2bb3337 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1177,6 +1177,8 @@ struct timer_rand_state {
  */
 void add_device_randomness(const void *buf, unsigned int size)
 {
+	return;
+
 	unsigned long time = random_get_entropy() ^ jiffies;
 	unsigned long flags;
 
@@ -1205,6 +1207,8 @@ static struct timer_rand_state input_timer_state = INIT_TIMER_RAND_STATE;
  */
 static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 {
+	return;
+
 	struct entropy_store	*r;
 	struct {
 		long jiffies;
@@ -1255,6 +1259,8 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 void add_input_randomness(unsigned int type, unsigned int code,
 				 unsigned int value)
 {
+	return;
+
 	static unsigned char last_value;
 
 	/* ignore autorepeat and the like */
@@ -1308,6 +1314,8 @@ static __u32 get_reg(struct fast_pool *f, struct pt_regs *regs)
 
 void add_interrupt_randomness(int irq, int irq_flags)
 {
+	return;
+
 	struct entropy_store	*r;
 	struct fast_pool	*fast_pool = this_cpu_ptr(&irq_randomness);
 	struct pt_regs		*regs = get_irq_regs();
@@ -1375,6 +1383,8 @@ EXPORT_SYMBOL_GPL(add_interrupt_randomness);
 #ifdef CONFIG_BLOCK
 void add_disk_randomness(struct gendisk *disk)
 {
+	return;
+
 	if (!disk || !disk->random)
 		return;
 	/* first major is 1, so we get >= 0x200 here */
@@ -2489,6 +2499,8 @@ randomize_page(unsigned long start, unsigned long range)
 void add_hwgenerator_randomness(const char *buffer, size_t count,
 				size_t entropy)
 {
+	return;
+
 	struct entropy_store *poolp = &input_pool;
 
 	if (unlikely(crng_init == 0)) {
@@ -2515,9 +2527,11 @@ EXPORT_SYMBOL_GPL(add_hwgenerator_randomness);
  */
 void add_bootloader_randomness(const void *buf, unsigned int size)
 {
+	return;
+
 	if (IS_ENABLED(CONFIG_RANDOM_TRUST_BOOTLOADER))
 		add_hwgenerator_randomness(buf, size, size * 8);
 	else

> Another test we should do is the
> multi-boot test. Testing the stream (with ent, or with my dieharder run)
> is mainly testing the RNG algo. I'd like to see if the first 8 bytes
> out of the kernel RNG change between multiple boots of the same system.
> e.g. read the first 8 bytes, for each of 100000 boots, and feed THAT
> byte "stream" into ent...
>

Oh, indeed, that's an excellent point... I'll prototype this and come
back.

thanks,

--
Ahmed Darwish