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random32: add a selftest for the prandom32 code

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Given that this code is new, let's add a selftest for it as well.
It doesn't rely on fixed sets, instead it picks 1024 numbers and
verifies that they're not more correlated than desired.

Link: https://lore.kernel.org/netdev/20200808152628.GA27941@SDF.ORG/
Cc: George Spelvin <lkml@sdf.org>
Cc: Amit Klein <aksecurity@gmail.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: tytso@mit.edu
Cc: Florian Westphal <fw@strlen.de>
Cc: Marc Plumb <lkml.mplumb@gmail.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
This commit is contained in:
Willy Tarreau 2020-10-24 18:36:27 +02:00
parent 3744741ada
commit c6e169bc14
1 changed files with 56 additions and 0 deletions
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56
lib/random32.c
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@ -38,6 +38,7 @@
#include <linux/jiffies.h>
#include <linux/random.h>
#include <linux/sched.h>
#include <linux/bitops.h>
#include <asm/unaligned.h>
#include <trace/events/random.h>
@ -556,6 +557,61 @@ static void prandom_timer_start(struct random_ready_callback *unused)
mod_timer(&seed_timer, jiffies);
}
#ifdef CONFIG_RANDOM32_SELFTEST
/* Principle: True 32-bit random numbers will all have 16 differing bits on
* average. For each 32-bit number, there are 601M numbers differing by 16
* bits, and 89% of the numbers differ by at least 12 bits. Note that more
* than 16 differing bits also implies a correlation with inverted bits. Thus
* we take 1024 random numbers and compare each of them to the other ones,
* counting the deviation of correlated bits to 16. Constants report 32,
* counters 32-log2(TEST_SIZE), and pure randoms, around 6 or lower. With the
* u32 total, TEST_SIZE may be as large as 4096 samples.
*/
#define TEST_SIZE 1024
static int __init prandom32_state_selftest(void)
{
unsigned int x, y, bits, samples;
u32 xor, flip;
u32 total;
u32 *data;
data = kmalloc(sizeof(*data) * TEST_SIZE, GFP_KERNEL);
if (!data)
return 0;
for (samples = 0; samples < TEST_SIZE; samples++)
data[samples] = prandom_u32();
flip = total = 0;
for (x = 0; x < samples; x++) {
for (y = 0; y < samples; y++) {
if (x == y)
continue;
xor = data[x] ^ data[y];
flip |= xor;
bits = hweight32(xor);
total += (bits - 16) * (bits - 16);
}
}
/* We'll return the average deviation as 2*sqrt(corr/samples), which
* is also sqrt(4*corr/samples) which provides a better resolution.
*/
bits = int_sqrt(total / (samples * (samples - 1)) * 4);
if (bits > 6)
pr_warn("prandom32: self test failed (at least %u bits"
" correlated, fixed_mask=%#x fixed_value=%#x\n",
bits, ~flip, data[0] & ~flip);
else
pr_info("prandom32: self test passed (less than %u bits"
" correlated)\n",
bits+1);
kfree(data);
return 0;
}
core_initcall(prandom32_state_selftest);
#endif /* CONFIG_RANDOM32_SELFTEST */
/*
* Start periodic full reseeding as soon as strong
* random numbers are available.