crypto: jitter - permanent and intermittent health errors

According to SP800-90B, two health failures are allowed: the intermittend
and the permanent failure. So far, only the intermittent failure was
implemented. The permanent failure was achieved by resetting the entire
entropy source including its health test state and waiting for two or
more back-to-back health errors.

This approach is appropriate for RCT, but not for APT as APT has a
non-linear cutoff value. Thus, this patch implements 2 cutoff values
for both RCT/APT. This implies that the health state is left untouched
when an intermittent failure occurs. The noise source is reset
and a new APT powerup-self test is performed. Yet, whith the unchanged
health test state, the counting of failures continues until a permanent
failure is reached.

Any non-failing raw entropy value causes the health tests to reset.

The intermittent error has an unchanged significance level of 2^-30.
The permanent error has a significance level of 2^-60. Considering that
this level also indicates a false-positive rate (see SP800-90B section 4.2)
a false-positive must only be incurred with a low probability when
considering a fleet of Linux kernels as a whole. Hitting the permanent
error may cause a panic(), the following calculation applies: Assuming
that a fleet of 10^9 Linux kernels run concurrently with this patch in
FIPS mode and on each kernel 2 health tests are performed every minute
for one year, the chances of a false positive is about 1:1000
based on the binomial distribution.

In addition, any power-up health test errors triggered with
jent_entropy_init are treated as permanent errors.

A permanent failure causes the entire entropy source to permanently
return an error. This implies that a caller can only remedy the situation
by re-allocating a new instance of the Jitter RNG. In a subsequent
patch, a transparent re-allocation will be provided which also changes
the implied heuristic entropy assessment.

In addition, when the kernel is booted with fips=1, the Jitter RNG
is defined to be part of a FIPS module. The permanent error of the
Jitter RNG is translated as a FIPS module error. In this case, the entire
FIPS module must cease operation. This is implemented in the kernel by
invoking panic().

The patch also fixes an off-by-one in the RCT cutoff value which is now
set to 30 instead of 31. This is because the counting of the values
starts with 0.

Reviewed-by: Vladis Dronov <vdronov@redhat.com>
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Reviewed-by: Marcelo Henrique Cerri <marcelo.cerri@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 24 insertions, 27 deletions

diff --git a/crypto/jitterentropy-kcapi.c b/crypto/jitterentropy-kcapi.c
index 2d115bec15ae..b9edfaa51b27 100644
--- a/crypto/jitterentropy-kcapi.c
+++ b/crypto/jitterentropy-kcapi.c
@@ -37,6 +37,7 @@
  * DAMAGE.
  */
 
+#include <linux/fips.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/slab.h>
@@ -59,11 +60,6 @@ void jent_zfree(void *ptr)
 	kfree_sensitive(ptr);
 }
 
-void jent_panic(char *s)
-{
-	panic("%s", s);
-}
-
 void jent_memcpy(void *dest, const void *src, unsigned int n)
 {
 	memcpy(dest, src, n);
@@ -102,7 +98,6 @@ void jent_get_nstime(__u64 *out)
 struct jitterentropy {
 	spinlock_t jent_lock;
 	struct rand_data *entropy_collector;
-	unsigned int reset_cnt;
 };
 
 static int jent_kcapi_init(struct crypto_tfm *tfm)
@@ -138,32 +133,30 @@ static int jent_kcapi_random(struct crypto_rng *tfm,
 
 	spin_lock(&rng->jent_lock);
 
-	/* Return a permanent error in case we had too many resets in a row. */
-	if (rng->reset_cnt > (1<<10)) {
-		ret = -EFAULT;
-		goto out;
-	}
-
 	ret = jent_read_entropy(rng->entropy_collector, rdata, dlen);
 
-	/* Reset RNG in case of health failures */
-	if (ret < -1) {
-		pr_warn_ratelimited("Reset Jitter RNG due to health test failure: %s failure\n",
-				    (ret == -2) ? "Repetition Count Test" :
-						  "Adaptive Proportion Test");
-
-		rng->reset_cnt++;
-
+	if (ret == -3) {
+		/* Handle permanent health test error */
+		/*
+		 * If the kernel was booted with fips=1, it implies that
+		 * the entire kernel acts as a FIPS 140 module. In this case
+		 * an SP800-90B permanent health test error is treated as
+		 * a FIPS module error.
+		 */
+		if (fips_enabled)
+			panic("Jitter RNG permanent health test failure\n");
+
+		pr_err("Jitter RNG permanent health test failure\n");
+		ret = -EFAULT;
+	} else if (ret == -2) {
+		/* Handle intermittent health test error */
+		pr_warn_ratelimited("Reset Jitter RNG due to intermittent health test failure\n");
 		ret = -EAGAIN;
-	} else {
-		rng->reset_cnt = 0;
-
-		/* Convert the Jitter RNG error into a usable error code */
-		if (ret == -1)
-			ret = -EINVAL;
+	} else if (ret == -1) {
+		/* Handle other errors */
+		ret = -EINVAL;
 	}
 
-out:
 	spin_unlock(&rng->jent_lock);
 
 	return ret;
@@ -197,6 +190,10 @@ static int __init jent_mod_init(void)
 
 	ret = jent_entropy_init();
 	if (ret) {
+		/* Handle permanent health test error */
+		if (fips_enabled)
+			panic("jitterentropy: Initialization failed with host not compliant with requirements: %d\n", ret);
+
 		pr_info("jitterentropy: Initialization failed with host not compliant with requirements: %d\n", ret);
 		return -EFAULT;
 	}