crypto: jitter - quit sample collection loop upon RCT failure

The jitterentropy collection loop in jent_gen_entropy() can in principle
run indefinitely without making any progress if it only receives stuck
measurements as determined by jent_stuck(). After 31 consecutive stuck
samples, the Repetition Count Test (RCT) would fail anyway and the
jitterentropy RNG instances moved into ->health_failure == 1 state.
jent_gen_entropy()'s caller, jent_read_entropy() would then check for
this ->health_failure condition and return an error if found set. It
follows that there's absolutely no point in continuing the collection loop
in jent_gen_entropy() once the RCT has failed.

Make the jitterentropy collection loop more robust by terminating it upon
jent_health_failure() so that it won't continue to run indefinitely without
making any progress.

Signed-off-by: Nicolai Stange <nstange@suse.de>
Reviewed-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/jitterentropy.c

@@ -547,7 +547,7 @@ static void jent_gen_entropy(struct rand_data *ec)
 	/* priming of the ->prev_time value */
 	jent_measure_jitter(ec);
 
-	while (1) {
+	while (!jent_health_failure(ec)) {
 		/* If a stuck measurement is received, repeat measurement */
 		if (jent_measure_jitter(ec))
 			continue;