linux-kernel - [PATCH 3/3] crypto: jitter - quit sample collection loop upon RCT failure

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Date:   Tue, 30 Nov 2021 15:10:09 +0100
From:   Nicolai Stange <nstange@...e.de>
To:     Stephan Müller <smueller@...onox.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Torsten Duwe <duwe@...e.de>, linux-crypto@...r.kernel.org,
        linux-kernel@...r.kernel.org, Nicolai Stange <nstange@...e.de>
Subject: [PATCH 3/3] 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@...e.de>
---
 crypto/jitterentropy.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/crypto/jitterentropy.c b/crypto/jitterentropy.c
index 24e087c3f526..8f5283f28ed3 100644
--- a/crypto/jitterentropy.c
+++ b/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;
-- 
2.26.2