From:   Stephan =?ISO-8859-1?Q?M=FCller?= <smueller@chronox.de>
To:     herbert@gondor.apana.org.au
Cc:     linux-crypto@vger.kernel.org
Subject: [PATCH] crypto: Jitter RNG - consider 32 LSB for APT
Date:   Sun, 21 Nov 2021 15:14:20 +0100
Message-ID: <2572116.vuYhMxLoTh@positron.chronox.de>
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Precedence: bulk

The APT compares the current time stamp with a pre-set value. The
current code only considered the 4 LSB only. Yet, after reviews by
mathematicians of the user space Jitter RNG version >= 3.1.0, it was
concluded that the APT can be calculated on the 32 LSB of the time
delta. Thi change is applied to the kernel.

This fixes a bug where an AMD EPYC fails this test as its RDTSC value
contains zeros in the LSB. The most appropriate fix would have been to
apply a GCD calculation and divide the time stamp by the GCD. Yet, this
is a significant code change that will be considered for a future
update. Note, tests showed that constantly the GCD always was 32 on
these systems, i.e. the 5 LSB were always zero (thus failing the APT
since it only considered the 4 LSB for its calculation).

Signed-off-by: Stephan Mueller <smueller@chronox.de>
---
 crypto/jitterentropy.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/crypto/jitterentropy.c b/crypto/jitterentropy.c
index a11b3208760f..f6d3a84e3c21 100644
--- a/crypto/jitterentropy.c
+++ b/crypto/jitterentropy.c
@@ -265,7 +265,6 @@ static int jent_stuck(struct rand_data *ec, __u64 current_delta)
 {
 	__u64 delta2 = jent_delta(ec->last_delta, current_delta);
 	__u64 delta3 = jent_delta(ec->last_delta2, delta2);
-	unsigned int delta_masked = current_delta & JENT_APT_WORD_MASK;
 
 	ec->last_delta = current_delta;
 	ec->last_delta2 = delta2;
@@ -274,7 +273,7 @@ static int jent_stuck(struct rand_data *ec, __u64 current_delta)
 	 * Insert the result of the comparison of two back-to-back time
 	 * deltas.
 	 */
-	jent_apt_insert(ec, delta_masked);
+	jent_apt_insert(ec, current_delta);
 
 	if (!current_delta || !delta2 || !delta3) {
 		/* RCT with a stuck bit */
-- 
2.33.1