crypto: jitter - consider 32 LSB for APT

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2164067
Upstream Status: merged into the linux.git

commit 552d03a
Author: Stephan Müller <smueller@chronox.de>
Date:   Sun Nov 21 15:14:20 2021 +0100

    crypto: jitter - consider 32 LSB for APT

    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>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Signed-off-by: Vladis Dronov <vdronov@redhat.com>

Author: nefigtut

crypto/jitterentropy.c

@@ -281,7 +281,6 @@ 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;
@@ -290,7 +289,7 @@ 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 */