crypto: dh - implement FIPS PCT

JIRA: https://issues.redhat.com/browse/RHEL-25845
Upstream Status: RHEL only

The below patch from Nicolai is not going to be accepted upstream.
Add a panic on a failed test per FIPS certification requirement.

    From: Nicolai Stange <nstange@suse.de>
    Date: Tue, 30 Nov 2021 16:51:12 +0100
    Subject: [PATCH] crypto: dh - implement FIPS PCT
    References: jsc#SLE-21132,bsc#1191256
    Patch-mainline: Never, not upstreamable

    SP800-56Arev3, 5.6.2.1.4 ("Owner Assurance of Pair-wise Consistency")
    requires that a pair-wise consistency check needs to be conducted on a
    keypair. A pair-wise consistency test (PCT) is meant to ensure that a
    some provided public key is indeed associated with the given private one.
    As the kernel's DH implementation always computes the public key from the
    private one, this is guaranteed already as per the API. However, in the
    course of the certification process, there had been a lengthy discussion
    regarding this topic, with the result that a PCT is nonetheless mandatory.
    Simply implement a PCT for DH and move on. As mandated by SP800-56Arev3,
    5.6.2.1.4, the PCT involves recomputing the public key and comparing it
    against the one under test.

    Signed-off-by: Nicolai Stange <nstange@suse.de>

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

Author: nefigtut

crypto/dh.c

@@ -228,10 +228,35 @@ static int dh_compute_value(struct kpp_request *req)
 
 		/* SP800-56A rev 3 5.6.2.1.3 key check */
 		} else {
+			MPI val_pct;
+
 			if (dh_is_pubkey_valid(ctx, val)) {
 				ret = -EAGAIN;
 				goto err_free_val;
 			}
+
+			/*
+			 * SP800-56Arev3, 5.6.2.1.4: ("Owner Assurance
+			 * of Pair-wise Consistency"): recompute the
+			 * public key and check if the results match.
+			 */
+			val_pct = mpi_alloc(0);
+			if (!val_pct) {
+				ret = -ENOMEM;
+				goto err_free_val;
+			}
+
+			ret = _compute_val(ctx, base, val_pct);
+			if (ret) {
+				mpi_free(val_pct);
+				goto err_free_val;
+			}
+
+			if (mpi_cmp(val, val_pct) != 0) {
+				fips_fail_notify();
+				panic("dh: pair-wise consistency test failed\n");
+			}
+			mpi_free(val_pct);
 		}
 	}