crypto: dh - add public key verification test

According to SP800-56A section 5.6.2.1, the public key to be processed
for the DH operation shall be checked for appropriateness. The check
shall covers the full verification test in case the domain parameter Q
is provided as defined in SP800-56A section 5.6.2.3.1. If Q is not
provided, the partial check according to SP800-56A section 5.6.2.3.2 is
performed.

The full verification test requires the presence of the domain parameter
Q. Thus, the patch adds the support to handle Q. It is permissible to
not provide the Q value as part of the domain parameters. This implies
that the interface is still backwards-compatible where so far only P and
G are to be provided. However, if Q is provided, it is imported.

Without the test, the NIST ACVP testing fails. After adding this check,
the NIST ACVP testing passes. Testing without providing the Q domain
parameter has been performed to verify the interface has not changed.

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

1 files changed, 63 insertions, 3 deletions

diff --git a/crypto/dh.c b/crypto/dh.c
index 5659fe7f446d..8f79269db2b7 100644
--- a/crypto/dh.c
+++ b/crypto/dh.c
@@ -16,14 +16,16 @@
 #include <linux/mpi.h>
 
 struct dh_ctx {
-	MPI p;
-	MPI g;
-	MPI xa;
+	MPI p;	/* Value is guaranteed to be set. */
+	MPI q;	/* Value is optional. */
+	MPI g;	/* Value is guaranteed to be set. */
+	MPI xa;	/* Value is guaranteed to be set. */
 };
 
 static void dh_clear_ctx(struct dh_ctx *ctx)
 {
 	mpi_free(ctx->p);
+	mpi_free(ctx->q);
 	mpi_free(ctx->g);
 	mpi_free(ctx->xa);
 	memset(ctx, 0, sizeof(*ctx));
@@ -60,6 +62,12 @@ static int dh_set_params(struct dh_ctx *ctx, struct dh *params)
 	if (!ctx->p)
 		return -EINVAL;
 
+	if (params->q && params->q_size) {
+		ctx->q = mpi_read_raw_data(params->q, params->q_size);
+		if (!ctx->q)
+			return -EINVAL;
+	}
+
 	ctx->g = mpi_read_raw_data(params->g, params->g_size);
 	if (!ctx->g)
 		return -EINVAL;
@@ -93,6 +101,55 @@ err_clear_ctx:
 	return -EINVAL;
 }
 
+/*
+ * SP800-56A public key verification:
+ *
+ * * If Q is provided as part of the domain paramenters, a full validation
+ *   according to SP800-56A section 5.6.2.3.1 is performed.
+ *
+ * * If Q is not provided, a partial validation according to SP800-56A section
+ *   5.6.2.3.2 is performed.
+ */
+static int dh_is_pubkey_valid(struct dh_ctx *ctx, MPI y)
+{
+	if (unlikely(!ctx->p))
+		return -EINVAL;
+
+	/*
+	 * Step 1: Verify that 2 <= y <= p - 2.
+	 *
+	 * The upper limit check is actually y < p instead of y < p - 1
+	 * as the mpi_sub_ui function is yet missing.
+	 */
+	if (mpi_cmp_ui(y, 1) < 1 || mpi_cmp(y, ctx->p) >= 0)
+		return -EINVAL;
+
+	/* Step 2: Verify that 1 = y^q mod p */
+	if (ctx->q) {
+		MPI val = mpi_alloc(0);
+		int ret;
+
+		if (!val)
+			return -ENOMEM;
+
+		ret = mpi_powm(val, y, ctx->q, ctx->p);
+
+		if (ret) {
+			mpi_free(val);
+			return ret;
+		}
+
+		ret = mpi_cmp_ui(val, 1);
+
+		mpi_free(val);
+
+		if (ret != 0)
+			return -EINVAL;
+	}
+
+	return 0;
+}
+
 static int dh_compute_value(struct kpp_request *req)
 {
 	struct crypto_kpp *tfm = crypto_kpp_reqtfm(req);
@@ -115,6 +172,9 @@ static int dh_compute_value(struct kpp_request *req)
 			ret = -EINVAL;
 			goto err_free_val;
 		}
+		ret = dh_is_pubkey_valid(ctx, base);
+		if (ret)
+			goto err_free_val;
 	} else {
 		base = ctx->g;
 	}