crypto: ecdh - fix to allow multi segment scatterlists

Apparently the ecdh use case was in bluetooth which always has single
element scatterlists, so the ecdh module was hard coded to expect
them.  Now we're using this in TPM, we need multi-element
scatterlists, so remove this limitation.
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/ecdh.c

@@ -89,12 +89,19 @@ static int ecdh_compute_value(struct kpp_request *req)
 		if (!shared_secret)
 			goto free_pubkey;
 
-		copied = sg_copy_to_buffer(req->src, 1, public_key,
-					   public_key_sz);
-		if (copied != public_key_sz) {
-			ret = -EINVAL;
+		/* from here on it's invalid parameters */
+		ret = -EINVAL;
+
+		/* must have exactly two points to be on the curve */
+		if (public_key_sz != req->src_len)
+			goto free_all;
+
+		copied = sg_copy_to_buffer(req->src,
+					   sg_nents_for_len(req->src,
+							    public_key_sz),
+					   public_key, public_key_sz);
+		if (copied != public_key_sz)
 			goto free_all;
-		}
 
 		ret = crypto_ecdh_shared_secret(ctx->curve_id, ctx->ndigits,
 						ctx->private_key, public_key,
@@ -111,7 +118,11 @@ static int ecdh_compute_value(struct kpp_request *req)
 	if (ret < 0)
 		goto free_all;
 
-	copied = sg_copy_from_buffer(req->dst, 1, buf, nbytes);
+	/* might want less than we've got */
+	nbytes = min_t(size_t, nbytes, req->dst_len);
+	copied = sg_copy_from_buffer(req->dst, sg_nents_for_len(req->dst,
+								nbytes),
+				     buf, nbytes);
 	if (copied != nbytes)
 		ret = -EINVAL;