[PATCH v2 2/2] nvme-auth: use hkdf_expand_label()

[Chris Leech <cleech@redhat.com>]

When generating keying material during an authentication transaction
(secure channel concatenation), the HKDF-Expand-Label function is part
of the specified key derivation process.

The current open-coded implementation misses the length prefix
requirements on the HkdfLabel label and context variable-length vectors
(RFC 8446 Section 3.4).

Instead, use the hkdf_expand_label() function.

Signed-off-by: Chris Leech <cleech@redhat.com>
Signed-off-by: Hannes Reinecke <hare@kernel.org>
---
 drivers/nvme/common/auth.c | 33 +++++++++++++--------------------
 1 file changed, 13 insertions(+), 20 deletions(-)

diff --git a/drivers/nvme/common/auth.c b/drivers/nvme/common/auth.c
index c6eae8e6b6f99..1f51fbebd9fac 100644
--- a/drivers/nvme/common/auth.c
+++ b/drivers/nvme/common/auth.c
@@ -768,10 +768,10 @@ int nvme_auth_derive_tls_psk(int hmac_id, u8 *psk, size_t psk_len,
 {
 	struct crypto_shash *hmac_tfm;
 	const char *hmac_name;
-	const char *psk_prefix = "tls13 nvme-tls-psk";
+	const char *label = "nvme-tls-psk";
 	static const char default_salt[HKDF_MAX_HASHLEN];
-	size_t info_len, prk_len;
-	char *info;
+	size_t prk_len;
+	const char *ctx;
 	unsigned char *prk, *tls_key;
 	int ret;
 
@@ -811,36 +811,29 @@ int nvme_auth_derive_tls_psk(int hmac_id, u8 *psk, size_t psk_len,
 	if (ret)
 		goto out_free_prk;
 
-	/*
-	 * 2 additional bytes for the length field from HDKF-Expand-Label,
-	 * 2 additional bytes for the HMAC ID, and one byte for the space
-	 * separator.
-	 */
-	info_len = strlen(psk_digest) + strlen(psk_prefix) + 5;
-	info = kzalloc(info_len + 1, GFP_KERNEL);
-	if (!info) {
+	ctx = kasprintf(GFP_KERNEL, "%02d %s", hmac_id, psk_digest);
+	if (!ctx) {
 		ret = -ENOMEM;
 		goto out_free_prk;
 	}
 
-	put_unaligned_be16(psk_len, info);
-	memcpy(info + 2, psk_prefix, strlen(psk_prefix));
-	sprintf(info + 2 + strlen(psk_prefix), "%02d %s", hmac_id, psk_digest);
-
 	tls_key = kzalloc(psk_len, GFP_KERNEL);
 	if (!tls_key) {
 		ret = -ENOMEM;
-		goto out_free_info;
+		goto out_free_ctx;
 	}
-	ret = hkdf_expand(hmac_tfm, info, info_len, tls_key, psk_len);
+	ret = hkdf_expand_label(hmac_tfm,
+				label, strlen(label),
+				ctx, strlen(ctx),
+				tls_key, psk_len);
 	if (ret) {
 		kfree(tls_key);
-		goto out_free_info;
+		goto out_free_ctx;
 	}
 	*ret_psk = tls_key;
 
-out_free_info:
-	kfree(info);
+out_free_ctx:
+	kfree(ctx);
 out_free_prk:
 	kfree(prk);
 out_free_shash:
-- 
2.50.1