[RFC PATCH 1/2] crypto: hkdf: add hkdf_expand_label()

[Chris Leech <cleech@redhat.com>]

Provide an implementation of RFC 8446 (TLS 1.3) HKDF-Expand-Label

Signed-off-by: Chris Leech <cleech@redhat.com>
---
 crypto/hkdf.c         | 55 +++++++++++++++++++++++++++++++++++++++++++
 include/crypto/hkdf.h |  4 ++++
 2 files changed, 59 insertions(+)

diff --git a/crypto/hkdf.c b/crypto/hkdf.c
index 82d1b32ca6ce4..465bad6e6c93e 100644
--- a/crypto/hkdf.c
+++ b/crypto/hkdf.c
@@ -11,6 +11,7 @@
 #include <crypto/sha2.h>
 #include <crypto/hkdf.h>
 #include <linux/module.h>
+#include <linux/unaligned.h>
 
 /*
  * HKDF consists of two steps:
@@ -129,6 +130,60 @@ int hkdf_expand(struct crypto_shash *hmac_tfm,
 }
 EXPORT_SYMBOL_GPL(hkdf_expand);
 
+/**
+ * hkdf_expand_label - HKDF-Expand-Label (RFC 8846 section 7.1)
+ * @hmac_tfm: hash context keyed with pseudorandom key
+ * @label: ASCII label without "tls13 " prefix
+ * @label_len: length of @label
+ * @context: context bytes
+ * @contextlen: length of @context
+ * @okm: output keying material
+ * @okmlen: length of @okm
+ *
+ * Build the TLS 1.3 HkdfLabel structure and invoke hkdf_expand().
+ *
+ * Returns 0 on success with output keying material stored in @okm,
+ * or a negative errno value otherwise.
+ */
+int hkdf_expand_label(struct crypto_shash *hmac_tfm,
+		const u8 *label, unsigned int labellen,
+		const u8 *context, unsigned int contextlen,
+		u8 *okm, unsigned int okmlen)
+{
+	int err;
+	u8 *info;
+	unsigned int infolen;
+	static const char tls13_prefix[] = "tls13 ";
+	unsigned int prefixlen = sizeof(tls13_prefix) - 1; /* exclude NUL */
+
+	if (WARN_ON(labellen > (255 - prefixlen)))
+		return -EINVAL;
+	if (WARN_ON(contextlen > 255))
+		return -EINVAL;
+
+	infolen = 2 + (1 + prefixlen + labellen) + (1 + contextlen);
+	info = kzalloc(infolen, GFP_KERNEL);
+	if (!info)
+		return -ENOMEM;
+
+	/* HkdfLabel.Length */
+	put_unaligned_be16(okmlen, info);
+
+	/* HkdfLabel.Label */
+	info[2] = prefixlen + labellen;
+	memcpy(info + 3, tls13_prefix, prefixlen);
+	memcpy(info + 3 + prefixlen, label, labellen);
+
+	/* HkdfLabel.Context */
+	info[3 + prefixlen + labellen] = contextlen;
+	memcpy(info + 4 + prefixlen + labellen, context, contextlen);
+
+	err = hkdf_expand(hmac_tfm, info, infolen, okm, okmlen);
+	kfree_sensitive(info);
+	return err;
+}
+EXPORT_SYMBOL_GPL(hkdf_expand_label);
+
 struct hkdf_testvec {
 	const char *test;
 	const u8 *ikm;
diff --git a/include/crypto/hkdf.h b/include/crypto/hkdf.h
index 6a9678f508f5d..5e75d17a58abe 100644
--- a/include/crypto/hkdf.h
+++ b/include/crypto/hkdf.h
@@ -17,4 +17,8 @@ int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm,
 int hkdf_expand(struct crypto_shash *hmac_tfm,
 		const u8 *info, unsigned int infolen,
 		u8 *okm, unsigned int okmlen);
+int hkdf_expand_label(struct crypto_shash *hmac_tfm,
+		const u8 *label, unsigned int labellen,
+		const u8 *context, unsigned int contextlen,
+		u8 *okm, unsigned int okmlen);
 #endif
-- 
2.49.0


From 17e645b802bd61b4f3182195c15935e2fa1155ad Mon Sep 17 00:00:00 2001
From: Chris Leech <cleech@redhat.com>
Date: Tue, 12 Aug 2025 12:41:08 -0700
Subject: [RFC PATCH 2/2] nvme-auth: use hkdf_expand_label()

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>
---
 drivers/nvme/common/auth.c | 34 ++++++++++++++--------------------
 1 file changed, 14 insertions(+), 20 deletions(-)

diff --git a/drivers/nvme/common/auth.c b/drivers/nvme/common/auth.c
index 91e273b89fea3..bfa6f0a82d3e9 100644
--- a/drivers/nvme/common/auth.c
+++ b/drivers/nvme/common/auth.c
@@ -715,10 +715,10 @@ 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";
 	static const char default_salt[HKDF_MAX_HASHLEN];
-	size_t info_len, prk_len;
-	char *info;
+	size_t prk_len;
+	const char *label = "nvme-tls-psk";
+	const char *ctx;
 	unsigned char *prk, *tls_key;
 	int ret;
 
@@ -758,36 +758,30 @@ 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) {
+	/* Context is ASCII: "<hash-id-two-digits> <base64-digest>" */
+	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:
-- 
2.49.0