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Subject: [PATCH 13/22] KEYS: asym_tpm: Implement pkey_query
From:   David Howells <dhowells@redhat.com>
To:     jmorris@namei.org
Cc:     denkenz@gmail.com, keyrings@vger.kernel.org,
        linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org
Date:   Wed, 05 Sep 2018 22:55:43 +0100
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From: Denis Kenzior <denkenz@gmail.com>

This commit implements the pkey_query operation.  This is accomplished
by utilizing the public key portion to obtain max encryption size
information for the operations that utilize the public key (encrypt,
verify).  The private key size extracted from the TPM_Key data structure
is used to fill the information where the private key is used (decrypt,
sign).

The kernel uses a DER/BER format for public keys and does not support
setting the key via the raw binary form.  To get around this a simple
DER/BER formatter is implemented which stores the DER/BER formatted key
and exponent in a temporary buffer for use by the crypto API.

The only exponent supported currently is 65537.  This holds true for
other Linux TPM tools such as 'create_tpm_key' and
trousers-openssl_tpm_engine.

Signed-off-by: Denis Kenzior <denkenz@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
---

 crypto/asymmetric_keys/asym_tpm.c |  135 +++++++++++++++++++++++++++++++++++++
 1 file changed, 135 insertions(+)

diff --git a/crypto/asymmetric_keys/asym_tpm.c b/crypto/asymmetric_keys/asym_tpm.c
index 308c51e055a4..837472d107d5 100644
--- a/crypto/asymmetric_keys/asym_tpm.c
+++ b/crypto/asymmetric_keys/asym_tpm.c
@@ -7,10 +7,24 @@
 #include <linux/seq_file.h>
 #include <linux/scatterlist.h>
 #include <linux/tpm.h>
+#include <crypto/akcipher.h>
 #include <asm/unaligned.h>
 #include <keys/asymmetric-subtype.h>
 #include <crypto/asym_tpm_subtype.h>
 
+/*
+ * Maximum buffer size for the BER/DER encoded public key.  The public key
+ * is of the form SEQUENCE { INTEGER n, INTEGER e } where n is a maximum 2048
+ * bit key and e is usually 65537
+ * The encoding overhead is:
+ * - max 4 bytes for SEQUENCE
+ *   - max 4 bytes for INTEGER n type/length
+ *     - 257 bytes of n
+ *   - max 2 bytes for INTEGER e type/length
+ *     - 3 bytes of e
+ */
+#define PUB_KEY_BUF_SIZE (4 + 4 + 257 + 2 + 3)
+
 /*
  * Provide a part of a description of the key for /proc/keys.
  */
@@ -38,6 +52,126 @@ static void asym_tpm_destroy(void *payload0, void *payload3)
 	kfree(tk);
 }
 
+/* How many bytes will it take to encode the length */
+static inline uint32_t definite_length(uint32_t len)
+{
+	if (len <= 127)
+		return 1;
+	if (len <= 255)
+		return 2;
+	return 3;
+}
+
+static inline uint8_t *encode_tag_length(uint8_t *buf, uint8_t tag,
+					 uint32_t len)
+{
+	*buf++ = tag;
+
+	if (len <= 127) {
+		buf[0] = len;
+		return buf + 1;
+	}
+
+	if (len <= 255) {
+		buf[0] = 0x81;
+		buf[1] = len;
+		return buf + 2;
+	}
+
+	buf[0] = 0x82;
+	put_unaligned_be16(len, buf + 1);
+	return buf + 3;
+}
+
+static uint32_t derive_pub_key(const void *pub_key, uint32_t len, uint8_t *buf)
+{
+	uint8_t *cur = buf;
+	uint32_t n_len = definite_length(len) + 1 + len + 1;
+	uint32_t e_len = definite_length(3) + 1 + 3;
+	uint8_t e[3] = { 0x01, 0x00, 0x01 };
+
+	/* SEQUENCE */
+	cur = encode_tag_length(cur, 0x30, n_len + e_len);
+	/* INTEGER n */
+	cur = encode_tag_length(cur, 0x02, len + 1);
+	cur[0] = 0x00;
+	memcpy(cur + 1, pub_key, len);
+	cur += len + 1;
+	cur = encode_tag_length(cur, 0x02, sizeof(e));
+	memcpy(cur, e, sizeof(e));
+	cur += sizeof(e);
+
+	return cur - buf;
+}
+
+/*
+ * Determine the crypto algorithm name.
+ */
+static int determine_akcipher(const char *encoding, const char *hash_algo,
+			      char alg_name[CRYPTO_MAX_ALG_NAME])
+{
+	/* TODO: We don't support hashing yet */
+	if (hash_algo)
+		return -ENOPKG;
+
+	if (strcmp(encoding, "pkcs1") == 0) {
+		strcpy(alg_name, "pkcs1pad(rsa)");
+		return 0;
+	}
+
+	if (strcmp(encoding, "raw") == 0) {
+		strcpy(alg_name, "rsa");
+		return 0;
+	}
+
+	return -ENOPKG;
+}
+
+/*
+ * Query information about a key.
+ */
+static int tpm_key_query(const struct kernel_pkey_params *params,
+			 struct kernel_pkey_query *info)
+{
+	struct tpm_key *tk = params->key->payload.data[asym_crypto];
+	int ret;
+	char alg_name[CRYPTO_MAX_ALG_NAME];
+	struct crypto_akcipher *tfm;
+	uint8_t der_pub_key[PUB_KEY_BUF_SIZE];
+	uint32_t der_pub_key_len;
+	int len;
+
+	/* TPM only works on private keys, public keys still done in software */
+	ret = determine_akcipher(params->encoding, params->hash_algo, alg_name);
+	if (ret < 0)
+		return ret;
+
+	tfm = crypto_alloc_akcipher(alg_name, 0, 0);
+	if (IS_ERR(tfm))
+		return PTR_ERR(tfm);
+
+	der_pub_key_len = derive_pub_key(tk->pub_key, tk->pub_key_len,
+					 der_pub_key);
+
+	ret = crypto_akcipher_set_pub_key(tfm, der_pub_key, der_pub_key_len);
+	if (ret < 0)
+		goto error_free_tfm;
+
+	len = crypto_akcipher_maxsize(tfm);
+
+	info->key_size = tk->key_len;
+	info->max_data_size = tk->key_len / 8;
+	info->max_sig_size = len;
+	info->max_enc_size = len;
+	info->max_dec_size = tk->key_len / 8;
+
+	ret = 0;
+error_free_tfm:
+	crypto_free_akcipher(tfm);
+	pr_devel("<==%s() = %d\n", __func__, ret);
+	return ret;
+}
+
 /*
  * Parse enough information out of TPM_KEY structure:
  * TPM_STRUCT_VER -> 4 bytes
@@ -194,6 +328,7 @@ struct asymmetric_key_subtype asym_tpm_subtype = {
 	.name_len		= sizeof("asym_tpm") - 1,
 	.describe		= asym_tpm_describe,
 	.destroy		= asym_tpm_destroy,
+	.query			= tpm_key_query,
 };
 EXPORT_SYMBOL_GPL(asym_tpm_subtype);