KEYS: Add parser for TPM-based keys [ver #2]

For TPM based keys, the only standard seems to be described here:
http://david.woodhou.se/draft-woodhouse-cert-best-practice.html#rfc.section.4.4

Quote from the relevant section:
"Rather, a common form of storage for "wrapped" keys is to encode the
binary TCPA_KEY structure in a single ASN.1 OCTET-STRING, and store the
result in PEM format with the tag "-----BEGIN TSS KEY BLOB-----". "

This patch implements the above behavior.  It is assumed that the PEM
encoding is stripped out by userspace and only the raw DER/BER format is
provided.  This is similar to how PKCS7, PKCS8 and X.509 keys are
handled.
Signed-off-by: Denis Kenzior <denkenz@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Marcel Holtmann <marcel@holtmann.org>
Reviewed-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: James Morris <james.morris@microsoft.com>

crypto/asymmetric_keys/Kconfig

@@ -52,6 +52,15 @@ config PKCS8_PRIVATE_KEY_PARSER
 	  private key data and provides the ability to instantiate a crypto key
 	  from that data.
 
+config TPM_KEY_PARSER
+	tristate "TPM private key parser"
+	depends on ASYMMETRIC_TPM_KEY_SUBTYPE
+	select ASN1
+	help
+	  This option provides support for parsing TPM format blobs for
+	  private key data and provides the ability to instantiate a crypto key
+	  from that data.
+
 config PKCS7_MESSAGE_PARSER
 	tristate "PKCS#7 message parser"
 	depends on X509_CERTIFICATE_PARSER

crypto/asymmetric_keys/Makefile

@@ -75,3 +75,14 @@ verify_signed_pefile-y := \
 
 $(obj)/mscode_parser.o: $(obj)/mscode.asn1.h $(obj)/mscode.asn1.h
 $(obj)/mscode.asn1.o: $(obj)/mscode.asn1.c $(obj)/mscode.asn1.h
+
+#
+# TPM private key parsing
+#
+obj-$(CONFIG_TPM_KEY_PARSER) += tpm_key_parser.o
+tpm_key_parser-y := \
+	tpm.asn1.o \
+	tpm_parser.o
+
+$(obj)/tpm_parser.o: $(obj)/tpm.asn1.h
+$(obj)/tpm.asn1.o: $(obj)/tpm.asn1.c $(obj)/tpm.asn1.h

crypto/asymmetric_keys/tpm.asn1

+--
+-- Unencryted TPM Blob.  For details of the format, see:
+-- http://david.woodhou.se/draft-woodhouse-cert-best-practice.html#I-D.mavrogiannopoulos-tpmuri
+--
+PrivateKeyInfo ::= OCTET STRING ({ tpm_note_key })

crypto/asymmetric_keys/tpm_parser.c

+// SPDX-License-Identifier: GPL-2.0
+#define pr_fmt(fmt) "TPM-PARSER: "fmt
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/export.h>
+#include <linux/slab.h>
+#include <linux/err.h>
+#include <keys/asymmetric-subtype.h>
+#include <keys/asymmetric-parser.h>
+#include <crypto/asym_tpm_subtype.h>
+#include "tpm.asn1.h"
+
+struct tpm_parse_context {
+	const void	*blob;
+	u32		blob_len;
+};
+
+/*
+ * Note the key data of the ASN.1 blob.
+ */
+int tpm_note_key(void *context, size_t hdrlen,
+		   unsigned char tag,
+		   const void *value, size_t vlen)
+{
+	struct tpm_parse_context *ctx = context;
+
+	ctx->blob = value;
+	ctx->blob_len = vlen;
+
+	return 0;
+}
+
+/*
+ * Parse a TPM-encrypted private key blob.
+ */
+static struct tpm_key *tpm_parse(const void *data, size_t datalen)
+{
+	struct tpm_parse_context ctx;
+	long ret;
+
+	memset(&ctx, 0, sizeof(ctx));
+
+	/* Attempt to decode the private key */
+	ret = asn1_ber_decoder(&tpm_decoder, &ctx, data, datalen);
+	if (ret < 0)
+		goto error;
+
+	return tpm_key_create(ctx.blob, ctx.blob_len);
+
+error:
+	return ERR_PTR(ret);
+}
+/*
+ * Attempt to parse a data blob for a key as a TPM private key blob.
+ */
+static int tpm_key_preparse(struct key_preparsed_payload *prep)
+{
+	struct tpm_key *tk;
+
+	/*
+	 * TPM 1.2 keys are max 2048 bits long, so assume the blob is no
+	 * more than 4x that
+	 */
+	if (prep->datalen > 256 * 4)
+		return -EMSGSIZE;
+
+	tk = tpm_parse(prep->data, prep->datalen);
+
+	if (IS_ERR(tk))
+		return PTR_ERR(tk);
+
+	/* We're pinning the module by being linked against it */
+	__module_get(asym_tpm_subtype.owner);
+	prep->payload.data[asym_subtype] = &asym_tpm_subtype;
+	prep->payload.data[asym_key_ids] = NULL;
+	prep->payload.data[asym_crypto] = tk;
+	prep->payload.data[asym_auth] = NULL;
+	prep->quotalen = 100;
+	return 0;
+}
+
+static struct asymmetric_key_parser tpm_key_parser = {
+	.owner	= THIS_MODULE,
+	.name	= "tpm_parser",
+	.parse	= tpm_key_preparse,
+};
+
+static int __init tpm_key_init(void)
+{
+	return register_asymmetric_key_parser(&tpm_key_parser);
+}
+
+static void __exit tpm_key_exit(void)
+{
+	unregister_asymmetric_key_parser(&tpm_key_parser);
+}
+
+module_init(tpm_key_init);
+module_exit(tpm_key_exit);
+
+MODULE_DESCRIPTION("TPM private key-blob parser");
+MODULE_LICENSE("GPL v2");