MODSIGN: Provide a utility to append a PKCS#7 signature to a module

Provide a utility that:

 (1) Digests a module using the specified hash algorithm (typically sha256).

     [The digest can be dumped into a file by passing the '-d' flag]

 (2) Generates a PKCS#7 message that:

     (a) Has detached data (ie. the module content).

     (b) Is signed with the specified private key.

     (c) Refers to the specified X.509 certificate.

     (d) Has an empty X.509 certificate list.

     [The PKCS#7 message can be dumped into a file by passing the '-p' flag]

 (3) Generates a signed module by concatenating the old module, the PKCS#7
     message, a descriptor and a magic string.  The descriptor contains the
     size of the PKCS#7 message and indicates the id_type as PKEY_ID_PKCS7.

 (4) Either writes the signed module to the specified destination or renames
     it over the source module.

This allows module signing to reuse the PKCS#7 handling code that was added
for PE file parsing for signed kexec.

Note that the utility is written in C and must be linked against the OpenSSL
crypto library.

Note further that I have temporarily dropped support for handling externally
created signatures until we can work out the best way to do those.  Hopefully,
whoever creates the signature can give me a PKCS#7 certificate.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Vivek Goyal <vgoyal@redhat.com>

include/crypto/public_key.h

@@ -33,6 +33,7 @@ extern const struct public_key_algorithm *pkey_algo[PKEY_ALGO__LAST];
 enum pkey_id_type {
 	PKEY_ID_PGP,		/* OpenPGP generated key ID */
 	PKEY_ID_X509,		/* X.509 arbitrary subjectKeyIdentifier */
+	PKEY_ID_PKCS7,		/* Signature in PKCS#7 message */
 	PKEY_ID_TYPE__LAST
 };
 

scripts/sign-file.c

@@ -0,0 +1,205 @@
+/* Sign a module file using the given key.
+ *
+ * Copyright (C) 2014 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public Licence
+ * as published by the Free Software Foundation; either version
+ * 2 of the Licence, or (at your option) any later version.
+ */
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <stdbool.h>
+#include <string.h>
+#include <getopt.h>
+#include <err.h>
+#include <arpa/inet.h>
+#include <openssl/bio.h>
+#include <openssl/evp.h>
+#include <openssl/pem.h>
+#include <openssl/pkcs7.h>
+#include <openssl/err.h>
+
+struct module_signature {
+	uint8_t		algo;		/* Public-key crypto algorithm [0] */
+	uint8_t		hash;		/* Digest algorithm [0] */
+	uint8_t		id_type;	/* Key identifier type [PKEY_ID_PKCS7] */
+	uint8_t		signer_len;	/* Length of signer's name [0] */
+	uint8_t		key_id_len;	/* Length of key identifier [0] */
+	uint8_t		__pad[3];
+	uint32_t	sig_len;	/* Length of signature data */
+};
+
+#define PKEY_ID_PKCS7 2
+
+static char magic_number[] = "~Module signature appended~\n";
+
+static __attribute__((noreturn))
+void format(void)
+{
+	fprintf(stderr,
+		"Usage: scripts/sign-file [-dp] <hash algo> <key> <x509> <module> [<dest>]\n");
+	exit(2);
+}
+
+static void display_openssl_errors(int l)
+{
+	const char *file;
+	char buf[120];
+	int e, line;
+
+	if (ERR_peek_error() == 0)
+		return;
+	fprintf(stderr, "At main.c:%d:\n", l);
+
+	while ((e = ERR_get_error_line(&file, &line))) {
+		ERR_error_string(e, buf);
+		fprintf(stderr, "- SSL %s: %s:%d\n", buf, file, line);
+	}
+}
+
+static void drain_openssl_errors(void)
+{
+	const char *file;
+	int line;
+
+	if (ERR_peek_error() == 0)
+		return;
+	while (ERR_get_error_line(&file, &line)) {}
+}
+
+#define ERR(cond, fmt, ...)				\
+	do {						\
+		bool __cond = (cond);			\
+		display_openssl_errors(__LINE__);	\
+		if (__cond) {				\
+			err(1, fmt, ## __VA_ARGS__);	\
+		}					\
+	} while(0)
+
+int main(int argc, char **argv)
+{
+	struct module_signature sig_info = { .id_type = PKEY_ID_PKCS7 };
+	char *hash_algo = NULL;
+	char *private_key_name, *x509_name, *module_name, *dest_name;
+	bool save_pkcs7 = false, replace_orig;
+	unsigned char buf[4096];
+	unsigned long module_size, pkcs7_size;
+	const EVP_MD *digest_algo;
+	EVP_PKEY *private_key;
+	PKCS7 *pkcs7;
+	X509 *x509;
+	BIO *b, *bd, *bm;
+	int opt, n;
+
+	ERR_load_crypto_strings();
+	ERR_clear_error();
+
+	do {
+		opt = getopt(argc, argv, "dp");
+		switch (opt) {
+		case 'p': save_pkcs7 = true; break;
+		case -1: break;
+		default: format();
+		}
+	} while (opt != -1);
+
+	argc -= optind;
+	argv += optind;
+	if (argc < 4 || argc > 5)
+		format();
+
+	hash_algo = argv[0];
+	private_key_name = argv[1];
+	x509_name = argv[2];
+	module_name = argv[3];
+	if (argc == 5) {
+		dest_name = argv[4];
+		replace_orig = false;
+	} else {
+		ERR(asprintf(&dest_name, "%s.~signed~", module_name) < 0,
+		    "asprintf");
+		replace_orig = true;
+	}
+
+	/* Read the private key and the X.509 cert the PKCS#7 message
+	 * will point to.
+	 */
+	b = BIO_new_file(private_key_name, "rb");
+	ERR(!b, "%s", private_key_name);
+        private_key = PEM_read_bio_PrivateKey(b, NULL, NULL, NULL);
+	BIO_free(b);
+
+	b = BIO_new_file(x509_name, "rb");
+	ERR(!b, "%s", x509_name);
+	x509 = d2i_X509_bio(b, NULL); /* Binary encoded X.509 */
+	if (!x509) {
+		BIO_reset(b);
+		x509 = PEM_read_bio_X509(b, NULL, NULL, NULL); /* PEM encoded X.509 */
+		if (x509)
+			drain_openssl_errors();
+	}
+	BIO_free(b);
+	ERR(!x509, "%s", x509_name);
+
+	/* Open the destination file now so that we can shovel the module data
+	 * across as we read it.
+	 */
+	bd = BIO_new_file(dest_name, "wb");
+	ERR(!bd, "%s", dest_name);
+
+	/* Digest the module data. */
+	OpenSSL_add_all_digests();
+	display_openssl_errors(__LINE__);
+	digest_algo = EVP_get_digestbyname(hash_algo);
+	ERR(!digest_algo, "EVP_get_digestbyname");
+
+	bm = BIO_new_file(module_name, "rb");
+	ERR(!bm, "%s", module_name);
+
+	/* Load the PKCS#7 message from the digest buffer. */
+	pkcs7 = PKCS7_sign(NULL, NULL, NULL, NULL,
+			   PKCS7_NOCERTS | PKCS7_PARTIAL | PKCS7_BINARY | PKCS7_DETACHED | PKCS7_STREAM);
+	ERR(!pkcs7, "PKCS7_sign");
+
+	ERR(!PKCS7_sign_add_signer(pkcs7, x509, private_key, digest_algo, PKCS7_NOCERTS | PKCS7_BINARY),
+	    "PKCS7_sign_add_signer");
+	ERR(PKCS7_final(pkcs7, bm, PKCS7_NOCERTS | PKCS7_BINARY) < 0,
+	    "PKCS7_final");
+
+	if (save_pkcs7) {
+		char *pkcs7_name;
+
+		ERR(asprintf(&pkcs7_name, "%s.pkcs7", module_name) < 0, "asprintf");
+		b = BIO_new_file(pkcs7_name, "wb");
+		ERR(!b, "%s", pkcs7_name);
+		ERR(i2d_PKCS7_bio_stream(b, pkcs7, NULL, 0) < 0, "%s", pkcs7_name);
+		BIO_free(b);
+	}
+
+	/* Append the marker and the PKCS#7 message to the destination file */
+	ERR(BIO_reset(bm) < 0, "%s", module_name);
+	while ((n = BIO_read(bm, buf, sizeof(buf))),
+	       n > 0) {
+		ERR(BIO_write(bd, buf, n) < 0, "%s", dest_name);
+	}
+	ERR(n < 0, "%s", module_name);
+	module_size = BIO_number_written(bd);
+
+	ERR(i2d_PKCS7_bio_stream(bd, pkcs7, NULL, 0) < 0, "%s", dest_name);
+	pkcs7_size = BIO_number_written(bd) - module_size;
+	sig_info.sig_len = htonl(pkcs7_size);
+	ERR(BIO_write(bd, &sig_info, sizeof(sig_info)) < 0, "%s", dest_name);
+	ERR(BIO_write(bd, magic_number, sizeof(magic_number) - 1) < 0, "%s", dest_name);
+
+	ERR(BIO_free(bd) < 0, "%s", dest_name);
+
+	/* Finally, if we're signing in place, replace the original. */
+	if (replace_orig)
+		ERR(rename(dest_name, module_name) < 0, "%s", dest_name);
+
+	return 0;
+}