cmd_sign.c

// SPDX-License-Identifier: GPL-2.0+
/*
 * The 'fsverity sign' command
 *
 * Copyright (C) 2018 Google LLC
 *
 * Written by Eric Biggers.
 */

#include <fcntl.h>
#include <getopt.h>
#include <limits.h>
#include <openssl/bio.h>
#include <openssl/err.h>
#include <openssl/pem.h>
#include <openssl/pkcs7.h>
#include <stdlib.h>
#include <string.h>

#include "commands.h"
#include "fsverity_uapi.h"
#include "hash_algs.h"

/*
 * Merkle tree properties.  The file measurement is the hash of this structure
 * excluding the signature and with the sig_size field set to 0.
 */
struct fsverity_descriptor {
	__u8 version;		/* must be 1 */
	__u8 hash_algorithm;	/* Merkle tree hash algorithm */
	__u8 log_blocksize;	/* log2 of size of data and tree blocks */
	__u8 salt_size;		/* size of salt in bytes; 0 if none */
	__le32 sig_size;	/* size of signature in bytes; 0 if none */
	__le64 data_size;	/* size of file the Merkle tree is built over */
	__u8 root_hash[64];	/* Merkle tree root hash */
	__u8 salt[32];		/* salt prepended to each hashed block */
	__u8 __reserved[144];	/* must be 0's */
	__u8 signature[];	/* optional PKCS#7 signature */
};

/*
 * Format in which verity file measurements are signed.  This is the same as
 * 'struct fsverity_digest', except here some magic bytes are prepended to
 * provide some context about what is being signed in case the same key is used
 * for non-fsverity purposes, and here the fields have fixed endianness.
 */
struct fsverity_signed_digest {
	char magic[8];			/* must be "FSVerity" */
	__le16 digest_algorithm;
	__le16 digest_size;
	__u8 digest[];
};

static void __printf(1, 2) __cold
error_msg_openssl(const char *format, ...)
{
	va_list va;

	va_start(va, format);
	do_error_msg(format, va, 0);
	va_end(va);

	if (ERR_peek_error() == 0)
		return;

	fprintf(stderr, "OpenSSL library errors:\n");
	ERR_print_errors_fp(stderr);
}

/* Read a PEM PKCS#8 formatted private key */
static EVP_PKEY *read_private_key(const char *keyfile)
{
	BIO *bio;
	EVP_PKEY *pkey;

	bio = BIO_new_file(keyfile, "r");
	if (!bio) {
		error_msg_openssl("can't open '%s' for reading", keyfile);
		return NULL;
	}

	pkey = PEM_read_bio_PrivateKey(bio, NULL, NULL, NULL);
	if (!pkey) {
		error_msg_openssl("Failed to parse private key file '%s'.\n"
				  "       Note: it must be in PEM PKCS#8 format.",
				  keyfile);
	}
	BIO_free(bio);
	return pkey;
}

/* Read a PEM X.509 formatted certificate */
static X509 *read_certificate(const char *certfile)
{
	BIO *bio;
	X509 *cert;

	bio = BIO_new_file(certfile, "r");
	if (!bio) {
		error_msg_openssl("can't open '%s' for reading", certfile);
		return NULL;
	}
	cert = PEM_read_bio_X509(bio, NULL, NULL, NULL);
	if (!cert) {
		error_msg_openssl("Failed to parse X.509 certificate file '%s'.\n"
				  "       Note: it must be in PEM format.",
				  certfile);
	}
	BIO_free(bio);
	return cert;
}

#ifdef OPENSSL_IS_BORINGSSL

static bool sign_pkcs7(const void *data_to_sign, size_t data_size,
		       EVP_PKEY *pkey, X509 *cert, const EVP_MD *md,
		       u8 **sig_ret, u32 *sig_size_ret)
{
	CBB out, outer_seq, wrapped_seq, seq, digest_algos_set, digest_algo,
		null, content_info, issuer_and_serial, signed_data,
		wrapped_signed_data, signer_infos, signer_info, sign_algo,
		signature;
	EVP_MD_CTX md_ctx;
	u8 *name_der = NULL, *sig = NULL, *pkcs7_data = NULL;
	size_t pkcs7_data_len, sig_len;
	int name_der_len, sig_nid;
	bool ok = false;

	EVP_MD_CTX_init(&md_ctx);
	BIGNUM *serial = ASN1_INTEGER_to_BN(X509_get_serialNumber(cert), NULL);

	if (!CBB_init(&out, 1024)) {
		error_msg("out of memory");
		goto out;
	}

	name_der_len = i2d_X509_NAME(X509_get_subject_name(cert), &name_der);
	if (name_der_len < 0) {
		error_msg_openssl("i2d_X509_NAME failed");
		goto out;
	}

	if (!EVP_DigestSignInit(&md_ctx, NULL, md, NULL, pkey)) {
		error_msg_openssl("EVP_DigestSignInit failed");
		goto out;
	}

	sig_len = EVP_PKEY_size(pkey);
	sig = xmalloc(sig_len);
	if (!EVP_DigestSign(&md_ctx, sig, &sig_len, data_to_sign, data_size)) {
		error_msg_openssl("EVP_DigestSign failed");
		goto out;
	}

	sig_nid = EVP_PKEY_id(pkey);
	/* To mirror OpenSSL behaviour, always use |NID_rsaEncryption| with RSA
	 * rather than the combined hash+pkey NID. */
	if (sig_nid != NID_rsaEncryption) {
		OBJ_find_sigid_by_algs(&sig_nid, EVP_MD_type(md),
				       EVP_PKEY_id(pkey));
	}

	// See https://tools.ietf.org/html/rfc2315#section-7
	if (!CBB_add_asn1(&out, &outer_seq, CBS_ASN1_SEQUENCE) ||
	    !OBJ_nid2cbb(&outer_seq, NID_pkcs7_signed) ||
	    !CBB_add_asn1(&outer_seq, &wrapped_seq, CBS_ASN1_CONTEXT_SPECIFIC |
			  CBS_ASN1_CONSTRUCTED | 0) ||
	    // See https://tools.ietf.org/html/rfc2315#section-9.1
	    !CBB_add_asn1(&wrapped_seq, &seq, CBS_ASN1_SEQUENCE) ||
	    !CBB_add_asn1_uint64(&seq, 1 /* version */) ||
	    !CBB_add_asn1(&seq, &digest_algos_set, CBS_ASN1_SET) ||
	    !CBB_add_asn1(&digest_algos_set, &digest_algo, CBS_ASN1_SEQUENCE) ||
	    !OBJ_nid2cbb(&digest_algo, EVP_MD_type(md)) ||
	    !CBB_add_asn1(&digest_algo, &null, CBS_ASN1_NULL) ||
	    !CBB_add_asn1(&seq, &content_info, CBS_ASN1_SEQUENCE) ||
	    !OBJ_nid2cbb(&content_info, NID_pkcs7_data) ||
	    !CBB_add_asn1(
		&content_info, &signed_data,
		CBS_ASN1_CONTEXT_SPECIFIC | CBS_ASN1_CONSTRUCTED | 0) ||
	    !CBB_add_asn1(&signed_data, &wrapped_signed_data,
			  CBS_ASN1_OCTETSTRING) ||
	    !CBB_add_bytes(&wrapped_signed_data, (const u8 *)data_to_sign,
			   data_size) ||
	    !CBB_add_asn1(&seq, &signer_infos, CBS_ASN1_SET) ||
	    !CBB_add_asn1(&signer_infos, &signer_info, CBS_ASN1_SEQUENCE) ||
	    !CBB_add_asn1_uint64(&signer_info, 1 /* version */) ||
	    !CBB_add_asn1(&signer_info, &issuer_and_serial,
			  CBS_ASN1_SEQUENCE) ||
	    !CBB_add_bytes(&issuer_and_serial, name_der, name_der_len) ||
	    !BN_marshal_asn1(&issuer_and_serial, serial) ||
	    !CBB_add_asn1(&signer_info, &digest_algo, CBS_ASN1_SEQUENCE) ||
	    !OBJ_nid2cbb(&digest_algo, EVP_MD_type(md)) ||
	    !CBB_add_asn1(&digest_algo, &null, CBS_ASN1_NULL) ||
	    !CBB_add_asn1(&signer_info, &sign_algo, CBS_ASN1_SEQUENCE) ||
	    !OBJ_nid2cbb(&sign_algo, sig_nid) ||
	    !CBB_add_asn1(&sign_algo, &null, CBS_ASN1_NULL) ||
	    !CBB_add_asn1(&signer_info, &signature, CBS_ASN1_OCTETSTRING) ||
	    !CBB_add_bytes(&signature, sig, sig_len) ||
	    !CBB_finish(&out, &pkcs7_data, &pkcs7_data_len)) {
		error_msg_openssl("failed to construct PKCS#7 data");
		goto out;
	}

	*sig_ret = xmemdup(pkcs7_data, pkcs7_data_len);
	*sig_size_ret = pkcs7_data_len;
	ok = true;
out:
	BN_free(serial);
	EVP_MD_CTX_cleanup(&md_ctx);
	CBB_cleanup(&out);
	free(sig);
	OPENSSL_free(name_der);
	OPENSSL_free(pkcs7_data);
	return ok;
}

#else /* OPENSSL_IS_BORINGSSL */

static BIO *new_mem_buf(const void *buf, size_t size)
{
	BIO *bio;

	ASSERT(size <= INT_MAX);
	/*
	 * Prior to OpenSSL 1.1.0, BIO_new_mem_buf() took a non-const pointer,
	 * despite still marking the resulting bio as read-only.  So cast away
	 * the const to avoid a compiler warning with older OpenSSL versions.
	 */
	bio = BIO_new_mem_buf((void *)buf, size);
	if (!bio)
		error_msg_openssl("out of memory");
	return bio;
}

static bool sign_pkcs7(const void *data_to_sign, size_t data_size,
		       EVP_PKEY *pkey, X509 *cert, const EVP_MD *md,
		       u8 **sig_ret, u32 *sig_size_ret)
{
	/*
	 * PKCS#7 signing flags:
	 *
	 * - PKCS7_BINARY	signing binary data, so skip MIME translation
	 *
	 * - PKCS7_NOATTR	omit extra authenticated attributes, such as
	 *			SMIMECapabilities
	 *
	 * - PKCS7_NOCERTS	omit the signer's certificate
	 *
	 * - PKCS7_PARTIAL	PKCS7_sign() creates a handle only, then
	 *			PKCS7_sign_add_signer() can add a signer later.
	 *			This is necessary to change the message digest
	 *			algorithm from the default of SHA-1.  Requires
	 *			OpenSSL 1.0.0 or later.
	 */
	int pkcs7_flags = PKCS7_BINARY | PKCS7_NOATTR | PKCS7_NOCERTS |
			  PKCS7_PARTIAL;
	u8 *sig;
	u32 sig_size;
	BIO *bio = NULL;
	PKCS7 *p7 = NULL;
	bool ok = false;

	bio = new_mem_buf(data_to_sign, data_size);
	if (!bio)
		goto out;

	p7 = PKCS7_sign(NULL, NULL, NULL, bio, pkcs7_flags);
	if (!p7) {
		error_msg_openssl("failed to initialize PKCS#7 signature object");
		goto out;
	}

	if (!PKCS7_sign_add_signer(p7, cert, pkey, md, pkcs7_flags)) {
		error_msg_openssl("failed to add signer to PKCS#7 signature object");
		goto out;
	}

	if (PKCS7_final(p7, bio, pkcs7_flags) != 1) {
		error_msg_openssl("failed to finalize PKCS#7 signature");
		goto out;
	}

	BIO_free(bio);
	bio = BIO_new(BIO_s_mem());
	if (!bio) {
		error_msg_openssl("out of memory");
		goto out;
	}

	if (i2d_PKCS7_bio(bio, p7) != 1) {
		error_msg_openssl("failed to DER-encode PKCS#7 signature object");
		goto out;
	}

	sig_size = BIO_get_mem_data(bio, &sig);
	*sig_ret = xmemdup(sig, sig_size);
	*sig_size_ret = sig_size;
	ok = true;
out:
	PKCS7_free(p7);
	BIO_free(bio);
	return ok;
}

#endif /* !OPENSSL_IS_BORINGSSL */

/*
 * Sign the specified @data_to_sign of length @data_size bytes using the private
 * key in @keyfile, the certificate in @certfile, and the hash algorithm
 * @hash_alg.  Returns the DER-formatted PKCS#7 signature, with the signed data
 * included (not detached), in @sig_ret and @sig_size_ret.
 */
static bool sign_data(const void *data_to_sign, size_t data_size,
		      const char *keyfile, const char *certfile,
		      const struct fsverity_hash_alg *hash_alg,
		      u8 **sig_ret, u32 *sig_size_ret)
{
	EVP_PKEY *pkey = NULL;
	X509 *cert = NULL;
	const EVP_MD *md;
	bool ok = false;

	pkey = read_private_key(keyfile);
	if (!pkey)
		goto out;

	cert = read_certificate(certfile);
	if (!cert)
		goto out;

	OpenSSL_add_all_digests();
	md = EVP_get_digestbyname(hash_alg->name);
	if (!md) {
		fprintf(stderr,
			"Warning: '%s' algorithm not found in OpenSSL library.\n"
			"         Falling back to SHA-256 signature.\n",
			hash_alg->name);
		md = EVP_sha256();
	}

	ok = sign_pkcs7(data_to_sign, data_size, pkey, cert, md,
			sig_ret, sig_size_ret);
out:
	EVP_PKEY_free(pkey);
	X509_free(cert);
	return ok;
}

static bool write_signature(const char *filename, const u8 *sig, u32 sig_size)
{
	struct filedes file;
	bool ok;

	if (!open_file(&file, filename, O_WRONLY|O_CREAT|O_TRUNC, 0644))
		return false;
	ok = full_write(&file, sig, sig_size);
	ok &= filedes_close(&file);
	return ok;
}

#define FS_VERITY_MAX_LEVELS	64

struct block_buffer {
	u32 filled;
	u8 *data;
};

/*
 * Hash a block, writing the result to the next level's pending block buffer.
 * Returns true if the next level's block became full, else false.
 */
static bool hash_one_block(struct hash_ctx *hash, struct block_buffer *cur,
			   u32 block_size, const u8 *salt, u32 salt_size)
{
	struct block_buffer *next = cur + 1;

	/* Zero-pad the block if it's shorter than block_size. */
	memset(&cur->data[cur->filled], 0, block_size - cur->filled);

	hash_init(hash);
	hash_update(hash, salt, salt_size);
	hash_update(hash, cur->data, block_size);
	hash_final(hash, &next->data[next->filled]);

	next->filled += hash->alg->digest_size;
	cur->filled = 0;

	return next->filled + hash->alg->digest_size > block_size;
}

/*
 * Compute the file's Merkle tree root hash using the given hash algorithm,
 * block size, and salt.
 */
static bool compute_root_hash(struct filedes *file, u64 file_size,
			      struct hash_ctx *hash, u32 block_size,
			      const u8 *salt, u32 salt_size, u8 *root_hash)
{
	const u32 hashes_per_block = block_size / hash->alg->digest_size;
	const u32 padded_salt_size = roundup(salt_size, hash->alg->block_size);
	u8 *padded_salt = xzalloc(padded_salt_size);
	u64 blocks;
	int num_levels = 0;
	int level;
	struct block_buffer _buffers[1 + FS_VERITY_MAX_LEVELS + 1] = {};
	struct block_buffer *buffers = &_buffers[1];
	u64 offset;
	bool ok = false;

	memcpy(padded_salt, salt, salt_size);

	/* Compute number of levels */
	for (blocks = DIV_ROUND_UP(file_size, block_size); blocks > 1;
	     blocks = DIV_ROUND_UP(blocks, hashes_per_block)) {
		ASSERT(num_levels < FS_VERITY_MAX_LEVELS);
		num_levels++;
	}

	/*
	 * Allocate the block buffers.  Buffer "-1" is for data blocks.
	 * Buffers 0 <= level < num_levels are for the actual tree levels.
	 * Buffer 'num_levels' is for the root hash.
	 */
	for (level = -1; level < num_levels; level++)
		buffers[level].data = xmalloc(block_size);
	buffers[num_levels].data = root_hash;

	/* Hash each data block, also hashing the tree blocks as they fill up */
	for (offset = 0; offset < file_size; offset += block_size) {
		buffers[-1].filled = min(block_size, file_size - offset);

		if (!full_read(file, buffers[-1].data, buffers[-1].filled))
			goto out;

		level = -1;
		while (hash_one_block(hash, &buffers[level], block_size,
				      padded_salt, padded_salt_size)) {
			level++;
			ASSERT(level < num_levels);
		}
	}
	/* Finish all nonempty pending tree blocks */
	for (level = 0; level < num_levels; level++) {
		if (buffers[level].filled != 0)
			hash_one_block(hash, &buffers[level], block_size,
				       padded_salt, padded_salt_size);
	}

	/* Root hash was filled by the last call to hash_one_block() */
	ASSERT(buffers[num_levels].filled == hash->alg->digest_size);
	ok = true;
out:
	for (level = -1; level < num_levels; level++)
		free(buffers[level].data);
	free(padded_salt);
	return ok;
}

/*
 * Compute the fs-verity measurement of the given file.
 *
 * The fs-verity measurement is the hash of the fsverity_descriptor, which
 * contains the Merkle tree properties including the root hash.
 */
static bool compute_file_measurement(const char *filename,
				     const struct fsverity_hash_alg *hash_alg,
				     u32 block_size, const u8 *salt,
				     u32 salt_size, u8 *measurement)
{
	struct filedes file = { .fd = -1 };
	struct hash_ctx *hash = hash_create(hash_alg);
	u64 file_size;
	struct fsverity_descriptor desc;
	bool ok = false;

	if (!open_file(&file, filename, O_RDONLY, 0))
		goto out;

	if (!get_file_size(&file, &file_size))
		goto out;

	memset(&desc, 0, sizeof(desc));
	desc.version = 1;
	desc.hash_algorithm = hash_alg - fsverity_hash_algs;

	ASSERT(is_power_of_2(block_size));
	desc.log_blocksize = ilog2(block_size);

	if (salt_size != 0) {
		if (salt_size > sizeof(desc.salt)) {
			error_msg("Salt too long (got %u bytes; max is %zu bytes)",
				  salt_size, sizeof(desc.salt));
			goto out;
		}
		memcpy(desc.salt, salt, salt_size);
		desc.salt_size = salt_size;
	}

	desc.data_size = cpu_to_le64(file_size);

	/* Root hash of empty file is all 0's */
	if (file_size != 0 &&
	    !compute_root_hash(&file, file_size, hash, block_size, salt,
			       salt_size, desc.root_hash))
		goto out;

	hash_full(hash, &desc, sizeof(desc), measurement);
	ok = true;
out:
	filedes_close(&file);
	hash_free(hash);
	return ok;
}

enum {
	OPT_HASH_ALG,
	OPT_BLOCK_SIZE,
	OPT_SALT,
	OPT_KEY,
	OPT_CERT,
};

static const struct option longopts[] = {
	{"hash-alg",	required_argument, NULL, OPT_HASH_ALG},
	{"block-size",	required_argument, NULL, OPT_BLOCK_SIZE},
	{"salt",	required_argument, NULL, OPT_SALT},
	{"key",		required_argument, NULL, OPT_KEY},
	{"cert",	required_argument, NULL, OPT_CERT},
	{NULL, 0, NULL, 0}
};

/* Sign a file for fs-verity by computing its measurement, then signing it. */
int fsverity_cmd_sign(const struct fsverity_command *cmd,
		      int argc, char *argv[])
{
	const struct fsverity_hash_alg *hash_alg = NULL;
	u32 block_size = 0;
	u8 *salt = NULL;
	u32 salt_size = 0;
	const char *keyfile = NULL;
	const char *certfile = NULL;
	struct fsverity_signed_digest *digest = NULL;
	char digest_hex[FS_VERITY_MAX_DIGEST_SIZE * 2 + 1];
	u8 *sig = NULL;
	u32 sig_size;
	int status;
	int c;

	while ((c = getopt_long(argc, argv, "", longopts, NULL)) != -1) {
		switch (c) {
		case OPT_HASH_ALG:
			if (hash_alg != NULL) {
				error_msg("--hash-alg can only be specified once");
				goto out_usage;
			}
			hash_alg = find_hash_alg_by_name(optarg);
			if (hash_alg == NULL)
				goto out_usage;
			break;
		case OPT_BLOCK_SIZE:
			if (!parse_block_size_option(optarg, &block_size))
				goto out_usage;
			break;
		case OPT_SALT:
			if (!parse_salt_option(optarg, &salt, &salt_size))
				goto out_usage;
			break;
		case OPT_KEY:
			if (keyfile != NULL) {
				error_msg("--key can only be specified once");
				goto out_usage;
			}
			keyfile = optarg;
			break;
		case OPT_CERT:
			if (certfile != NULL) {
				error_msg("--cert can only be specified once");
				goto out_usage;
			}
			certfile = optarg;
			break;
		default:
			goto out_usage;
		}
	}

	argv += optind;
	argc -= optind;

	if (argc != 2)
		goto out_usage;

	if (hash_alg == NULL)
		hash_alg = &fsverity_hash_algs[FS_VERITY_HASH_ALG_DEFAULT];

	if (block_size == 0)
		block_size = get_default_block_size();

	if (keyfile == NULL) {
		error_msg("Missing --key argument");
		goto out_usage;
	}
	if (certfile == NULL)
		certfile = keyfile;

	digest = xzalloc(sizeof(*digest) + hash_alg->digest_size);
	memcpy(digest->magic, "FSVerity", 8);
	digest->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
	digest->digest_size = cpu_to_le16(hash_alg->digest_size);

	if (!compute_file_measurement(argv[0], hash_alg, block_size,
				      salt, salt_size, digest->digest))
		goto out_err;

	if (!sign_data(digest, sizeof(*digest) + hash_alg->digest_size,
		       keyfile, certfile, hash_alg, &sig, &sig_size))
		goto out_err;

	if (!write_signature(argv[1], sig, sig_size))
		goto out_err;

	bin2hex(digest->digest, hash_alg->digest_size, digest_hex);
	printf("Signed file \"%s\" (%s:%s)\n", argv[0], hash_alg->name,
	       digest_hex);
	status = 0;
out:
	free(salt);
	free(digest);
	free(sig);
	return status;

out_err:
	status = 1;
	goto out;

out_usage:
	usage(cmd, stderr);
	status = 2;
	goto out;
}