programs/fsverity: Add digest subcommand

Add a 'fsverity digest' subcommand that prints the hex-encoded digest of
the specified file(s), ready to be signed offline.  By default the
output is like 'fsverity measure', but if the --for-builtin-sig option
is given, the output is the hex-encoded "struct fsverity_signed_digest"
that the kernel expects to be signed for builtin signatures.

This subcommand is useful in case the integrated signing mechanism with
local cert/key cannot be used.

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>
Link: https://lore.kernel.org/r/20201026191839.3329948-1-luca.boccassi@gmail.com
[EB: commit message, comment, and whitespace tweaks]
Signed-off-by: Eric Biggers <ebiggers@google.com>

programs/cmd_digest.c

@@ -0,0 +1,161 @@
+// SPDX-License-Identifier: MIT
+/*
+ * The 'fsverity digest' command
+ *
+ * Copyright 2020 Microsoft
+ *
+ * Use of this source code is governed by an MIT-style
+ * license that can be found in the LICENSE file or at
+ * https://opensource.org/licenses/MIT.
+ */
+
+#include "fsverity.h"
+
+#include <fcntl.h>
+#include <getopt.h>
+
+enum {
+	OPT_HASH_ALG,
+	OPT_BLOCK_SIZE,
+	OPT_SALT,
+	OPT_COMPACT,
+	OPT_FOR_BUILTIN_SIG,
+};
+
+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},
+	{"compact",		no_argument,	   NULL, OPT_COMPACT},
+	{"for-builtin-sig",	no_argument,	   NULL, OPT_FOR_BUILTIN_SIG},
+	{NULL, 0, NULL, 0}
+};
+
+struct fsverity_signed_digest {
+	char magic[8];			/* must be "FSVerity" */
+	__le16 digest_algorithm;
+	__le16 digest_size;
+	__u8 digest[];
+};
+
+/*
+ * Compute the fs-verity measurement of the given file(s), for offline signing.
+ */
+int fsverity_cmd_digest(const struct fsverity_command *cmd,
+		      int argc, char *argv[])
+{
+	u8 *salt = NULL;
+	struct filedes file = { .fd = -1 };
+	struct libfsverity_merkle_tree_params tree_params = { .version = 1 };
+	bool compact = false, for_builtin_sig = false;
+	int status;
+	int c;
+
+	while ((c = getopt_long(argc, argv, "", longopts, NULL)) != -1) {
+		switch (c) {
+		case OPT_HASH_ALG:
+			if (!parse_hash_alg_option(optarg,
+						   &tree_params.hash_algorithm))
+				goto out_usage;
+			break;
+		case OPT_BLOCK_SIZE:
+			if (!parse_block_size_option(optarg,
+						     &tree_params.block_size))
+				goto out_usage;
+			break;
+		case OPT_SALT:
+			if (!parse_salt_option(optarg, &salt,
+					       &tree_params.salt_size))
+				goto out_usage;
+			tree_params.salt = salt;
+			break;
+		case OPT_COMPACT:
+			compact = true;
+			break;
+		case OPT_FOR_BUILTIN_SIG:
+			for_builtin_sig = true;
+			break;
+		default:
+			goto out_usage;
+		}
+	}
+
+	argv += optind;
+	argc -= optind;
+
+	if (argc < 1)
+		goto out_usage;
+
+	if (tree_params.hash_algorithm == 0)
+		tree_params.hash_algorithm = FS_VERITY_HASH_ALG_DEFAULT;
+
+	if (tree_params.block_size == 0)
+		tree_params.block_size = get_default_block_size();
+
+	for (int i = 0; i < argc; i++) {
+		struct fsverity_signed_digest *d = NULL;
+		struct libfsverity_digest *digest = NULL;
+		char digest_hex[FS_VERITY_MAX_DIGEST_SIZE * 2 +
+				sizeof(struct fsverity_signed_digest) * 2 + 1];
+
+		if (!open_file(&file, argv[i], O_RDONLY, 0))
+			goto out_err;
+
+		if (!get_file_size(&file, &tree_params.file_size))
+			goto out_err;
+
+		if (libfsverity_compute_digest(&file, read_callback,
+					       &tree_params, &digest) != 0) {
+			error_msg("failed to compute digest");
+			goto out_err;
+		}
+
+		ASSERT(digest->digest_size <= FS_VERITY_MAX_DIGEST_SIZE);
+
+		if (for_builtin_sig) {
+			/*
+			 * Format the digest for use with the built-in signature
+			 * support.
+			 */
+			d = xzalloc(sizeof(*d) + digest->digest_size);
+			memcpy(d->magic, "FSVerity", 8);
+			d->digest_algorithm =
+				cpu_to_le16(digest->digest_algorithm);
+			d->digest_size = cpu_to_le16(digest->digest_size);
+			memcpy(d->digest, digest->digest, digest->digest_size);
+
+			bin2hex((const u8 *)d, sizeof(*d) + digest->digest_size,
+				digest_hex);
+		} else {
+			bin2hex(digest->digest, digest->digest_size,
+				digest_hex);
+		}
+
+		if (compact)
+			printf("%s\n", digest_hex);
+		else if (for_builtin_sig)
+			printf("%s %s\n", digest_hex, argv[i]);
+		else
+			printf("%s:%s %s\n",
+			       libfsverity_get_hash_name(tree_params.hash_algorithm),
+			       digest_hex, argv[i]);
+
+		filedes_close(&file);
+		free(digest);
+		free(d);
+	}
+	status = 0;
+out:
+	free(salt);
+	return status;
+
+out_err:
+	filedes_close(&file);
+	status = 1;
+	goto out;
+
+out_usage:
+	usage(cmd, stderr);
+	status = 2;
+	goto out;
+}

programs/cmd_sign.c

@@ -43,14 +43,6 @@ static const struct option longopts[] = {
 	{NULL, 0, NULL, 0}
 };
 
-static int read_callback(void *file, void *buf, size_t count)
-{
-	errno = 0;
-	if (!full_read(file, buf, count))
-		return errno ? -errno : -EIO;
-	return 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[])

programs/fsverity.c

@@ -21,6 +21,15 @@ static const struct fsverity_command {
 	const char *usage_str;
 } fsverity_commands[] = {
 	{
+		.name = "digest",
+		.func = fsverity_cmd_digest,
+		.short_desc =
+"Compute the fs-verity measurement of the given file(s), for offline signing",
+		.usage_str =
+"    fsverity digest FILE...\n"
+"               [--hash-alg=HASH_ALG] [--block-size=BLOCK_SIZE] [--salt=SALT]\n"
+"               [--compact] [--for-builtin-sig]\n"
+	}, {
 		.name = "enable",
 		.func = fsverity_cmd_enable,
 		.short_desc = "Enable fs-verity on a file",

programs/fsverity.h

@@ -25,6 +25,10 @@
 
 struct fsverity_command;
 
+/* cmd_digest.c */
+int fsverity_cmd_digest(const struct fsverity_command *cmd,
+			int argc, char *argv[]);
+
 /* cmd_enable.c */
 int fsverity_cmd_enable(const struct fsverity_command *cmd,
 			int argc, char *argv[]);

programs/utils.c

@@ -175,6 +175,14 @@ bool filedes_close(struct filedes *file)
 	return res == 0;
 }
 
+int read_callback(void *file, void *buf, size_t count)
+{
+	errno = 0;
+	if (!full_read(file, buf, count))
+		return errno ? -errno : -EIO;
+	return 0;
+}
+
 /* ========== String utilities ========== */
 
 static int hex2bin_char(char c)

programs/utils.h

@@ -43,6 +43,7 @@ bool get_file_size(struct filedes *file, u64 *size_ret);
 bool full_read(struct filedes *file, void *buf, size_t count);
 bool full_write(struct filedes *file, const void *buf, size_t count);
 bool filedes_close(struct filedes *file);
+int read_callback(void *file, void *buf, size_t count);
 
 bool hex2bin(const char *hex, u8 *bin, size_t bin_len);
 void bin2hex(const u8 *bin, size_t bin_len, char *hex);