btrfs-progs: optionally enforce chroot for btrfs receive

This patch forces btrfs receive to issue chroot before
parsing the btrfs stream using command-line flag -C
to confine the process and minimize damage that could
be done via malicious btrfs stream.

Signed-off-by: Lauri Võsandi <lauri.vosandi@gmail.com>
[added long option variant, added docs]
Signed-off-by: David Sterba <dsterba@suse.cz>

Documentation/btrfs-receive.asciidoc

@@ -7,7 +7,7 @@ btrfs-receive - receive subvolumes from stdin/file.
 
 SYNOPSIS
 --------
-*btrfs receive* [-ve] [-f <infile>] [--max-errors <N>] <mount>
+*btrfs receive* [options] <mount>
 
 DESCRIPTION
 -----------
@@ -34,6 +34,8 @@ verbose level more.
 -f <infile>::
 By default, btrfs receive uses stdin to receive the subvolumes.
 Use this option to specify a file to use instead.
+-C|--chroot::
+Confine the process to <mount> using chroot.
 -e::
 Terminate after receiving an <end cmd> in the data stream.
 Without this option, the receiver terminates only if an error is recognized

cmds-receive.c

@@ -61,6 +61,7 @@ struct btrfs_receive
 	char *root_path;
 	char *dest_dir_path; /* relative to root_path */
 	char *full_subvol_path;
+	int dest_dir_chroot;
 
 	struct subvol_info *cur_subvol;
 
@@ -858,14 +859,38 @@ static int do_receive(struct btrfs_receive *r, const char *tomnt, int r_fd,
 		goto out;
 	}
 
-	/*
-	 * find_mount_root returns a root_path that is a subpath of
-	 * dest_dir_full_path. Now get the other part of root_path,
-	 * which is the destination dir relative to root_path.
-	 */
-	r->dest_dir_path = dest_dir_full_path + strlen(r->root_path);
-	while (r->dest_dir_path[0] == '/')
-		r->dest_dir_path++;
+	if (r->dest_dir_chroot) {
+		if (chroot(dest_dir_full_path)) {
+			ret = -errno;
+			fprintf(stderr,
+				"ERROR: failed to chroot to %s, %s\n",
+				dest_dir_full_path,
+				strerror(-ret));
+			goto out;
+		}
+		if (chdir("/")) {
+			ret = -errno;
+			fprintf(stderr,
+				"ERROR: failed to chdir to /, %s\n",
+				strerror(-ret));
+			goto out;
+		}
+		if (g_verbose >= 1) {
+			fprintf(stderr, "chrooted to %s\n",
+				dest_dir_full_path);
+		}
+		r->root_path = strdup("/");
+		r->dest_dir_path = r->root_path;
+	} else {
+		/*
+		 * find_mount_root returns a root_path that is a subpath of
+		 * dest_dir_full_path. Now get the other part of root_path,
+		 * which is the destination dir relative to root_path.
+		 */
+		r->dest_dir_path = dest_dir_full_path + strlen(r->root_path);
+		while (r->dest_dir_path[0] == '/')
+			r->dest_dir_path++;
+	}
 
 	ret = subvol_uuid_search_init(r->mnt_fd, &r->sus);
 	if (ret < 0)
@@ -930,15 +955,17 @@ int cmd_receive(int argc, char **argv)
 	r.mnt_fd = -1;
 	r.write_fd = -1;
 	r.dest_dir_fd = -1;
+	r.dest_dir_chroot = 0;
 
 	while (1) {
 		int c;
 		static const struct option long_opts[] = {
 			{ "max-errors", required_argument, NULL, 'E' },
+			{ "chroot", no_argument, NULL, 'C' },
 			{ NULL, 0, NULL, 0 }
 		};
 
-		c = getopt_long(argc, argv, "evf:", long_opts, NULL);
+		c = getopt_long(argc, argv, "Cevf:", long_opts, NULL);
 		if (c < 0)
 			break;
 
@@ -952,6 +979,9 @@ int cmd_receive(int argc, char **argv)
 		case 'e':
 			r.honor_end_cmd = 1;
 			break;
+		case 'C':
+			r.dest_dir_chroot = 1;
+			break;
 		case 'E':
 			max_errors = arg_strtou64(optarg);
 			break;
@@ -1001,6 +1031,7 @@ const char * const cmd_receive_usage[] = {
 	"                 in the data stream. Without this option,",
 	"                 the receiver terminates only if an error",
 	"                 is recognized or on EOF.",
+	"-C|--chroot      confine the process to <mount> using chroot",
 	"--max-errors <N> Terminate as soon as N errors happened while",
 	"                 processing commands from the send stream.",
 	"                 Default value is 1. A value of 0 means no limit.",