daemon: un-backslash escaped option args (#829)

Without --secluded-args, the client's safe_arg() backslash-escapes shell
and wildcard chars in option values before sending them to the server, so
--chown's --usermap=*:user is transmitted as --usermap=\*:user.  Over ssh a
remote shell removes the backslashes before rsync parses the args, but a
daemon has no shell and read_args() stored option args verbatim -- so the
receiver saw the literal "\*", the usermap/groupmap wildcard never matched,
and the module's configured uid/gid won instead.  A regression from the
secluded-args hardening; rsync 3.2.3 (protocol 31) worked.

Un-backslash option args in read_args() on the daemon's first
(non-protected) read, mirroring what the ssh-side shell does.  File args
after the dot are already handled by glob_expand(); the protected (NUL,
already-unescaped) re-read and the server's stdin read pass unescape=0 so
their raw args are left untouched.

Thanks to @elcamlost for the report (#829).

Fixes: #829

clientserver.c

@@ -1070,7 +1070,7 @@ static int rsync_module(int f_in, int f_out, int i, const char *addr, const char
 
 	io_printf(f_out, "@RSYNCD: OK\n");
 
-	read_args(f_in, name, line, sizeof line, rl_nulls, &argv, &argc, &request);
+	read_args(f_in, name, line, sizeof line, rl_nulls, 1, &argv, &argc, &request);
 	orig_argv = argv;
 
 	save_munge_symlinks = munge_symlinks;
@@ -1080,7 +1080,7 @@ static int rsync_module(int f_in, int f_out, int i, const char *addr, const char
 	if (protect_args && ret) {
 		orig_early_argv = orig_argv;
 		protect_args = 2;
-		read_args(f_in, name, line, sizeof line, 1, &argv, &argc, &request);
+		read_args(f_in, name, line, sizeof line, 1, 0, &argv, &argc, &request);
 		orig_argv = argv;
 		ret = parse_arguments(&argc, (const char ***) &argv);
 	} else

io.c

@@ -1292,8 +1292,21 @@ int read_line(int fd, char *buf, size_t bufsiz, int flags)
 	return s - buf;
 }
 
+/* Reverse safe_arg()'s backslash escaping of a daemon option arg, the way a
+ * remote shell un-escapes args for the ssh transport.  In place; \X -> X. */
+static void unbackslash_arg(char *s)
+{
+	char *f = s, *t = s;
+	while (*f) {
+		if (*f == '\\' && f[1])
+			f++;
+		*t++ = *f++;
+	}
+	*t = '\0';
+}
+
 void read_args(int f_in, char *mod_name, char *buf, size_t bufsiz, int rl_nulls,
-	       char ***argv_p, int *argc_p, char **request_p)
+	       int unescape, char ***argv_p, int *argc_p, char **request_p)
 {
 	int maxargs = MAX_ARGS;
 	int dot_pos = 0, argc = 0, request_len = 0;
@@ -1335,6 +1348,11 @@ void read_args(int f_in, char *mod_name, char *buf, size_t bufsiz, int rl_nulls,
 				glob_expand(buf, &argv, &argc, &maxargs);
 		} else {
 			p = strdup(buf);
+			/* An option arg the client escaped with safe_arg() (no
+			 * remote shell un-escapes it for a daemon).  File args
+			 * after the dot are handled by glob_expand() below. */
+			if (unescape)
+				unbackslash_arg(p);
 			argv[argc++] = p;
 			if (*p == '.' && p[1] == '\0')
 				dot_pos = argc;

main.c

@@ -1840,7 +1840,7 @@ int main(int argc,char *argv[])
 	if (am_server && protect_args) {
 		char buf[MAXPATHLEN];
 		protect_args = 2;
-		read_args(STDIN_FILENO, NULL, buf, sizeof buf, 1, &argv, &argc, NULL);
+		read_args(STDIN_FILENO, NULL, buf, sizeof buf, 1, 0, &argv, &argc, NULL);
 		if (!parse_arguments(&argc, (const char ***) &argv)) {
 			option_error();
 			exit_cleanup(RERR_SYNTAX);