touches to shell proxy

reworked the privilege escalation works by changing the way target user info
is passed around, now we pass struct passwd* and struct group* instead of
strings wich allows much greater flexibility.
Also now us mimics the behavior of sudo and doas regarding the shell proxies:
- commands are started under a subshell, that shell is the user's
- if no command is specified just the shell is executed

TODO

@@ -3,3 +3,5 @@
 	* LOGNAME -> to target user
 	* SHELL -> to the target user's SHELL
 	* HOME -> to the target user's HOME
+
+- Reconsider executing programs in a subshell

us.c

@@ -32,8 +32,10 @@
 #include <security/pam_misc.h>
 
 static void usage (void);
-static int perm_set (const char *, const char *);
+static int perm_set (struct passwd *, struct group *);
 static int authenticate (const char *);
+static struct passwd* user_to_passwd (const char *);
+static struct group* group_to_grp (const char *);
 
 // FIXME: misc_conv is a separate library, should stick to plain PAM or make
 // our own pam module
@@ -43,9 +45,12 @@ int main (int argc, char *argv[])
 {
 	// TODO: Add arguments
 	// FIXME: change the default program to execute SHELL
-	char *t_usr = NULL, *t_grp = NULL;
+	char *t_usr = "root", *t_grp = NULL;
+	struct passwd *t_pw;
+	struct group *t_gr;
 	int opt;
-	while ((opt = getopt(argc, argv, "A:u:g:C:")) != -1) {
+	int shellflag = 0;
+	while ((opt = getopt(argc, argv, "A:u:g:C:s")) != -1) {
 		switch (opt) {
 		case 'A':
 			printf("-A is not yet implemented\n");
@@ -61,6 +66,9 @@ int main (int argc, char *argv[])
 			printf("-C is not yet implemented\n");
 			exit(EXIT_FAILURE);
 			break;
+		case 's':
+			shellflag = 1;
+			break;
 		case '?':
 			usage();
 			exit(EINVAL);
@@ -68,64 +76,51 @@ int main (int argc, char *argv[])
 		}
 	}
 
-/*
+	/* Get user info */
-	int c_argc = argc - optind + 2;
+	const char *uname;
-	char **c_argv = malloc((c_argc + 1) * sizeof(char *));
+	char *shell;
-	if (!c_argv) {
+	uid_t ruid = getuid();
-		fprintf(stderr, "malloc: %s\n", strerror(errno));
+	struct passwd *my_pw = getpwuid(ruid);
-		goto fail_end;
+	if (!my_pw) {
-	}
-	c_argv[0] = shell;
-	c_argv[1] = "-c";
-	for (int i = 2; optind < argc; i++, optind++) {
-		c_argv[i] = strdup(argv[optind]);
-		if (!c_argv[i]) {
 		fprintf(stderr, "getpwid: %s\n", strerror(errno));
-			exit(errno);
+		return errno;
 	}
+	uname = my_pw->pw_name;
+
+	t_pw = user_to_passwd(t_usr);
+	if (!t_pw) {
+		fprintf(stderr, "user_to_passwd: %s\n", strerror(errno));
+		return errno;
 	}
-	c_argc[c_argv] = NULL;
+	t_gr = group_to_grp(t_grp);
-*/
+
+	/* Get target user's shell */
+	if (!shellflag)
+		shell = t_pw->pw_shell;
+	else
+		shell = getenv("SHELL");
+	if (!shell)
+		shell = "/bin/sh";
 
 	/* Set argc and argv */
-	// TODO: get the right shell for the user (it is in the passwd struct)
+	char *command = NULL;
-	char *shell = "/bin/sh";
-	char *command, **c_argv;
 	int size, popind = optind;
-	int c_argc = 3; // /bin/sh -c command
+	if (argc - optind) {
-
-	c_argv = malloc((c_argc + 1) * sizeof(char *));
-	if (!c_argv) {
-		fprintf(stderr, "malloc: %s\n", strerror(errno));
-		exit(errno);
-	}
-
 		for (size = 0; optind < argc; optind++)
 			size += strlen(argv[optind]) + 1;
-	command = malloc(sizeof(char) * size);
+		command = malloc(sizeof(char) * size + 1);
 		if (!command) {
 			fprintf(stderr, "malloc: %s\n", strerror(errno));
 			exit(errno);
 		}
+		memset(command, 0, size + 1);
 		for (optind = popind; optind < argc; optind++) {
 			strcat(command, argv[optind]);
 			strcat(command, " ");
 		}
-
+	}
-	c_argv[0] = shell;
-	// FIXME: this should be optional if size is zero: we want just a shell
-	c_argv[1] = "-c";
-	c_argv[2] = command;
-	c_argv[c_argc] = NULL;
 
 	/* Authenticate */
-	uid_t ruid = getuid();
-	struct passwd *pw = getpwuid(ruid);
-	if (!pw) {
-		fprintf(stderr, "getpwid: %s\n", strerror(errno));
-		return errno;
-	}
-	const char *uname = pw->pw_name;
 	if (authenticate(uname) != PAM_SUCCESS)
 		exit(EXIT_FAILURE);
 
@@ -133,21 +128,24 @@ int main (int argc, char *argv[])
 
 	errno = 0;
 	/* Set permissions */
-	if (perm_set(t_usr, t_grp) == -1) { // 0 = root
+	if (perm_set(t_pw, t_gr) == -1) {
 		fprintf(stderr, "perm_set: %s\n", strerror(errno));
 		goto fail_end;
 	}
 
 	/* Execute the command */
-	if (execv(*c_argv, c_argv) == -1) // execvp searches in path
+	int err;
-		fprintf(stderr, "execv: %s\n", strerror(errno));
+	if (command)
+		err = execl(shell, shell, "-c", command, (char *)NULL);
+	else
+		err = execl(shell, shell, (char *)NULL);
+	if (err == -1)
+		fprintf(stderr, "execl: %s\n", strerror(errno));
 
 	/* Cleanup and return */
 	fail_end:
 	/* Free up the copied argv */
-	for (int i = 0; i < c_argc; i++)
+	free(command);
-		free(c_argv[i]);
-	free(c_argv);
 	return errno;
 }
 
@@ -161,60 +159,24 @@ static inline void usage (void)
 	// -c [file]: manually select config file
 	// something about environment
 	// something about non interactiveness
-	printf("usage: us [-u user] [-g group] command [args]\n");
+	printf("usage: us [-s] [-u user] [-g group] command [args]\n");
 }
 
-static int perm_set (const char	*user, const char *group)
+static int perm_set (struct passwd *pw, struct group *gr)
 {
-	if (!user)
-		user = "root";
-
-	struct passwd *pw;
-	struct group *gr;
-	uid_t uid;
-	gid_t gid;
-	long uid_l, gid_l;
-
-	errno = 0;
-	if (user[0] != '#') {
-		pw = getpwnam(user);
-	} else {
-		uid_l = strtol(&user[1], NULL, 10);
-		if (uid_l < 0 || errno) {
-			errno = errno ? errno : EINVAL;
-			return -1;
-		}
-		pw = getpwuid((uid_t)uid_l);
-	}
-
 	if (!pw) {
-		if (!errno)
 		errno = EINVAL;
 		return -1;
 	}
 
+	uid_t uid;
+	gid_t gid;
+
 	uid = pw->pw_uid;
 	gid = pw->pw_gid;
 
-	if (group) {
+	if (gr)
-		if (group[0] != '#') {
-			gr = getgrnam(group);
-		} else {
-			gid_l = strtol(&group[1], NULL, 10);
-			if (gid_l < 0 || errno) {
-				errno = errno ? errno : EINVAL;
-				return -1;
-			}
-			gr = getgrgid((gid_t)gid_l);
-		}
-
-		if (!gr) {
-			if (!errno)
-				errno = EINVAL;
-			return -1;
-		}
 		gid = gr->gr_gid;
-	}
 
 	/* Set permissions, setting group perms first because in the case of
 	 * dropping from higher permissions setting the uid first results in
@@ -272,3 +234,62 @@ static int authenticate (const char *uname)
 
 	return pam_end(pamh, pam_err);
 }
+
+static struct passwd* user_to_passwd (const char *user)
+{
+	if (!user) {
+		errno = EINVAL;
+		return NULL;
+	}
+
+	struct passwd* pw;
+	long uid_l;
+	errno = 0;
+	if (user[0] != '#') {
+		pw = getpwnam(user);
+	} else {
+		uid_l = strtol(&user[1], NULL, 10);
+		if (uid_l < 0 || errno) {
+			errno = errno ? errno : EINVAL;
+			return NULL;
+		}
+		pw = getpwuid((uid_t)uid_l);
+	}
+
+	if (!pw) {
+		if (!errno)
+			errno = EINVAL;
+		return NULL;
+	}
+
+	return pw;
+}
+
+static struct group* group_to_grp (const char *group)
+{
+	if (!group) {
+		errno = EINVAL;
+		return NULL;
+	}
+
+	struct group* gr;
+	long gid_l;
+	errno = 0;
+	if (group[0] != '#') {
+		gr = getgrnam(group);
+	} else {
+		gid_l = strtol(&group[1], NULL, 10);
+		if (gid_l < 0 || errno) {
+			errno = errno ? errno : EINVAL;
+			return NULL;
+		}
+		gr = getgrgid((gid_t)gid_l);
+	}
+
+	if (!gr) {
+		if (!errno)
+			errno = EINVAL;
+		return NULL;
+	}
+	return gr;
+}