From e63109a763e75f294582f2af6cb10f466a8a4e81 Mon Sep 17 00:00:00 2001
From: Alessandro Mauri <alemauri001@gmail.com>
Date: Tue, 13 Jul 2021 13:25:34 +0200
Subject: [PATCH] improvements and fixes

---
 us.c | 52 +++++++++++++++++++++++++++++++---------------------
 1 file changed, 31 insertions(+), 21 deletions(-)

diff --git a/us.c b/us.c
index 97dfba2..4048275 100644
--- a/us.c
+++ b/us.c
@@ -38,6 +38,7 @@
 #include <termios.h>
 #include <stdarg.h>
 #include <ctype.h>
+#include <signal.h>
 
 #if !defined(_XOPEN_CRYPT) || _XOPEN_CRYPT == -1
 #include <crypt.h>
@@ -48,7 +49,6 @@
 #endif
 
 #define MAX_HASH 1024
-#define CONF_FILE "/etc/us.conf"
 #define CONF_LINE_MAX 1024
 #define GROUPS_MAX 256
 #define FLAG_PERSIST 0x1
@@ -81,20 +81,19 @@ static struct group* group_to_grp(const char *);
 static int get_config(struct config **, int *);
 
 extern char **environ;
+char *config_file = "/etc/us.conf";
 
 int main(int argc, char *argv[])
 {
-	// TODO: modify signal handlers to ignore sigchld to not make zombies
 	char *t_usr = "root", *t_grp = NULL;
 	struct passwd *t_pw;
 	struct group *t_gr;
 	int opt, err;
-	int shellflag = 0, envflag = 0;
-	while ((opt = getopt(argc, argv, "A:u:g:C:se")) != -1) {
+	int shellflag = 0, envflag = 0, askpass = 0;
+	while ((opt = getopt(argc, argv, "Au:g:C:se")) != -1) {
 		switch (opt) {
 		case 'A':
-			printf("-A is not yet implemented\n");
-			exit(EXIT_FAILURE);
+			askpass = 1;
 			break;
 		case 'u':
 			t_usr = optarg;
@@ -103,8 +102,7 @@ int main(int argc, char *argv[])
 			t_grp = optarg;
 			break;
 		case 'C':
-			printf("-C is not yet implemented\n");
-			exit(EXIT_FAILURE);
+			config_file = optarg;
 			break;
 		case 's':
 			shellflag = 1;
@@ -139,6 +137,13 @@ int main(int argc, char *argv[])
 		die("user_to_passwd:");
 	t_gr = group_to_grp(t_grp);
 
+	/* Don't have to wait for children */
+	struct sigaction sa = {0};
+	sa.sa_handler = SIG_DFL;
+	sa.sa_flags = SA_NOCLDWAIT;
+	if (sigaction(SIGCHLD, &sa, NULL) == -1)
+		die("sigaction:");
+
 	/* From now on most actions require root */
 	if (setuid(0) == -1)
 		die("setuid:");
@@ -210,7 +215,7 @@ int main(int argc, char *argv[])
 
 	/* Authenticate, we will be root from now on */
 	if (!(conf_flags & FLAG_NOPASS))
-		if (authenticate(my_pw->pw_uid, 0))
+		if (authenticate(my_pw->pw_uid, askpass))
 			exit(EXIT_FAILURE);
 
 	/* Get target user's shell */
@@ -323,7 +328,7 @@ static inline void usage(void)
 	// -c [file]: manually select config file
 	// something about environment
 	// something about non interactiveness
-	printf("usage: us [-se] [-u user] [-g group] command [args]\n");
+	printf("usage: us [-seA] [-u user] [-g group] [-C config] command [args]\n");
 }
 
 static int perm_set(struct passwd *pw, struct group *gr)
@@ -388,7 +393,6 @@ static int authenticate(uid_t uid, char ask)
 		endspent();
 		ulckpwdf();
 #elif defined(__OpenBSD__)
-		// TODO: openbsd has getpwuid_passwd
 		struct passwd *op = getpwuid_shadow(uid);
 		if (!op)
 			die("getpwuid_shadow:");
@@ -407,7 +411,7 @@ static int authenticate(uid_t uid, char ask)
 	char pass[1024] = {0};
 	struct termios tio_before, tio_pass;
 	if (ask && askpass) {
-		pid_t pid;
+		pid_t pid, parent = getpid();
 		int pipefd[2];
 		if (pipe(pipefd) == -1)
 			die("pipe:");
@@ -416,23 +420,28 @@ static int authenticate(uid_t uid, char ask)
 			die("fork:");
 			break;
 		case 0:
+			/* we are still root, drop off permissions before
+			 * disasters happen */
+			if (setuid(uid) == -1) {
+				kill(parent, SIGTERM);
+				die("askpass: setuid:");
+			}
 			if (dup2(pipefd[1], STDOUT_FILENO) == -1) {
-				// TODO: send signal to parent dup failed
-				die("dup2:");
+				kill(parent, SIGTERM);
+				die("askpass: dup2:");
 			}
 			close(pipefd[0]);
-			execl(askpass, "", (char *)NULL);
-			die("execl:");
+			execl("/bin/sh", "sh", "-c", askpass, (char *)NULL);
+			kill(parent, SIGTERM);
+			die("askpass: execl:");
 			break;
 		default:
 			fd = pipefd[0];
 			close(pipefd[1]);
 			break;
 		}
-	} else if (ask) {
-		die("askpass: no valid askpass specified");
 	} else {
-		printf("Password: ");
+		printf("Password (%s): ", pw->pw_name);
 		fflush(stdout);
 		if (tcgetattr(tty_fd, &tio_before) == -1)
 			die("tcgetattr:");
@@ -459,7 +468,7 @@ static int authenticate(uid_t uid, char ask)
 	if (pass[l-1] == '\n')
 		pass[--l] = '\0';
 
-	if (ask) {
+	if (ask && askpass) {
 		close(fd);
 	} else {
 		if (tcsetattr(tty_fd, TCSANOW, &tio_before) == -1)
@@ -467,6 +476,7 @@ static int authenticate(uid_t uid, char ask)
 	}
 
 	char *enc = crypt(pass, hash);
+	/* clean pass from memory */
 	memset(pass, 0, 1024);
 	if (strncmp(hash, enc, 1024)) {
 		printf("Authentication failure\n");
@@ -591,7 +601,7 @@ static int get_config(struct config **conf, int *num)
 {
 	if (!conf || !num)
 		return -1;
-	FILE *fp = fopen(CONF_FILE, "r");
+	FILE *fp = fopen(config_file, "r");
 	if (!fp)
 		die("fopen:");
 	struct stat st;