pam authentication

us now attempts to verify the authentication using PAM, after three failed
attempts it stops execution

makefile

@@ -1,5 +1,6 @@
 CC ?= gcc
 CFLAGS = -Wall -pedantic --std=c99 -O2
+LDFLAGS = -lpam -lpam_misc
 PREFIX = /usr/local
 MANPREFIX = ${PREFIX}/share/man
 

us.c

@@ -6,9 +6,15 @@
 #include <errno.h>
 #include <string.h>
 #include <sys/types.h>
+#include <pwd.h>
+#include <security/pam_appl.h>
+#include <security/pam_misc.h>
 
 static void usage (void);
-static int perm_set (uid_t);
+static int perm_set (uid_t, gid_t);
+
+// FIXME: misc_conv is a separate library, should stick to plain PAM
+static struct pam_conv conv = {misc_conv, NULL};
 
 int main (int argc, char *argv[])
 {
@@ -18,6 +24,42 @@ int main (int argc, char *argv[])
 	}
 
 	uid_t ruid = getuid();
+	gid_t rgid = getgid();
+	struct passwd *pw = getpwuid(ruid);
+	if (!pw) {
+		fprintf(stderr, "getpwid: %s\n", strerror(errno));
+		exit(errno);
+	}
+	const char *uname = pw->pw_name;
+
+	pam_handle_t *pamh;
+	int pam_err, count = 0;
+
+	// TODO: Add arguments
+
+	// TODO: add PAM authentication
+	pam_err = pam_start("User Switcher", uname, &conv, &pamh);
+	if (pam_err != PAM_SUCCESS) {
+		fprintf(stderr, "pam_start: %s\n", pam_strerror(pamh, pam_err));
+		exit(pam_err);
+	}
+
+	do {
+		pam_err = pam_authenticate(pamh, 0);
+		if (pam_err != PAM_SUCCESS)
+			printf("Auth failed: %s\n", pam_strerror(pamh, pam_err));
+		count++;
+	} while (pam_err != PAM_SUCCESS && count < 4);
+	if (pam_err != PAM_SUCCESS) {
+		fprintf(stderr, "better luck next time\n");
+		pam_end(pamh, pam_err);
+		exit(pam_err);
+	}
+	// FIXME: check again for the validity of the login for more security
+	// as in: https://docs.oracle.com/cd/E19120-01/open.solaris/819-2145/pam-20/index.html
+
+	pam_end(pamh, pam_err);
+	// TODO: clean up env
 
 	// Copy argv and argc
 	int c_argc = argc - 1;
@@ -30,20 +72,31 @@ int main (int argc, char *argv[])
 		c_argv[i] = strdup(argv[i+1]);
 	c_argc[c_argv] = NULL;
 
-	if (perm_set(0) == -1) { // 0 = root
+	errno = 0;
+	/* Set permissions */
+	if (perm_set(0, 0) == -1) { // 0 = root
 		fprintf(stderr, "perm_set: %s\n", strerror(errno));
 		goto fail_end;
 	}
+
+	/* Execute the command */
 	if (execvp(*c_argv, c_argv) == -1) // execvp searches in path
 		fprintf(stderr, "execv: %s\n", strerror(errno));
-	// if exec fails reset the permissions
-	if (perm_set(ruid) == -1) { // 0 = root
+
+	/* If exec fails reset the permissions */
+	if (perm_set(ruid, rgid) == -1) { // 0 = root
 		fprintf(stderr, "perm_set: %s\n", strerror(errno));
 		goto fail_end;
 	}
 
+	/* Cleanup and return */
 	fail_end:
-	return 0;
+
+	/* Free up the copied argv */
+	for (int i = 0; i < c_argc; i++)
+		free(c_argv[i]);
+	free(c_argv);
+	return errno;
 }
 
 static inline void usage (void)
@@ -51,7 +104,21 @@ static inline void usage (void)
 	printf("usage: us [command]\n");
 }
 
-static inline int perm_set (uid_t id)
+static int perm_set (uid_t uid, gid_t gid)
 {
-	return seteuid(id);	
+	// FIXME: This doesn't change the uid and gid, it only changes the
+	// e(ffective)uid and egid keeping the original gid, uid and groups list
+	// to make it work like sudo, the uid gid and groups list must be all
+	// set to 0 (or the corresponding value for other users).
+	// An option could be to add an argument to keep the groups list,
+	// another to change only the euid/egid.
+	// For more information run:
+	// $ sudo id
+	// and check the output
+	int err = 0;
+	if ((err = seteuid(uid)) == -1)
+		return err;
+	else if ((err = setegid(gid)) == -1)
+		return err;
+	return err;
 }