write(1): Capsicumify

Enter Capsicum capability sandbox pretty early in this setuid program.

Some minor modifications were needed to cache directory fds and use
relative lookups.

Rights restriction of the stdio descriptors is unfortunately pretty messy
because we need an ioctl capability not present in the current libcapsicum
helpers (FIODGNAME).

Reviewed by:	ed
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D7999

1 files changed, 81 insertions, 33 deletions

diff --git a/usr.bin/write/write.c b/usr.bin/write/write.c
index 2008fd2d80fae..0bf08b4b2b197 100644
--- a/usr.bin/write/write.c
+++ b/usr.bin/write/write.c
@@ -46,12 +46,16 @@ static char sccsid[] = "@(#)write.c	8.1 (Berkeley) 6/6/93";
 __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
+#include <sys/capsicum.h>
+#include <sys/filio.h>
 #include <sys/signal.h>
 #include <sys/stat.h>
-#include <sys/file.h>
 #include <sys/time.h>
+
+#include <capsicum_helpers.h>
 #include <ctype.h>
 #include <err.h>
+#include <errno.h>
 #include <locale.h>
 #include <paths.h>
 #include <pwd.h>
@@ -64,23 +68,76 @@ __FBSDID("$FreeBSD$");
 #include <wctype.h>
 
 void done(int);
-void do_write(char *, char *, uid_t);
+void do_write(int, char *, char *, const char *);
 static void usage(void);
-int term_chk(char *, int *, time_t *, int);
+int term_chk(int, char *, int *, time_t *, int);
 void wr_fputs(wchar_t *s);
-void search_utmp(char *, char *, char *, uid_t);
+void search_utmp(int, char *, char *, char *, uid_t);
 int utmp_chk(char *, char *);
 
 int
 main(int argc, char **argv)
 {
+	unsigned long cmds[] = { TIOCGETA, TIOCGWINSZ, FIODGNAME };
+	cap_rights_t rights;
+	struct passwd *pwd;
 	time_t atime;
 	uid_t myuid;
 	int msgsok, myttyfd;
 	char tty[MAXPATHLEN], *mytty;
+	const char *login;
+	int devfd;
 
 	(void)setlocale(LC_CTYPE, "");
 
+	devfd = open(_PATH_DEV, O_RDONLY);
+	if (devfd < 0)
+		err(1, "open(/dev)");
+	cap_rights_init(&rights, CAP_FCNTL, CAP_FSTAT, CAP_IOCTL, CAP_LOOKUP,
+	    CAP_PWRITE);
+	if (cap_rights_limit(devfd, &rights) < 0 && errno != ENOSYS)
+		err(1, "can't limit devfd rights");
+
+	/*
+	 * Can't use capsicum helpers here because we need the additional
+	 * FIODGNAME ioctl.
+	 */
+	cap_rights_init(&rights, CAP_FCNTL, CAP_FSTAT, CAP_IOCTL, CAP_READ,
+	    CAP_WRITE);
+	if ((cap_rights_limit(STDIN_FILENO, &rights) < 0 && errno != ENOSYS) ||
+	    (cap_rights_limit(STDOUT_FILENO, &rights) < 0 && errno != ENOSYS) ||
+	    (cap_rights_limit(STDERR_FILENO, &rights) < 0 && errno != ENOSYS) ||
+	    (cap_ioctls_limit(STDIN_FILENO, cmds, nitems(cmds)) < 0 && errno != ENOSYS) ||
+	    (cap_ioctls_limit(STDOUT_FILENO, cmds, nitems(cmds)) < 0 && errno != ENOSYS) ||
+	    (cap_ioctls_limit(STDERR_FILENO, cmds, nitems(cmds)) < 0 && errno != ENOSYS) ||
+	    (cap_fcntls_limit(STDIN_FILENO, CAP_FCNTL_GETFL) < 0 && errno != ENOSYS) ||
+	    (cap_fcntls_limit(STDOUT_FILENO, CAP_FCNTL_GETFL) < 0 && errno != ENOSYS) ||
+	    (cap_fcntls_limit(STDERR_FILENO, CAP_FCNTL_GETFL) < 0 && errno != ENOSYS))
+		err(1, "can't limit stdio rights");
+
+	caph_cache_catpages();
+	caph_cache_tzdata();
+
+	/*
+	 * Cache UTX database fds.
+	 */
+	setutxent();
+
+	/*
+	 * Determine our login name before we reopen() stdout
+	 * and before entering capability sandbox.
+	 */
+	myuid = getuid();
+	if ((login = getlogin()) == NULL) {
+		if ((pwd = getpwuid(myuid)))
+			login = pwd->pw_name;
+		else
+			login = "???";
+	}
+
+	if (cap_enter() < 0 && errno != ENOSYS)
+		err(1, "cap_enter");
+
 	while (getopt(argc, argv, "") != -1)
 		usage();
 	argc -= optind;
@@ -99,29 +156,27 @@ main(int argc, char **argv)
 		errx(1, "can't find your tty's name");
 	if (!strncmp(mytty, _PATH_DEV, strlen(_PATH_DEV)))
 		mytty += strlen(_PATH_DEV);
-	if (term_chk(mytty, &msgsok, &atime, 1))
+	if (term_chk(devfd, mytty, &msgsok, &atime, 1))
 		exit(1);
 	if (!msgsok)
 		errx(1, "you have write permission turned off");
 
-	myuid = getuid();
-
 	/* check args */
 	switch (argc) {
 	case 1:
-		search_utmp(argv[0], tty, mytty, myuid);
-		do_write(tty, mytty, myuid);
+		search_utmp(devfd, argv[0], tty, mytty, myuid);
+		do_write(devfd, tty, mytty, login);
 		break;
 	case 2:
 		if (!strncmp(argv[1], _PATH_DEV, strlen(_PATH_DEV)))
 			argv[1] += strlen(_PATH_DEV);
 		if (utmp_chk(argv[0], argv[1]))
 			errx(1, "%s is not logged in on %s", argv[0], argv[1]);
-		if (term_chk(argv[1], &msgsok, &atime, 1))
+		if (term_chk(devfd, argv[1], &msgsok, &atime, 1))
 			exit(1);
 		if (myuid && !msgsok)
 			errx(1, "%s has messages disabled on %s", argv[0], argv[1]);
-		do_write(argv[1], mytty, myuid);
+		do_write(devfd, argv[1], mytty, login);
 		break;
 	default:
 		usage();
@@ -170,7 +225,7 @@ utmp_chk(char *user, char *tty)
  * writing from, unless that's the only terminal with messages enabled.
  */
 void
-search_utmp(char *user, char *tty, char *mytty, uid_t myuid)
+search_utmp(int devfd, char *user, char *tty, char *mytty, uid_t myuid)
 {
 	struct utmpx *u;
 	time_t bestatime, atime;
@@ -185,7 +240,7 @@ search_utmp(char *user, char *tty, char *mytty, uid_t myuid)
 		if (u->ut_type == USER_PROCESS &&
 		    strcmp(user, u->ut_user) == 0) {
 			++nloggedttys;
-			if (term_chk(u->ut_line, &msgsok, &atime, 0))
+			if (term_chk(devfd, u->ut_line, &msgsok, &atime, 0))
 				continue;	/* bad term? skip */
 			if (myuid && !msgsok)
 				continue;	/* skip ttys with msgs off */
@@ -219,15 +274,13 @@ search_utmp(char *user, char *tty, char *mytty, uid_t myuid)
  *     and the access time
  */
 int
-term_chk(char *tty, int *msgsokP, time_t *atimeP, int showerror)
+term_chk(int devfd, char *tty, int *msgsokP, time_t *atimeP, int showerror)
 {
 	struct stat s;
-	char path[MAXPATHLEN];
 
-	(void)snprintf(path, sizeof(path), "%s%s", _PATH_DEV, tty);
-	if (stat(path, &s) < 0) {
+	if (fstatat(devfd, tty, &s, 0) < 0) {
 		if (showerror)
-			warn("%s", path);
+			warn("%s%s", _PATH_DEV, tty);
 		return(1);
 	}
 	*msgsokP = (s.st_mode & (S_IWRITE >> 3)) != 0;	/* group write bit */
@@ -239,26 +292,21 @@ term_chk(char *tty, int *msgsokP, time_t *atimeP, int showerror)
  * do_write - actually make the connection
  */
 void
-do_write(char *tty, char *mytty, uid_t myuid)
+do_write(int devfd, char *tty, char *mytty, const char *login)
 {
-	const char *login;
 	char *nows;
-	struct passwd *pwd;
 	time_t now;
-	char path[MAXPATHLEN], host[MAXHOSTNAMELEN];
+	char host[MAXHOSTNAMELEN];
 	wchar_t line[512];
+	int fd;
 
-	/* Determine our login name before we reopen() stdout */
-	if ((login = getlogin()) == NULL) {
-		if ((pwd = getpwuid(myuid)))
-			login = pwd->pw_name;
-		else
-			login = "???";
-	}
-
-	(void)snprintf(path, sizeof(path), "%s%s", _PATH_DEV, tty);
-	if ((freopen(path, "w", stdout)) == NULL)
-		err(1, "%s", path);
+	fd = openat(devfd, tty, O_WRONLY);
+	if (fd < 0)
+		err(1, "openat(%s%s)", _PATH_DEV, tty);
+	fclose(stdout);
+	stdout = fdopen(fd, "w");
+	if (stdout == NULL)
+		err(1, "%s%s", _PATH_DEV, tty);
 
 	(void)signal(SIGINT, done);
 	(void)signal(SIGHUP, done);