Give the code around chroot(2)/chdir(2) a major overhaul by

separating its part around chroot(2) from that around initial
chdir(2).  This makes the below changes really easy.

Move seteuid(to user's uid) to before calling chdir(2).  There are
two goals to achieve by that.  First, NFS mounted home directories
with restrictive permissions become accessible (local superuser
can't access them if not mapped to uid 0 on the remote side
explicitly.)  Second, all the permissions to the home directory
pathname components become effective; previously a user could be
carried to any local directory despite its permissions since the
chdir(2) was done with euid 0.  This reduces possible impact from
FTP server misconfiguration, e.g., assigning a wrong home directory
to a user.

Implement the "/./" feature.  Now a guest or user subject to chrooting
may have "/./" in his login directory, which separates his chroot
directory from his home directory inside the chrooted environment.
This works for ftpchroot(5) as well.

PR:		bin/17843 bin/23944

libexec/ftpd/ftpchroot.5

@@ -66,9 +66,16 @@ A username is specified otherwise.
 The optional second field describes the directory for the user
 or each member of the group to be locked up in using
 .Xr chroot 2 .
+Be it omitted, the user's login directory will be used.
 If it is not an absolute pathname, then it will be relative
 to the user's login directory.
-Be this field omitted, the user's login directory will be used.
+If it contains the
+.Qq \&/./
+seprator,
+.Xr ftpd 8
+will treat its left-hand side as the name of the directory to do
+.Xr chroot 2
+to, and its right-hand side to change the current directory to afterwards.
 .Sh FILES
 .Bl -tag -width /etc/ftpchroot -compact
 .It Pa /etc/ftpchroot
@@ -86,12 +93,17 @@ webuser
 @hostee
 .Ed
 .Pp
-And this line will lock up the user
+And this line will tell
+.Xr ftpd 8
+to lock up the user
 .Qq joe
 in
-.Pa /var/spool/ftp :
+.Pa /var/spool/ftp
+and then to change the current directory to
+.Pa /joe ,
+which is relative to the session's new root:
 .Bd -literal -offset indent
-joe	/var/spool/ftp
+joe	/var/spool/ftp/./joe
 .Ed
 .Pp
 And finally the following line will lock up every user connecting

libexec/ftpd/ftpcmd.y

@@ -78,6 +78,7 @@ extern	union sockunion data_dest, his_addr;
 extern	int logged_in;
 extern	struct passwd *pw;
 extern	int guest;
+extern	char *homedir;
 extern 	int paranoid;
 extern	int logging;
 extern	int type;
@@ -535,10 +536,7 @@ cmd
 	| CWD check_login CRLF
 		{
 			if ($2) {
-				if (guest)
-					cwd("/");
-				else
-					cwd(pw->pw_dir);
+				cwd(homedir);
 			}
 		}
 	| CWD check_login SP pathname CRLF

libexec/ftpd/ftpd.8

@@ -401,6 +401,17 @@ The server performs a
 to the home directory of the
 .Dq ftp
 user.
+As a special case if the
+.Dq ftp
+user's home directory pathname contains the
+.Dq \&/./
+separator,
+.Nm
+uses its left-hand side as the name of the directory to do
+.Xr chroot 2
+to, and its right-hand side to change the current directory to afterwards.
+A typical example for this case would be
+.Pa /usr/local/ftp/./pub .
 In order that system security is not breached, it is recommended
 that the
 .Dq ftp

libexec/ftpd/ftpd.c

@@ -118,6 +118,7 @@ int	data;
 int	dataport;
 int	logged_in;
 struct	passwd *pw;
+char	*homedir;
 int	ftpdebug;
 int	timeout = 900;    /* timeout after 15 minutes of inactivity */
 int	maxtimeout = 7200;/* don't allow idle time to be set beyond 2 hours */
@@ -1349,6 +1350,7 @@ pass(char *passwd)
 #ifdef USE_PAM
 	int e;
 #endif
+	char *chrootdir;
 	char *residue = NULL;
 	char *xpasswd;
 
@@ -1471,50 +1473,79 @@ pass(char *passwd)
 		|| login_getcapbool(lc, "ftp-chroot", 0)
 #endif
 	;
-	if (guest) {
-		/*
-		 * We MUST do a chdir() after the chroot. Otherwise
-		 * the old current directory will be accessible as "."
-		 * outside the new root!
-		 */
-		if (chroot(pw->pw_dir) < 0 || chdir("/") < 0) {
-			reply(550, "Can't set guest privileges.");
-			goto bad;
-		}
-	} else if (dochroot) {
-		char *chrootdir = NULL;
-
-		if (residue &&
-		    (chrootdir = strtok(residue, " \t")) != NULL &&
-		    chrootdir[0] != '/') {
-			asprintf(&chrootdir, "%s/%s", pw->pw_dir, chrootdir);
-			if (chrootdir == NULL)
-				fatalerror("Ran out of memory.");
-			free(residue);
-			residue = chrootdir;
-		}
+	chrootdir = NULL;
+	/*
+	 * For a chrooted local user,
+	 * a) see whether ftpchroot(5) specifies a chroot directory,
+	 * b) extract the directory pathname from the line,
+	 * c) expand it to the absolute pathname if necessary.
+	 */
+	if (dochroot && residue &&
+	    (chrootdir = strtok(residue, " \t")) != NULL &&
+	    chrootdir[0] != '/') {
+		asprintf(&chrootdir, "%s/%s", pw->pw_dir, chrootdir);
 		if (chrootdir == NULL)
-			chrootdir = pw->pw_dir;
-		if (chroot(chrootdir) < 0 || chdir("/") < 0) {
+			fatalerror("Ran out of memory.");
+		
+	}
+	if (guest || dochroot) {
+		/*
+		 * If no chroot directory set yet, use the login directory.
+		 * Copy it so it can be modified while pw->pw_dir stays intact.
+		 */
+		if (chrootdir == NULL &&
+		    (chrootdir = strdup(pw->pw_dir)) == NULL)
+			fatalerror("Ran out of memory.");
+		/*
+		 * Check for the "/chroot/./home" syntax,
+		 * separate the chroot and home directory pathnames.
+		 */
+		if ((homedir = strstr(chrootdir, "/./")) != NULL) {
+			*(homedir++) = '\0';	/* wipe '/' */
+			homedir++;		/* skip '.' */
+			/* so chrootdir can be freed later */
+			if ((homedir = strdup(homedir)) == NULL)
+				fatalerror("Ran out of memory.");
+		} else {
+			/*
+			 * We MUST do a chdir() after the chroot. Otherwise
+			 * the old current directory will be accessible as "."
+			 * outside the new root!
+			 */
+			homedir = "/";
+		}
+		/*
+		 * Finally, do chroot()
+		 */
+		if (chroot(chrootdir) < 0) {
 			reply(550, "Can't change root.");
-			if (residue)
-				free(residue);
 			goto bad;
 		}
-		if (residue)
-			free(residue);
-	} else if (chdir(pw->pw_dir) < 0) {
-		if (chdir("/") < 0) {
-			reply(530, "User %s: can't change directory to %s.",
-			    pw->pw_name, pw->pw_dir);
-			goto bad;
-		} else
-			lreply(230, "No directory! Logging in with home=/");
-	}
+	} else	/* real user w/o chroot */
+		homedir = pw->pw_dir;
+	/*
+	 * Set euid *before* doing chdir() so
+	 * a) the user won't be carried to a directory that he couldn't reach
+	 *    on his own due to no permission to upper path components,
+	 * b) NFS mounted homedirs w/restrictive permissions will be accessible
+	 *    (uid 0 has no root power over NFS if not mapped explicitly.)
+	 */
 	if (seteuid((uid_t)pw->pw_uid) < 0) {
 		reply(550, "Can't set uid.");
 		goto bad;
 	}
+	if (chdir(homedir) < 0) {
+		if (guest || dochroot) {
+			reply(550, "Can't change to base directory.");
+			goto bad;
+		} else {
+			if (chdir("/") < 0) {
+				reply(550, "Root is inaccessible.");
+				goto bad;
+			}
+			lreply(230, "No directory! Logging in with home=/");
+		}
+	}
 
 	/*
 	 * Display a login message, if it exists.
@@ -1577,12 +1608,20 @@ pass(char *passwd)
 #ifdef	LOGIN_CAP
 	login_close(lc);
 #endif
+	if (chrootdir)
+		free(chrootdir);
+	if (residue)
+		free(residue);
 	return;
 bad:
 	/* Forget all about it... */
 #ifdef	LOGIN_CAP
 	login_close(lc);
 #endif
+	if (chrootdir)
+		free(chrootdir);
+	if (residue)
+		free(residue);
 	end_login();
 }