Fix jail rc.d script privilege escalation via symlink attack against

/var/log/console.log and mount points.

Security:	FreeBSD-SA-07:01.jail
Approved by:	re (bmah)

etc/rc.d/jail

@@ -67,6 +67,8 @@ init_variables()
 	[ -z "${_fstab}" ] && _fstab="/etc/fstab.${_j}"
 	eval _flags=\"\${jail_${_j}_flags:-${jail_flags}}\"
 	[ -z "${_flags}" ] && _flags="-l -U root"
+	eval _consolelog=\"\${jail_${_j}_consolelog:-${jail_consolelog}}\"
+	[ -z "${_consolelog}" ] && _consolelog="/var/log/jail_${_j}_console.log"
 
 	# Debugging aid
 	#
@@ -86,6 +88,7 @@ init_variables()
 	debug "$_j exec start: $_exec_start"
 	debug "$_j exec stop: $_exec_stop"
 	debug "$_j flags: $_flags"
+	debug "$_j consolelog: $_consolelog"
 
 	if [ -z "${_hostname}" ]; then
 		err 3 "$name: No hostname has been defined for ${_j}"
@@ -126,34 +129,122 @@ set_sysctl()
 	fi
 }
 
+# is_current_mountpoint()
+#	Is the directory mount point for a currently mounted file
+#	system?
+#
+is_current_mountpoint()
+{
+	local _dir _dir2
+
+	_dir=$1
+
+	_dir=`echo $_dir | sed -Ee 's#//+#/#g' -e 's#/$##'`
+	[ ! -d "${_dir}" ] && return 1
+	_dir2=`df ${_dir} | tail +2 | awk '{ print $6 }'`
+	[ "${_dir}" = "${_dir2}" ]
+	return $?
+}
+
+# is_symlinked_mountpoint()
+#	Is a mount point, or any of its parent directories, a symlink?
+#
+is_symlinked_mountpoint()
+{
+	local _dir
+
+	_dir=$1
+
+	[ -L "$_dir" ] && return 0
+	[ "$_dir" = "/" ] && return 1
+	is_symlinked_mountpoint `dirname $_dir`
+	return $?
+}
+
+# secure_umount
+#	Try to unmount a mount point without being vulnerable to
+#	symlink attacks.
+#
+secure_umount()
+{
+	local _dir
+
+	_dir=$1
+
+	if is_current_mountpoint ${_dir}; then
+		umount -f ${_dir} >/dev/null 2>&1
+	else
+		debug "Nothing mounted on ${_dir} - not unmounting"
+	fi
+}
+
+
 # jail_umount_fs
 #	This function unmounts certain special filesystems in the
 #	currently selected jail. The caller must call the init_variables()
 #	routine before calling this one.
 #
 jail_umount_fs()
 {
+	local _device _mountpt _rest
+
 	if checkyesno _fdescfs; then
 		if [ -d "${_fdescdir}" ] ; then
-			umount -f ${_fdescdir} >/dev/null 2>&1
+			secure_umount ${_fdescdir}
 		fi
 	fi
 	if checkyesno _devfs; then
 		if [ -d "${_devdir}" ] ; then
-			umount -f ${_devdir} >/dev/null 2>&1
+			secure_umount ${_devdir}
 		fi
 	fi
 	if checkyesno _procfs; then
 		if [ -d "${_procdir}" ] ; then
-			umount -f ${_procdir} >/dev/null 2>&1
+			secure_umount ${_procdir}
 		fi
 	fi
 	if checkyesno _mount; then
 		[ -f "${_fstab}" ] || warn "${_fstab} does not exist"
-		umount -a -F "${_fstab}" >/dev/null 2>&1
+		tail -r ${_fstab} | while read _device _mountpt _rest; do
+			case ":${_device}" in
+			:#* | :)
+				continue
+				;;
+			esac
+			secure_umount ${_mountpt}
+		done
 	fi
 }
 
+# jail_mount_fstab()
+#	Mount file systems from a per jail fstab while trying to
+#	secure against symlink attacks at the mount points.
+#
+#	If we are certain we cannot secure against symlink attacks we
+#	do not mount all of the file systems (since we cannot just not
+#	mount the file system with the problematic mount point).
+#
+#	The caller must call the init_variables() routine before
+#	calling this one.
+#
+jail_mount_fstab()
+{
+	local _device _mountpt _rest
+
+	while read _device _mountpt _rest; do
+		case ":${_device}" in
+		:#* | :)
+			continue
+			;;
+		esac
+		if is_symlinked_mountpoint ${_mountpt}; then
+			warn "${_mountpt} has symlink as parent - not mounting from ${_fstab}"
+			return
+		fi
+	done <${_fstab}
+	mount -a -F "${_fstab}"
+}
+
 jail_start()
 {
 	echo -n 'Configuring jails:'
@@ -183,12 +274,16 @@ jail_start()
 			if [ ! -f "${_fstab}" ]; then
 				err 3 "$name: ${_fstab} does not exist"
 			fi
-			mount -a -F "${_fstab}"
+			jail_mount_fstab
 		fi
 		if checkyesno _devfs; then
 			# If devfs is already mounted here, skip it.
 			df -t devfs "${_devdir}" >/dev/null
 			if [ $? -ne 0 ]; then
+				if is_symlinked_mountpoint ${_devdir}; then
+					warn "${_devdir} has symlink as parent - not starting jail ${_jail}"
+					continue
+				fi
 				info "Mounting devfs on ${_devdir}"
 				devfs_mount_jail "${_devdir}" ${_ruleset}
 				# Transitional symlink for old binaries
@@ -209,13 +304,21 @@ jail_start()
 			#	cd "$__pwd"
 		fi
 		if checkyesno _fdescfs; then
-			info "Mounting fdescfs on ${_fdescdir}"
-			mount -t fdescfs fdesc "${_fdescdir}"
+			if is_symlinked_mountpoint ${_fdescdir}; then
+				warn "${_fdescdir} has symlink as parent, not mounting"
+			else
+				info "Mounting fdescfs on ${_fdescdir}"
+				mount -t fdescfs fdesc "${_fdescdir}"
+			fi
 		fi
 		if checkyesno _procfs; then
-			info "Mounting procfs onto ${_procdir}"
-			if [ -d "${_procdir}" ] ; then
-				mount -t procfs proc "${_procdir}"
+			if is_symlinked_mountpoint ${_procdir}; then
+				warn "${_procdir} has symlink as parent, not mounting"
+			else
+				info "Mounting procfs onto ${_procdir}"
+				if [ -d "${_procdir}" ] ; then
+					mount -t procfs proc "${_procdir}"
+				fi
 			fi
 		fi
 		_tmp_jail=${_tmp_dir}/jail.$$
@@ -224,7 +327,7 @@ jail_start()
 			if [ "$?" -eq 0 ] ; then
 				echo -n " $_hostname"
 				_jail_id=$(head -1 ${_tmp_jail})
-				tail +2 ${_tmp_jail} >${_rootdir}/var/log/console.log
+				tail +2 ${_tmp_jail} >${_consolelog}
 				echo ${_jail_id} > /var/run/jail_${_jail}.id
 			else
 				jail_umount_fs
@@ -251,7 +354,7 @@ jail_stop()
 				init_variables $_jail
 				if [ -n "${_exec_stop}" ]; then
 					eval env -i /usr/sbin/jexec ${_jail_id} ${_exec_stop} \
-						>> ${_rootdir}/var/log/console.log 2>&1
+						>> ${_consolelog} 2>&1
 				fi
 				killall -j ${_jail_id} -TERM > /dev/null 2>&1
 				sleep 1