SELinux: properly handle empty tty_files list

SELinux has wrongly (since 2004) had an incorrect test for an empty
tty->tty_files list.  With an empty list selinux would be pointing to part
of the tty struct itself and would then proceed to dereference that value
and again dereference that result.  An F10 change to plymouth on a ppc64
system is actually currently triggering this bug.  This patch uses
list_empty() to handle empty lists rather than looking at a meaningless
location.

[note, this fixes the oops reported in
https://bugzilla.redhat.com/show_bug.cgi?id=469079]
Signed-off-by: NEric Paris <eparis@redhat.com>
Signed-off-by: NJames Morris <jmorris@namei.org>

security/selinux/hooks.c

@@ -2126,14 +2126,16 @@ static inline void flush_unauthorized_files(struct files_struct *files)
 	tty = get_current_tty();
 	if (tty) {
 		file_list_lock();
-		file = list_entry(tty->tty_files.next, typeof(*file), f_u.fu_list);
-		if (file) {
+		if (!list_empty(&tty->tty_files)) {
+			struct inode *inode;
+
 			/* Revalidate access to controlling tty.
 			   Use inode_has_perm on the tty inode directly rather
 			   than using file_has_perm, as this particular open
 			   file may belong to another process and we are only
 			   interested in the inode-based check here. */
-			struct inode *inode = file->f_path.dentry->d_inode;
+			file = list_first_entry(&tty->tty_files, struct file, f_u.fu_list);
+			inode = file->f_path.dentry->d_inode;
 			if (inode_has_perm(current, inode,
 					   FILE__READ | FILE__WRITE, NULL)) {
 				drop_tty = 1;