pty: make sure super_block is still valid in final /dev/tty close

Considering current pty code and multiple devpts instances, it's possible
to umount a devpts file system while a program still has /dev/tty opened
pointing to a previosuly closed pty pair in that instance. In the case all
ptmx and pts/N files are closed, umount can be done. If the program closes
/dev/tty after umount is done, devpts_kill_index will use now an invalid
super_block, which was already destroyed in the umount operation after
running ->kill_sb. This is another "use after free" type of issue, but now
related to the allocated super_block instance.

To avoid the problem (warning at ida_remove and potential crashes) for
this specific case, I added two functions in devpts which grabs additional
references to the super_block, which pty code now uses so it makes sure
the super block structure is still valid until pty shutdown is done.
I also moved the additional inode references to the same functions, which
also covered similar case with inode being freed before /dev/tty final
close/shutdown.

Signed-off-by: Herton R. Krzesinski <herton@redhat.com>
Cc: stable@vger.kernel.org # 2.6.29+
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/pty.c

@@ -688,7 +688,7 @@ static void pty_unix98_shutdown(struct tty_struct *tty)
 	else
 		ptmx_inode = tty->link->driver_data;
 	devpts_kill_index(ptmx_inode, tty->index);
-	iput(ptmx_inode); /* drop reference we acquired at ptmx_open */
+	devpts_del_ref(ptmx_inode);
 }
 
 static const struct tty_operations ptm_unix98_ops = {
@@ -785,9 +785,12 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 	 * still have /dev/tty opened pointing to the master/slave pair (ptmx
 	 * is closed/released before /dev/tty), we must make sure that the inode
 	 * is still valid when we call the final pty_unix98_shutdown, thus we
-	 * hold an additional reference to the ptmx inode
+	 * hold an additional reference to the ptmx inode. For the same /dev/tty
+	 * last close case, we also need to make sure the super_block isn't
+	 * destroyed (devpts instance unmounted), before /dev/tty is closed and
+	 * on its release devpts_kill_index is called.
 	 */
-	ihold(inode);
+	devpts_add_ref(inode);
 
 	tty_add_file(tty, filp);
 

fs/devpts/inode.c

@@ -575,6 +575,26 @@ void devpts_kill_index(struct inode *ptmx_inode, int idx)
 	mutex_unlock(&allocated_ptys_lock);
 }
 
+/*
+ * pty code needs to hold extra references in case of last /dev/tty close
+ */
+
+void devpts_add_ref(struct inode *ptmx_inode)
+{
+	struct super_block *sb = pts_sb_from_inode(ptmx_inode);
+
+	atomic_inc(&sb->s_active);
+	ihold(ptmx_inode);
+}
+
+void devpts_del_ref(struct inode *ptmx_inode)
+{
+	struct super_block *sb = pts_sb_from_inode(ptmx_inode);
+
+	iput(ptmx_inode);
+	deactivate_super(sb);
+}
+
 /**
  * devpts_pty_new -- create a new inode in /dev/pts/
  * @ptmx_inode: inode of the master

include/linux/devpts_fs.h

@@ -19,6 +19,8 @@
 
 int devpts_new_index(struct inode *ptmx_inode);
 void devpts_kill_index(struct inode *ptmx_inode, int idx);
+void devpts_add_ref(struct inode *ptmx_inode);
+void devpts_del_ref(struct inode *ptmx_inode);
 /* mknod in devpts */
 struct inode *devpts_pty_new(struct inode *ptmx_inode, dev_t device, int index,
 		void *priv);
@@ -32,6 +34,8 @@ void devpts_pty_kill(struct inode *inode);
 /* Dummy stubs in the no-pty case */
 static inline int devpts_new_index(struct inode *ptmx_inode) { return -EINVAL; }
 static inline void devpts_kill_index(struct inode *ptmx_inode, int idx) { }
+static inline void devpts_add_ref(struct inode *ptmx_inode) { }
+static inline void devpts_del_ref(struct inode *ptmx_inode) { }
 static inline struct inode *devpts_pty_new(struct inode *ptmx_inode,
 		dev_t device, int index, void *priv)
 {