Patch "pty: fix possible use after free of tty->driver_data" has been added to the 3.14-stable tree

This is a note to let you know that I've just added the patch titled

    pty: fix possible use after free of tty->driver_data

to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
pty-fix-possible-use-after-free-of-tty-driver_data.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Herton R. Krzesinski" <[email protected]>
Date: Mon, 11 Jan 2016 12:07:43 -0200
Subject: pty: fix possible use after free of tty->driver_data

From: Herton R. Krzesinski <[email protected]>

commit 2831c89f42dcde440cfdccb9fee9f42d54bbc1ef upstream.

This change fixes a bug for a corner case where we have the the last release from a pty master/slave coming from a previously opened /dev/tty file. When this happens, the tty->driver_data can be stale, due to all ptmx or pts/N files having already been closed before (and thus the inode related to these files, which tty->driver_data points to, being already freed/destroyed).

The fix here is to keep a reference on the opened master ptmx inode. We maintain the inode referenced until the final pty_unix98_shutdown, and only pass this inode to devpts_kill_index.

---
 drivers/tty/pty.c |   18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

@@ -617,7 +617,14 @@ static void pty_unix98_remove(struct tty
 /* this is called once with whichever end is closed last */
 static void pty_unix98_shutdown(struct tty_struct *tty)
 {
-	devpts_kill_index(tty->driver_data, tty->index);
+	struct inode *ptmx_inode;
+
+	if (tty->driver->subtype == PTY_TYPE_MASTER)
+		ptmx_inode = tty->driver_data;
+	else
+		ptmx_inode = tty->link->driver_data;
+	devpts_kill_index(ptmx_inode, tty->index);
+	iput(ptmx_inode); /* drop reference we acquired at ptmx_open */
 }
 
 static const struct tty_operations ptm_unix98_ops = {

@@ -708,6 +715,15 @@ static int ptmx_open(struct inode *inode
 	set_bit(TTY_PTY_LOCK, &tty->flags); /* LOCK THE SLAVE */
 	tty->driver_data = inode;
 
+	/*
+	 * In the case where all references to ptmx inode are dropped and we
+	 * 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
+	 */
+	ihold(inode);
+
 	tty_add_file(tty, filp);
 
 	slave_inode = devpts_pty_new(inode,

Patches currently in stable-queue which might be from [email protected] are

queue-3.14/pty-make-sure-super_block-is-still-valid-in-final-dev-tty-close.patch queue-3.14/pty-fix-possible-use-after-free-of-tty-driver_data.patch