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

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.

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>

1 files changed, 17 insertions, 1 deletions

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index b3110040164a..3b5cde833ee5 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -681,7 +681,14 @@ static void pty_unix98_remove(struct tty_driver *driver, struct tty_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 = {
@@ -773,6 +780,15 @@ static int ptmx_open(struct inode *inode, struct file *filp)
         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,