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

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.

Signed-off-by: Herton R. Krzesinski <herton@redhat.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 7cb36813aac2bbded76575f36620d4ec510b1a98..a251351236fc5842251fdb694f78e961f31ea0a0 100644 (file)
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -623,7 +623,14 @@ 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 = {
@@ -714,6 +721,15 @@ 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,