[PATCH] pty: fix use after free of tty->driver_data

["Herton R. Krzesinski" <herton@redhat.com>]

pty_unix98_shutdown allows a potential use after free of inode from
slave tty->driver_data: if final pty close is called with slave
tty_struct, and inode was released already by devpts_pty_kill at
pty_close, pty_unix98_shutdown will access stale data. If the evicted
inode is quickly reused again as another inode instance, this can
potentially break in the case the inode is on a different devpts
instance than the default devpts mount, not to mention a possible ops
if the inode_cache slab is destroyed and reused as something else.

Also there is an evident problem in case the last close is from a
opened "/dev/tty" which points to the master/slave pty: since in this
case any of the tty->driver_data can be stale, due to all references/
files being closed before (files related to ptmx/pts inodes set at
tty->driver_data), we have the possibility of referencing an already
freed inode.

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

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index a45660f..90743b0 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 rare case all references to ptmx inode are dropped (files
+	 * closed), and we still have a device opened pointing to the
+	 * master/slave pair (eg., "/dev/tty" opened), we must make sure that
+	 * the inode is still valid when we call the final pty_unix98_shutdown:
+	 * thus we must hold an additional reference to the ptmx inode here
+	 */
+	ihold(inode);
+
 	tty_add_file(tty, filp);
 
 	slave_inode = devpts_pty_new(inode,
-- 
2.4.3