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

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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>
This commit is contained in:
Herton R. Krzesinski 2016-01-11 12:07:43 -02:00 committed by Greg Kroah-Hartman
parent 7dde55787b
commit 2831c89f42
1 changed files with 17 additions and 1 deletions
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18
drivers/tty/pty.c
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@ -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,