linux-kernel - [PATCH 1/2 v2] pty: fix possible use after free of tty->driver

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Message-Id: <1452521264-21766-2-git-send-email-herton@redhat.com>
Date:	Mon, 11 Jan 2016 12:07:43 -0200
From:	"Herton R. Krzesinski" <herton@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Peter Hurley <peter@...leysoftware.com>,
	Alan Cox <alan@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Josh Triplett <josh@...htriplett.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	David Howells <dhowells@...hat.com>
Subject: [PATCH 1/2 v2] 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@...hat.com>
Cc: <stable@...r.kernel.org> # 2.6.29+
---
 drivers/tty/pty.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

v2: fixed patch changelog

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index a45660f..96016e5 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,
-- 
2.4.3