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 <her...@redhat.com>
Cc: <sta...@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
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