tty: n_gsm: fix deadlock in gsmtty_open()

In the current implementation the user may open a virtual tty which then
could fail to establish the underlying DLCI. The function gsmtty_open()
gets stuck in tty_port_block_til_ready() while waiting for a carrier rise.
This happens if the remote side fails to acknowledge the link establishment
request in time or completely. At some point gsm_dlci_close() is called
to abort the link establishment attempt. The function tries to inform the
associated virtual tty by performing a hangup. But the blocking loop within
tty_port_block_til_ready() is not informed about this event.
The patch proposed here fixes this by resetting the initialization state of
the virtual tty to ensure the loop exits and triggering it to make
tty_port_block_til_ready() return.

Fixes: e1eaea4 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220218073123.2121-7-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: dstarke-siemens

drivers/tty/n_gsm.c

@@ -1457,6 +1457,9 @@ static void gsm_dlci_close(struct gsm_dlci *dlci)
 	if (dlci->addr != 0) {
 		tty_port_tty_hangup(&dlci->port, false);
 		kfifo_reset(&dlci->fifo);
+		/* Ensure that gsmtty_open() can return. */
+		tty_port_set_initialized(&dlci->port, 0);
+		wake_up_interruptible(&dlci->port.open_wait);
 	} else
 		dlci->gsm->dead = true;
 	/* Unregister gsmtty driver,report gsmtty dev remove uevent for user */