[PATCH 5.15 159/177] tty: n_gsm: fix mux cleanup after unregister tty device

[Greg Kroah-Hartman]

From: Daniel Starke <daniel.starke@xxxxxxxxxxx>

commit 284260f278b706364fb4c88a7b56ba5298d5973c upstream.

Internally, we manage the alive state of the mux channels and mux itself
with the field member 'dead'. This makes it possible to notify the user
if the accessed underlying link is already gone. On the other hand,
however, removing the virtual ttys before terminating the channels may
result in peer messages being received without any internal target. Move
the mux cleanup procedure from gsmld_detach_gsm() to gsmld_close() to fix
this by keeping the virtual ttys open until the mux has been cleaned up.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Daniel Starke <daniel.starke@xxxxxxxxxxx>
Link: https://lore.kernel.org/r/20220414094225.4527-4-daniel.starke@xxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/tty/n_gsm.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2418,7 +2418,6 @@ static void gsmld_detach_gsm(struct tty_
 	WARN_ON(tty != gsm->tty);
 	for (i = 1; i < NUM_DLCI; i++)
 		tty_unregister_device(gsm_tty_driver, base + i);
-	gsm_cleanup_mux(gsm, false);
 	tty_kref_put(gsm->tty);
 	gsm->tty = NULL;
 }
@@ -2483,6 +2482,12 @@ static void gsmld_close(struct tty_struc
 {
 	struct gsm_mux *gsm = tty->disc_data;
 
+	/* The ldisc locks and closes the port before calling our close. This
+	 * means we have no way to do a proper disconnect. We will not bother
+	 * to do one.
+	 */
+	gsm_cleanup_mux(gsm, false);
+
 	gsmld_detach_gsm(tty, gsm);
 
 	gsmld_flush_buffer(tty);