From: daniel.starke@xxxxxxxxxxx <daniel.starke@xxxxxxxxxxx> commit 96b169f05cdcc844b400695184d77e42071d14f2 upstream. The here fixed commit made the tty hangup asynchronous to avoid a circular locking warning. I could not reproduce this warning. Furthermore, due to the asynchronous hangup the function call now gets queued up while the underlying tty is being freed. Depending on the timing this results in a NULL pointer access in the global work queue scheduler. To be precise in process_one_work(). Therefore, the previous commit made the issue worse which it tried to fix. This patch fixes this by falling back to the old behavior which uses a blocking tty hangup call before freeing up the associated tty. Fixes: 7030082a7415 ("tty: n_gsm: avoid recursive locking with async port hangup") Cc: stable@xxxxxxxxxxxxxxx Signed-off-by: Daniel Starke <daniel.starke@xxxxxxxxxxx> Link: https://lore.kernel.org/r/20220218073123.2121-4-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 @@ -1722,7 +1722,12 @@ static void gsm_dlci_release(struct gsm_ gsm_destroy_network(dlci); mutex_unlock(&dlci->mutex); - tty_hangup(tty); + /* We cannot use tty_hangup() because in tty_kref_put() the tty + * driver assumes that the hangup queue is free and reuses it to + * queue release_one_tty() -> NULL pointer panic in + * process_one_work(). + */ + tty_vhangup(tty); tty_port_tty_set(&dlci->port, NULL); tty_kref_put(tty); Patches currently in stable-queue which might be from daniel.starke@xxxxxxxxxxx are queue-5.4/tty-n_gsm-fix-encoding-of-control-signal-octet-bit-dv.patch queue-5.4/tty-n_gsm-fix-null-pointer-access-due-to-dlci-release.patch queue-5.4/tty-n_gsm-fix-proper-link-termination-after-failed-open.patch