linux-kernel - [PATCH 5.10 76/80] tty: n_gsm: fix NULL pointer access due to DLCI release

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Message-Id: <20220228172320.940705546@linuxfoundation.org>
Date:   Mon, 28 Feb 2022 18:24:57 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Daniel Starke <daniel.starke@...mens.com>
Subject: [PATCH 5.10 76/80] tty: n_gsm: fix NULL pointer access due to DLCI release

From: daniel.starke@...mens.com <daniel.starke@...mens.com>

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@...r.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@...mens.com>
Link: https://lore.kernel.org/r/20220218073123.2121-4-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 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
@@ -1719,7 +1719,12 @@ 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);