linux-kernel - [PATCH] tty: n_gsm: restrict tty devices to attach

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Date: Wed, 24 Jan 2024 19:06:04 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Jiri Slaby <jirislaby@...nel.org>,
        syzbot <syzbot+06fa1063cca8163ea541@...kaller.appspotmail.com>,
        syzkaller-bugs@...glegroups.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Starke, Daniel" <daniel.starke@...mens.com>,
        Lee Jones <lee@...nel.org>, Fedor Pchelkin <pchelkin@...ras.ru>
Cc: linux-kernel@...r.kernel.org, linux-serial <linux-serial@...r.kernel.org>
Subject: [PATCH] tty: n_gsm: restrict tty devices to attach

syzbot is reporting sleep in atomic context, for gsmld_write() is calling
con_write() with spinlock held and IRQs disabled.

Since n_gsm is designed to be used for serial port, reject attaching to
virtual consoles and PTY devices, by checking tty's device major/minor
numbers at gsmld_open().

Link: https://www.kernel.org/doc/html/v6.7/driver-api/tty/n_gsm.html
Reported-by: syzbot+06fa1063cca8163ea541@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=06fa1063cca8163ea541
Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
---
 drivers/tty/n_gsm.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 4036566febcb..14581483af78 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -3623,6 +3623,7 @@ static void gsmld_close(struct tty_struct *tty)
 static int gsmld_open(struct tty_struct *tty)
 {
 	struct gsm_mux *gsm;
+	int major;
 
 	if (!capable(CAP_NET_ADMIN))
 		return -EPERM;
@@ -3630,6 +3631,17 @@ static int gsmld_open(struct tty_struct *tty)
 	if (tty->ops->write == NULL)
 		return -EINVAL;
 
+	major = tty->driver->major;
+	/* Reject Virtual consoles */
+	if (major == 4 && tty->driver->minor_start == 1)
+		return -EINVAL;
+	/* Reject Unix98 PTY masters/slaves */
+	if (major >= 128 && major <= 143)
+		return -EINVAL;
+	/* Reject BSD PTY masters/slaves */
+	if (major >= 2 && major <= 3)
+		return -EINVAL;
+
 	/* Attach our ldisc data */
 	gsm = gsm_alloc_mux();
 	if (gsm == NULL)
-- 
2.18.4