[PATCH] tty: n_gsm: restrict tty devices to attach

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

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@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=06fa1063cca8163ea541
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.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