[PATCH 4.19 154/287] tty: n_gsm: fix race condition in gsmld_write()

[Greg Kroah-Hartman]

From: Daniel Starke <daniel.starke@xxxxxxxxxxx>

[ Upstream commit 32dd59f96924f45e33bc79854f7a00679c0fa28e ]

The function may be used by the user directly and also by the n_gsm
internal functions. They can lead into a race condition which results in
interleaved frames if both are writing at the same time. The receiving side
is not able to decode those interleaved frames correctly.

Add a lock around the low side tty write to avoid race conditions and frame
interleaving between user originated writes and n_gsm writes.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Signed-off-by: Daniel Starke <daniel.starke@xxxxxxxxxxx>
Link: https://lore.kernel.org/r/20220701061652.39604-9-daniel.starke@xxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/tty/n_gsm.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 2c34a024b75f..3d45999fad1b 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2519,11 +2519,24 @@ static ssize_t gsmld_read(struct tty_struct *tty, struct file *file,
 static ssize_t gsmld_write(struct tty_struct *tty, struct file *file,
 			   const unsigned char *buf, size_t nr)
 {
-	int space = tty_write_room(tty);
+	struct gsm_mux *gsm = tty->disc_data;
+	unsigned long flags;
+	int space;
+	int ret;
+
+	if (!gsm)
+		return -ENODEV;
+
+	ret = -ENOBUFS;
+	spin_lock_irqsave(&gsm->tx_lock, flags);
+	space = tty_write_room(tty);
 	if (space >= nr)
-		return tty->ops->write(tty, buf, nr);
-	set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
-	return -ENOBUFS;
+		ret = tty->ops->write(tty, buf, nr);
+	else
+		set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+	spin_unlock_irqrestore(&gsm->tx_lock, flags);
+
+	return ret;
 }
 
 /**
-- 
2.35.1