linux-kernel - [PATCH v2] tty: rpmsg: Fix race condition releasing tty port

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Message-ID: <20211214170646.25775-1-arnaud.pouliquen@foss.st.com>
Date:   Tue, 14 Dec 2021 18:06:46 +0100
From:   Arnaud Pouliquen <arnaud.pouliquen@...s.st.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>
CC:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        <linux-remoteproc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        <arnaud.pouliquen@...s.st.com>
Subject: [PATCH v2] tty: rpmsg: Fix race condition releasing tty port

In current implementation the tty_port struct is part of the
rpmsg_tty_port structure.The issue is that the rpmsg_tty_port structure is
freed on rpmsg_tty_remove but also referenced in the tty_struct.
Its release is not predictable due to workqueues.

For instance following ftrace shows that rpmsg_tty_close is called after
rpmsg_tty_release_cport:

     nr_test.sh-389     [000] .....   212.093752: rpmsg_tty_remove <-rpmsg_dev_
remove
             cat-1191    [001] .....   212.095697: tty_release <-__fput
      nr_test.sh-389     [000] .....   212.099166: rpmsg_tty_release_cport <-rpm
sg_tty_remove
             cat-1191    [001] .....   212.115352: rpmsg_tty_close <-tty_release
             cat-1191    [001] .....   212.115371: release_tty <-tty_release_str

As consequence, the port must be free only when user has released the TTY
interface.

This path :
- manages the port refcounting to trig the .destruct port ops,
- introduces the rpmsg_tty_cleanup function to ensure that the TTY is
  removed before decreasing the port refcount.
- calls rpmsg_tty_release_cport function in the rpmsg_tty_destruct_port
  function instead of in the rpmsg_tty_remove function.
- uses tty_vhangup and tty_port_hangup instead of tty_port_tty_hangup.

Fixes: 7c0408d80579 ("tty: add rpmsg driver")
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@...s.st.com>
---
diff vs V1:
 - rework patch based on port refcounting.
Applied and tested on fa55b7dcdc43 ("Linux 5.16-rc1", 2021-11-14)
---
 drivers/tty/rpmsg_tty.c | 43 +++++++++++++++++++++++++++++++++++------
 1 file changed, 37 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/rpmsg_tty.c b/drivers/tty/rpmsg_tty.c
index dae2a4e44f38..69272ad92266 100644
--- a/drivers/tty/rpmsg_tty.c
+++ b/drivers/tty/rpmsg_tty.c
@@ -53,9 +53,19 @@ static int rpmsg_tty_install(struct tty_driver *driver, struct tty_struct *tty)
 
 	tty->driver_data = cport;
 
+	tty_port_get(&cport->port);
 	return tty_port_install(&cport->port, driver, tty);
 }
 
+static void rpmsg_tty_cleanup(struct tty_struct *tty)
+{
+	struct tty_port *port = tty->port;
+
+	WARN_ON(!port);
+
+	tty_port_put(port);
+}
+
 static int rpmsg_tty_open(struct tty_struct *tty, struct file *filp)
 {
 	return tty_port_open(tty->port, tty, filp);
@@ -106,12 +116,19 @@ static unsigned int rpmsg_tty_write_room(struct tty_struct *tty)
 	return size;
 }
 
+static void rpmsg_tty_hangup(struct tty_struct *tty)
+{
+	tty_port_hangup(tty->port);
+}
+
 static const struct tty_operations rpmsg_tty_ops = {
 	.install	= rpmsg_tty_install,
 	.open		= rpmsg_tty_open,
 	.close		= rpmsg_tty_close,
 	.write		= rpmsg_tty_write,
 	.write_room	= rpmsg_tty_write_room,
+	.hangup		= rpmsg_tty_hangup,
+	.cleanup	= rpmsg_tty_cleanup,
 };
 
 static struct rpmsg_tty_port *rpmsg_tty_alloc_cport(void)
@@ -139,6 +156,8 @@ static struct rpmsg_tty_port *rpmsg_tty_alloc_cport(void)
 
 static void rpmsg_tty_release_cport(struct rpmsg_tty_port *cport)
 {
+	tty_port_destroy(&cport->port);
+
 	mutex_lock(&idr_lock);
 	idr_remove(&tty_idr, cport->id);
 	mutex_unlock(&idr_lock);
@@ -146,7 +165,17 @@ static void rpmsg_tty_release_cport(struct rpmsg_tty_port *cport)
 	kfree(cport);
 }
 
-static const struct tty_port_operations rpmsg_tty_port_ops = { };
+static void rpmsg_tty_destruct_port(struct tty_port *port)
+{
+	struct rpmsg_tty_port *cport = container_of(port, struct rpmsg_tty_port, port);
+
+	rpmsg_tty_release_cport(cport);
+}
+
+static const struct tty_port_operations rpmsg_tty_port_ops = {
+	.destruct = rpmsg_tty_destruct_port,
+};
+
 
 static int rpmsg_tty_probe(struct rpmsg_device *rpdev)
 {
@@ -179,7 +208,6 @@ static int rpmsg_tty_probe(struct rpmsg_device *rpdev)
 	return 0;
 
 err_destroy:
-	tty_port_destroy(&cport->port);
 	rpmsg_tty_release_cport(cport);
 
 	return ret;
@@ -188,17 +216,20 @@ static int rpmsg_tty_probe(struct rpmsg_device *rpdev)
 static void rpmsg_tty_remove(struct rpmsg_device *rpdev)
 {
 	struct rpmsg_tty_port *cport = dev_get_drvdata(&rpdev->dev);
+	struct tty_struct *tty;
 
 	dev_dbg(&rpdev->dev, "Removing rpmsg tty device %d\n", cport->id);
 
 	/* User hang up to release the tty */
-	if (tty_port_initialized(&cport->port))
-		tty_port_tty_hangup(&cport->port, false);
+	tty = tty_port_tty_get(&cport->port);
+	if (tty) {
+		tty_vhangup(tty);
+		tty_kref_put(tty);
+	}
 
 	tty_unregister_device(rpmsg_tty_driver, cport->id);
 
-	tty_port_destroy(&cport->port);
-	rpmsg_tty_release_cport(cport);
+	tty_port_put(&cport->port);
 }
 
 static struct rpmsg_device_id rpmsg_driver_tty_id_table[] = {
-- 
2.17.1