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Subject[PATCH v2] tty: rpmsg: Fix race condition releasing tty port
Date
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@foss.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
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