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Subject: [PATCH] usb: gadget: u_serial: fix use-after-free of gs_port on concurrent open
Author: lingrain580@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

gserial_free_line() frees the gs_port while the tty layer may still be
about to use the tty_port embedded in it.  gs_tty_driver->ports[] keeps
pointing at the freed memory, because tty_unregister_device() only clears
cdevs[] and never touches ports[].

A concurrent open() then picks up that stale pointer in tty_init_dev():

	if (!tty->port)
		tty->port = driver->ports[idx];
	...
	tty->port->itty = tty;

gs_open() does check ports[port_num].port and returns -ENODEV, but that
check runs after tty->port has already been assigned, so even the failed
open dereferences freed memory on its release path:

  BUG: KASAN: slab-use-after-free in release_tty+0x589/0x5d0
  Write of size 8 at addr ffff88804b450120 by task repro/4930

  Allocated by task 4929:
   gs_port_alloc
   gserial_alloc_line_no_console
   acm_alloc_instance
   configfs_mkdir

  Freed by task 4929:
   gserial_free_port
   gserial_free_line
   acm_free_instance
   config_item_cleanup

The existing wait_event(port->close_wait, gs_closed(port)) does not help
here: a failed open never increments port.count, so gs_closed() reports
the port as closed and the wait returns immediately.

Give gs_port proper reference counting, the way cdc-acm.c already does on
the host side.  Add a tty_port_operations::destruct callback that performs
the kfree(), acquire a reference in a new ->install callback while holding
ports[].lock, and release it in ->cleanup.  gserial_free_port() now calls
tty_port_put() instead of freeing directly, so the memory is released only
once the last user is gone.

Reproduced with the syzbot C reproducer on v7.2-rc6 with KASAN and
PREEMPT_RT enabled; the report is gone after this change and the
WARN_ON(port->itty) in tty_port_destructor() does not trigger, confirming
the references are balanced.

Reported-by: syzbot+bca09f5d8b843bbf7571@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=bca09f5d8b843bbf7571
Cc: stable@vger.kernel.org
Signed-off-by: Tianqi Dong <lingrain580@gmail.com>
---
 drivers/usb/gadget/function/u_serial.c | 48 ++++++++++++++++++++++++--
 1 file changed, 46 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
index cdd1dfc66..6eabccf9b 100644
--- a/drivers/usb/gadget/function/u_serial.c
+++ b/drivers/usb/gadget/function/u_serial.c
@@ -908,7 +908,51 @@ static int gs_get_icount(struct tty_struct *tty,
 	return 0;
 }
 
+static void gs_port_destruct(struct tty_port *port)
+{
+	struct gs_port	*gs = container_of(port, struct gs_port, port);
+
+	kfree(gs);
+}
+
+static const struct tty_port_operations gs_port_ops = {
+	.destruct =		gs_port_destruct,
+};
+
+/*
+ * Take a reference to the port before the tty core stores it in tty->port.
+ * Otherwise gserial_free_line() may free the port while a concurrent open()
+ * is about to dereference the stale pointer left in gs_tty_driver->ports[].
+ */
+static int gs_install(struct tty_driver *driver, struct tty_struct *tty)
+{
+	struct gs_port	*port;
+	int		status;
+
+	mutex_lock(&ports[tty->index].lock);
+	port = ports[tty->index].port;
+	if (!port) {
+		mutex_unlock(&ports[tty->index].lock);
+		return -ENODEV;
+	}
+	tty_port_get(&port->port);
+	mutex_unlock(&ports[tty->index].lock);
+
+	status = tty_port_install(&port->port, driver, tty);
+	if (status)
+		tty_port_put(&port->port);
+
+	return status;
+}
+
+static void gs_cleanup(struct tty_struct *tty)
+{
+	tty_port_put(tty->port);
+}
+
 static const struct tty_operations gs_tty_ops = {
+	.install =		gs_install,
+	.cleanup =		gs_cleanup,
 	.open =			gs_open,
 	.close =		gs_close,
 	.write =		gs_write,
@@ -1222,6 +1266,7 @@ gs_port_alloc(unsigned port_num, struct usb_cdc_line_coding *coding)
 	}
 
 	tty_port_init(&port->port);
+	port->port.ops = &gs_port_ops;
 	spin_lock_init(&port->port_lock);
 	init_waitqueue_head(&port->drain_wait);
 	init_waitqueue_head(&port->close_wait);
@@ -1258,8 +1303,7 @@ static void gserial_free_port(struct gs_port *port)
 	/* wait for old opens to finish */
 	wait_event(port->close_wait, gs_closed(port));
 	WARN_ON(port->port_usb != NULL);
-	tty_port_destroy(&port->port);
-	kfree(port);
+	tty_port_put(&port->port);
 }
 
 void gserial_free_line(unsigned char port_num)
-- 
2.43.0