[PATCH v2 3/4] tty: don't oops in tty_unregister_device() when no cdev is registered

[Karl Mehltretter <kmehltretter@gmail.com>]

serial_core_add_one_port() keeps a uart_port when tty device registration
fails so setserial can still use it. It marks the port UPF_DEAD and
returns success. Removing the port later reaches tty_unregister_device(),
which unconditionally passes driver->cdevs[index] to cdev_del().

The slot does not always contain a live cdev. A serdev registration error
other than -ENODEV returns before tty_register_device_attr(), leaving the
slot NULL. If cdev_add() fails, tty_cdev_add() drops the cdev reference
but leaves the slot pointing at freed memory. The later cdev_del() is
therefore a NULL dereference or use-after-free.

The NULL path was reproduced with failslab during UART bind on qemu's
mcimx6ul-evk and raspi1ap boards:

  Unhandled fault: page domain fault (0x01b) at 0x00000038
  PC is at cdev_del+0x14/0x34

Clear the slot after cdev_add() fails and only call cdev_del() when it is
non-NULL. This makes a non-NULL slot mean that a live cdev is registered.

Fixes: c1a752ba2d6b ("tty: don't leak cdev in tty_cdev_add()")
Fixes: 8cde11b2baa1 ("tty/serdev: add serdev registration interface")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-fable-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 drivers/tty/tty_io.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 6b283fd03ff8..4889076b975f 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -3167,8 +3167,10 @@ static int tty_cdev_add(struct tty_driver *driver, dev_t dev,
 	driver->cdevs[index]->ops = &tty_fops;
 	driver->cdevs[index]->owner = driver->owner;
 	err = cdev_add(driver->cdevs[index], dev, count);
-	if (err)
+	if (err) {
 		kobject_put(&driver->cdevs[index]->kobj);
+		driver->cdevs[index] = NULL;
+	}
 	return err;
 }
 
@@ -3305,7 +3307,7 @@ EXPORT_SYMBOL_GPL(tty_register_device_attr);
 void tty_unregister_device(struct tty_driver *driver, unsigned index)
 {
 	device_destroy(&tty_class, MKDEV(driver->major, driver->minor_start) + index);
-	if (!(driver->flags & TTY_DRIVER_DYNAMIC_ALLOC)) {
+	if (!(driver->flags & TTY_DRIVER_DYNAMIC_ALLOC) && driver->cdevs[index]) {
 		cdev_del(driver->cdevs[index]);
 		driver->cdevs[index] = NULL;
 	}
-- 
2.53.0