[PATCH] comedi: include poll wait state in busy assessment

[Jens Axboe <axboe@kernel.dk>]

syzbot reports a use-after-free in comedi in the below link, which is
due to comedi gladly removing the allocated async area even though poll
requests are still active on the wait_queue_head inside of it. This can
cause a use-after-free when the poll entries are later triggered or
removed, as the memory for the wait_queue_head has been freed. The
notion of being busy in comedi seems mostly centered around sync
syscalls, where the comedi subdevice stores the file in 'busy' as a
marker. This is obviously broken for things like poll which can be
persistent across syscalls.

Rename is_device_busy() to start_detach(), and move it under the
dev->attach_lock. The latter serializes it with poll attempts. If
start_detach() is successful, then it will have marked the device with
->detaching == 1 and this will prevent further poll attempts.

Similarly, have start_detach() check for active polls on the device, and
return busy for that case.

Cc: stable@vger.kernel.org
Fixes: 8ffdff6a8cfb ("staging: comedi: move out of staging directory")
Link: https://lore.kernel.org/all/687bd5fe.a70a0220.693ce.0091.GAE@google.com/
Reported-by: syzbot+01523a0ae5600aef5895@syzkaller.appspotmail.com
Tested-by: syzbot+01523a0ae5600aef5895@syzkaller.appspotmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

diff --git a/drivers/comedi/comedi_fops.c b/drivers/comedi/comedi_fops.c
index c83fd14dd7ad..58b034e45283 100644
--- a/drivers/comedi/comedi_fops.c
+++ b/drivers/comedi/comedi_fops.c
@@ -782,24 +782,33 @@ void comedi_device_cancel_all(struct comedi_device *dev)
 	}
 }
 
-static int is_device_busy(struct comedi_device *dev)
+static int start_detach(struct comedi_device *dev)
 {
 	struct comedi_subdevice *s;
-	int i;
+	int i, is_busy = 0;
 
 	lockdep_assert_held(&dev->mutex);
+	lockdep_assert_held(&dev->attach_lock);
 	if (!dev->attached)
 		return 0;
 
 	for (i = 0; i < dev->n_subdevices; i++) {
 		s = &dev->subdevices[i];
-		if (s->busy)
-			return 1;
-		if (s->async && comedi_buf_is_mmapped(s))
-			return 1;
+		if (s->busy) {
+			is_busy = 1;
+			break;
+		}
+		if (!s->async)
+			continue;
+		if (comedi_buf_is_mmapped(s) ||
+		    wq_has_sleeper(&s->async->wait_head)) {
+			is_busy = 1;
+			break;
+		}
 	}
-
-	return 0;
+	if (!is_busy)
+		dev->detaching = 1;
+	return is_busy;
 }
 
 /*
@@ -825,8 +834,13 @@ static int do_devconfig_ioctl(struct comedi_device *dev,
 		return -EPERM;
 
 	if (!arg) {
-		if (is_device_busy(dev))
+		/* prevent new polls */
+		down_write(&dev->attach_lock);
+		if (start_detach(dev)) {
+			up_write(&dev->attach_lock);
 			return -EBUSY;
+		}
+		up_write(&dev->attach_lock);
 		if (dev->attached) {
 			struct module *driver_module = dev->driver->module;
 
@@ -2479,7 +2493,7 @@ static __poll_t comedi_poll(struct file *file, poll_table *wait)
 
 	down_read(&dev->attach_lock);
 
-	if (!dev->attached) {
+	if (!dev->attached || dev->detaching) {
 		dev_dbg(dev->class_dev, "no driver attached\n");
 		goto done;
 	}
diff --git a/include/linux/comedi/comedidev.h b/include/linux/comedi/comedidev.h
index 4cb0400ad616..b2bec668785f 100644
--- a/include/linux/comedi/comedidev.h
+++ b/include/linux/comedi/comedidev.h
@@ -545,6 +545,7 @@ struct comedi_device {
 	const char *board_name;
 	const void *board_ptr;
 	unsigned int attached:1;
+	unsigned int detaching:1;
 	unsigned int ioenabled:1;
 	spinlock_t spinlock;	/* generic spin-lock for low-level driver */
 	struct mutex mutex;	/* generic mutex for COMEDI core */

-- 
Jens Axboe