staging: comedi: check s->async for poll(), read() and write()

commit cc400e18 upstream.

Some low-level comedi drivers (incorrectly) point `dev->read_subdev` or
`dev->write_subdev` to a subdevice that does not support asynchronous
commands.  Comedi's poll(), read() and write() file operation handlers
assume these subdevices do support asynchronous commands.  In
particular, they assume `s->async` is valid (where `s` points to the
read or write subdevice), which it won't be if it has been set
incorrectly.  This can lead to a NULL pointer dereference.

Check `s->async` is non-NULL in `comedi_poll()`, `comedi_read()` and
`comedi_write()` to avoid the bug.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -1779,7 +1779,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
 
 	mask = 0;
 	read_subdev = comedi_get_read_subdevice(dev_file_info);
-	if (read_subdev) {
+	if (read_subdev && read_subdev->async) {
 		poll_wait(file, &read_subdev->async->wait_head, wait);
 		if (!read_subdev->busy
 		    || comedi_buf_read_n_available(read_subdev->async) > 0
@@ -1789,7 +1789,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
 		}
 	}
 	write_subdev = comedi_get_write_subdevice(dev_file_info);
-	if (write_subdev) {
+	if (write_subdev && write_subdev->async) {
 		poll_wait(file, &write_subdev->async->wait_head, wait);
 		comedi_buf_write_alloc(write_subdev->async,
 				       write_subdev->async->prealloc_bufsz);
@@ -1831,7 +1831,7 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
 	}
 
 	s = comedi_get_write_subdevice(dev_file_info);
-	if (s == NULL) {
+	if (s == NULL || s->async == NULL) {
 		retval = -EIO;
 		goto done;
 	}
@@ -1942,7 +1942,7 @@ static ssize_t comedi_read(struct file *file, char __user *buf, size_t nbytes,
 	}
 
 	s = comedi_get_read_subdevice(dev_file_info);
-	if (s == NULL) {
+	if (s == NULL || s->async == NULL) {
 		retval = -EIO;
 		goto done;
 	}