[ 42/46] staging: comedi: check s->async for poll(), read() and write()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

3.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ian Abbott <abbotti@mev.co.uk>

commit cc400e185c07c15a42d2635995f422de5b94b696 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 |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -1577,7 +1577,7 @@ static unsigned int comedi_poll(struct f
 
 	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
@@ -1587,7 +1587,7 @@ static unsigned int comedi_poll(struct f
 		}
 	}
 	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);
@@ -1629,7 +1629,7 @@ static ssize_t comedi_write(struct file
 	}
 
 	s = comedi_get_write_subdevice(dev_file_info);
-	if (s == NULL) {
+	if (s == NULL || s->async == NULL) {
 		retval = -EIO;
 		goto done;
 	}
@@ -1740,7 +1740,7 @@ static ssize_t comedi_read(struct file *
 	}
 
 	s = comedi_get_read_subdevice(dev_file_info);
-	if (s == NULL) {
+	if (s == NULL || s->async == NULL) {
 		retval = -EIO;
 		goto done;
 	}