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

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>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -1839,7 +1839,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
 	}
 
 	s = comedi_read_subdevice(info);
-	if (s) {
+	if (s && s->async) {
 		poll_wait(file, &s->async->wait_head, wait);
 		if (!s->busy || !comedi_is_subdevice_running(s) ||
 		    comedi_buf_read_n_available(s->async) > 0)
@@ -1847,7 +1847,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
 	}
 
 	s = comedi_write_subdevice(info);
-	if (s) {
+	if (s && s->async) {
 		unsigned int bps = bytes_per_sample(s->async->subdevice);
 
 		poll_wait(file, &s->async->wait_head, wait);
@@ -1882,7 +1882,7 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
 	}
 
 	s = comedi_write_subdevice(info);
-	if (!s)
+	if (!s || !s->async)
 		return -EIO;
 
 	async = s->async;
@@ -1977,7 +1977,7 @@ static ssize_t comedi_read(struct file *file, char __user *buf, size_t nbytes,
 	}
 
 	s = comedi_read_subdevice(info);
-	if (!s)
+	if (!s || !s->async)
 		return -EIO;
 
 	async = s->async;