staging: comedi: fix a race between do_cmd_ioctl() and read/write

`do_cmd_ioctl()` is called with the comedi device's mutex locked to
process the `COMEDI_CMD` ioctl to set up comedi's asynchronous command
handling on a comedi subdevice. `comedi_read()` and `comedi_write()`
are the `read` and `write` handlers for the comedi device, but do not
lock the mutex (for performance reasons, as some things can hold the
mutex for quite a long time).

There is a race condition if `comedi_read()` or `comedi_write()` is
running at the same time and for the same file object and comedi
subdevice as `do_cmd_ioctl()`. `do_cmd_ioctl()` sets the subdevice's
`busy` pointer to the file object way before it sets the `SRF_RUNNING` flag
in the subdevice's `runflags` member. `comedi_read() and
`comedi_write()` check the subdevice's `busy` pointer is pointing to the
current file object, then if the `SRF_RUNNING` flag is not set, will call
`do_become_nonbusy()` to shut down the asyncronous command. Bad things
can happen if the asynchronous command is being shutdown and set up at
the same time.

To prevent the race, don't set the `busy` pointer until
after the `SRF_RUNNING` flag has been set. Also, make sure the mutex is
held in `comedi_read()` and `comedi_write()` while calling
`do_become_nonbusy()` in order to avoid moving the race condition to a
point within that function.

Change some error handling `goto cleanup` statements in `do_cmd_ioctl()`
to simple `return -ERRFOO` statements as a result of changing when the
`busy` pointer is set.

Signed-off-by: Ian Abbott <[e-mail]>
Cc: stable <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/comedi/comedi_fops.c

···
 		DPRINTK("subdevice busy\n");
 		return -EBUSY;
 	}
-	s->busy = file;
 
 	/* make sure channel/gain list isn't too long */
 	if (cmd.chanlist_len > s->len_chanlist) {
 		DPRINTK("channel/gain list too long %u > %d\n",
 			cmd.chanlist_len, s->len_chanlist);
-		ret = -EINVAL;
-		goto cleanup;
+		return -EINVAL;
 	}
 
 	/* make sure channel/gain list isn't too short */
 	if (cmd.chanlist_len < 1) {
 		DPRINTK("channel/gain list too short %u < 1\n",
 			cmd.chanlist_len);
-		ret = -EINVAL;
-		goto cleanup;
+		return -EINVAL;
 	}
 
 	async->cmd = cmd;
···
 	    kmalloc(async->cmd.chanlist_len * sizeof(int), GFP_KERNEL);
 	if (!async->cmd.chanlist) {
 		DPRINTK("allocation failed\n");
-		ret = -ENOMEM;
-		goto cleanup;
+		return -ENOMEM;
 	}
 
 	if (copy_from_user(async->cmd.chanlist, user_chanlist,
···
 
 	comedi_set_subdevice_runflags(s, ~0, SRF_USER | SRF_RUNNING);
 
+	/* set s->busy _after_ setting SRF_RUNNING flag to avoid race with
+	 * comedi_read() or comedi_write() */
+	s->busy = file;
 	ret = s->do_cmd(dev, s);
 	if (ret == 0)
 		return 0;
···
 
 		if (!comedi_is_subdevice_running(s)) {
 			if (count == 0) {
+				mutex_lock(&dev->mutex);
 				if (comedi_is_subdevice_in_error(s))
 					retval = -EPIPE;
 				else
 					retval = 0;
 				do_become_nonbusy(dev, s);
+				mutex_unlock(&dev->mutex);
 			}
 			break;
 		}
···
 
 		if (n == 0) {
 			if (!comedi_is_subdevice_running(s)) {
+				mutex_lock(&dev->mutex);
 				do_become_nonbusy(dev, s);
 				if (comedi_is_subdevice_in_error(s))
 					retval = -EPIPE;
 				else
 					retval = 0;
+				mutex_unlock(&dev->mutex);
 				break;
 			}
 			if (file->f_flags & O_NONBLOCK) {
···
 		buf += n;
 		break;		/* makes device work like a pipe */
 	}
-	if (comedi_is_subdevice_idle(s) &&
-	    async->buf_read_count - async->buf_write_count == 0) {
-		do_become_nonbusy(dev, s);
+	if (comedi_is_subdevice_idle(s)) {
+		mutex_lock(&dev->mutex);
+		if (async->buf_read_count - async->buf_write_count == 0)
+			do_become_nonbusy(dev, s);
+		mutex_unlock(&dev->mutex);
 	}
 	set_current_state(TASK_RUNNING);
 	remove_wait_queue(&async->wait_head, &wait);