[PATCH] iio: trigger: close race condition in acquiring trigger reference

[Alison Schofield <amsfield22@gmail.com>]

In iio_trigger_write_current() we find the trigger we want while
holding mutex on the list of triggers, but we don't actually do a
get on it while holding mutex.  We wait until further validations
are completed and we're sure it's the one we want.  Race condition
is that it could be freed by the time we do the get.

Solution is to grab the trigger (iio_trigger_get) as soon as we
find it while holding mutex on the list of triggers.  If later
we decide it's not the right one, put it back. (iio_trigger_put).


Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Suggested-by: Lars-Peter Clausen <lars@metafoo.de>

---
Not directly related to this patch, but wondering about the
behavior when userspace tries to set an invalid current_trigger.

If the given trigger name is not found on the list, we don't
simply quit, we proceed.  Net result is the old trigger is
removed, the current trigger is set to NULL, and we return
success to userspace.  This looks like desired behavior 
since it is very intentionally coded.  Perhaps it is the
method used to remove a trigger?

Is that what we want?  


 drivers/iio/industrialio-trigger.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/drivers/iio/industrialio-trigger.c b/drivers/iio/industrialio-trigger.c
index 978729f..d0d869e 100644
--- a/drivers/iio/industrialio-trigger.c
+++ b/drivers/iio/industrialio-trigger.c
@@ -147,8 +147,7 @@ static struct iio_trigger *__iio_trigger_find_by_name(const char *name)
 	return NULL;
 }
 
-static struct iio_trigger *iio_trigger_find_by_name(const char *name,
-						    size_t len)
+static struct iio_trigger *iio_trigger_acquire_by_name(const char *name)
 {
 	struct iio_trigger *trig = NULL, *iter;
 
@@ -156,6 +155,7 @@ static struct iio_trigger *iio_trigger_find_by_name(const char *name,
 	list_for_each_entry(iter, &iio_trigger_list, list)
 		if (sysfs_streq(iter->name, name)) {
 			trig = iter;
+			iio_trigger_get(trig);
 			break;
 		}
 	mutex_unlock(&iio_trigger_list_lock);
@@ -416,20 +416,22 @@ static ssize_t iio_trigger_write_current(struct device *dev,
 	}
 	mutex_unlock(&indio_dev->mlock);
 
-	trig = iio_trigger_find_by_name(buf, len);
-	if (oldtrig == trig)
-		return len;
+	trig = iio_trigger_acquire_by_name(buf);
+	if (oldtrig == trig) {
+		ret = len;
+		goto out_trigger_put;
+	}
 
 	if (trig && indio_dev->info->validate_trigger) {
 		ret = indio_dev->info->validate_trigger(indio_dev, trig);
 		if (ret)
-			return ret;
+			goto out_trigger_put;
 	}
 
 	if (trig && trig->ops->validate_device) {
 		ret = trig->ops->validate_device(trig, indio_dev);
 		if (ret)
-			return ret;
+			goto out_trigger_put;
 	}
 
 	indio_dev->trig = trig;
@@ -441,13 +443,16 @@ static ssize_t iio_trigger_write_current(struct device *dev,
 		iio_trigger_put(oldtrig);
 	}
 	if (indio_dev->trig) {
-		iio_trigger_get(indio_dev->trig);
 		if (indio_dev->modes & INDIO_EVENT_TRIGGERED)
 			iio_trigger_attach_poll_func(indio_dev->trig,
 						     indio_dev->pollfunc_event);
 	}
 
 	return len;
+
+out_trigger_put:
+	iio_trigger_put(trig);
+	return ret;
 }
 
 static DEVICE_ATTR(current_trigger, S_IRUGO | S_IWUSR,
-- 
2.1.4