[PATCH 3.18 070/120] media: v4l: event: Prevent freeing event subscriptions while accessed

Greg Kroah-Hartman
2018-10-11T15:40:41Z

From: Sakari Ailus <sakari.ailus@linux.intel.com>

commit ad608fbcf166fec809e402d548761768f602702c upstream.

The event subscriptions are added to the subscribed event list while
holding a spinlock, but that lock is subsequently released while still
accessing the subscription object. This makes it possible to unsubscribe
the event --- and freeing the subscription object's memory --- while
the subscription object is simultaneously accessed.

Prevent this by adding a mutex to serialise the event subscription and
unsubscription. This also gives a guarantee to the callback ops that the
add op has returned before the del op is called.

This change also results in making the elems field less special:
subscriptions are only added to the event list once they are fully
initialised.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: stable@vger.kernel.org # for 4.14 and up
Fixes: c3b5b0241f62 ("V4L/DVB: V4L: Events: Add backend")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/media/v4l2-core/v4l2-event.c | 37 +++++++++++++++++------------------
drivers/media/v4l2-core/v4l2-fh.c | 2 +
include/media/v4l2-fh.h | 1
3 files changed, 22 insertions(+), 18 deletions(-)

--- a/drivers/media/v4l2-core/v4l2-event.c
+++ b/drivers/media/v4l2-core/v4l2-event.c
@@ -119,14 +119,6 @@ static void __v4l2_event_queue_fh(struct
if (sev == NULL)
return;

- /*
- * If the event has been added to the fh->subscribed list, but its
- * add op has not completed yet elems will be 0, treat this as
- * not being subscribed.
- */
- if (!sev->elems)
- return;
-
/* Increase event sequence number on fh. */
fh->sequence++;

@@ -209,6 +201,7 @@ int v4l2_event_subscribe(struct v4l2_fh
struct v4l2_subscribed_event *sev, *found_ev;
unsigned long flags;
unsigned i;
+ int ret = 0;

if (sub->type == V4L2_EVENT_ALL)
return -EINVAL;
@@ -226,31 +219,36 @@ int v4l2_event_subscribe(struct v4l2_fh
sev->flags = sub->flags;
sev->fh = fh;
sev->ops = ops;
+ sev->elems = elems;
+
+ mutex_lock(&fh->subscribe_lock);

spin_lock_irqsave(&fh->vdev->fh_lock, flags);
found_ev = v4l2_event_subscribed(fh, sub->type, sub->id);
- if (!found_ev)
- list_add(&sev->list, &fh->subscribed);
spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);

if (found_ev) {
+ /* Already listening */
kfree(sev);
- return 0; /* Already listening */
+ goto out_unlock;
}

if (sev->ops && sev->ops->add) {
- int ret = sev->ops->add(sev, elems);
+ ret = sev->ops->add(sev, elems);
if (ret) {
- sev->ops = NULL;
- v4l2_event_unsubscribe(fh, sub);
- return ret;
+ kfree(sev);
+ goto out_unlock;
}
}

- /* Mark as ready for use */
- sev->elems = elems;
+ spin_lock_irqsave(&fh->vdev->fh_lock, flags);
+ list_add(&sev->list, &fh->subscribed);
+ spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);

- return 0;
+out_unlock:
+ mutex_unlock(&fh->subscribe_lock);
+
+ return ret;
}
EXPORT_SYMBOL_GPL(v4l2_event_subscribe);

@@ -289,6 +287,8 @@ int v4l2_event_unsubscribe(struct v4l2_f
return 0;
}

+ mutex_lock(&fh->subscribe_lock);
+
spin_lock_irqsave(&fh->vdev->fh_lock, flags);

sev = v4l2_event_subscribed(fh, sub->type, sub->id);
@@ -307,6 +307,7 @@ int v4l2_event_unsubscribe(struct v4l2_f
sev->ops->del(sev);

kfree(sev);
+ mutex_unlock(&fh->subscribe_lock);

return 0;
}
--- a/drivers/media/v4l2-core/v4l2-fh.c
+++ b/drivers/media/v4l2-core/v4l2-fh.c
@@ -49,6 +49,7 @@ void v4l2_fh_init(struct v4l2_fh *fh, st
INIT_LIST_HEAD(&fh->available);
INIT_LIST_HEAD(&fh->subscribed);
fh->sequence = -1;
+ mutex_init(&fh->subscribe_lock);
}
EXPORT_SYMBOL_GPL(v4l2_fh_init);

@@ -93,6 +94,7 @@ void v4l2_fh_exit(struct v4l2_fh *fh)
if (fh->vdev == NULL)
return;
v4l2_event_unsubscribe_all(fh);
+ mutex_destroy(&fh->subscribe_lock);
fh->vdev = NULL;
}
EXPORT_SYMBOL_GPL(v4l2_fh_exit);
--- a/include/media/v4l2-fh.h
+++ b/include/media/v4l2-fh.h
@@ -43,6 +43,7 @@ struct v4l2_fh {
wait_queue_head_t wait;
struct list_head subscribed; /* Subscribed events */
struct list_head available; /* Dequeueable event */
+ struct mutex subscribe_lock;
unsigned int navailable;
u32 sequence;