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Subject: media: v4l: event: Prevent freeing event subscriptions while accessed Author: Sakari Ailus sakari.ailus@linux.intel.com Date: Tue Sep 11 05:32:37 2018 -0400

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

drivers/media/v4l2-core/v4l2-event.c | 38 +++++++++++++++++++----------------- drivers/media/v4l2-core/v4l2-fh.c | 2 ++ include/media/v4l2-fh.h | 4 ++++ 3 files changed, 26 insertions(+), 18 deletions(-)

---

diff --git a/drivers/media/v4l2-core/v4l2-event.c b/drivers/media/v4l2-core/v4l2-event.c index 127fe6eb91d9..a3ef1f50a4b3 100644 --- a/drivers/media/v4l2-core/v4l2-event.c +++ b/drivers/media/v4l2-core/v4l2-event.c @@ -115,14 +115,6 @@ static void __v4l2_event_queue_fh(struct v4l2_fh *fh, const struct v4l2_event *e 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++;

@@ -208,6 +200,7 @@ int v4l2_event_subscribe(struct v4l2_fh *fh, struct v4l2_subscribed_event *sev, *found_ev; unsigned long flags; unsigned i; + int ret = 0;

if (sub->type == V4L2_EVENT_ALL) return -EINVAL; @@ -225,31 +218,36 @@ int v4l2_event_subscribe(struct v4l2_fh *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 */ kvfree(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; + kvfree(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);

@@ -288,6 +286,8 @@ int v4l2_event_unsubscribe(struct v4l2_fh *fh, return 0; }

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

sev = v4l2_event_subscribed(fh, sub->type, sub->id); @@ -305,6 +305,8 @@ int v4l2_event_unsubscribe(struct v4l2_fh *fh, if (sev && sev->ops && sev->ops->del) sev->ops->del(sev);

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

return 0; diff --git a/drivers/media/v4l2-core/v4l2-fh.c b/drivers/media/v4l2-core/v4l2-fh.c index 3895999bf880..c91a7bd3ecfc 100644 --- a/drivers/media/v4l2-core/v4l2-fh.c +++ b/drivers/media/v4l2-core/v4l2-fh.c @@ -45,6 +45,7 @@ void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev) INIT_LIST_HEAD(&fh->available); INIT_LIST_HEAD(&fh->subscribed); fh->sequence = -1; + mutex_init(&fh->subscribe_lock); } EXPORT_SYMBOL_GPL(v4l2_fh_init);

@@ -90,6 +91,7 @@ void v4l2_fh_exit(struct v4l2_fh *fh) return; v4l_disable_media_source(fh->vdev); v4l2_event_unsubscribe_all(fh); + mutex_destroy(&fh->subscribe_lock); fh->vdev = NULL; } EXPORT_SYMBOL_GPL(v4l2_fh_exit); diff --git a/include/media/v4l2-fh.h b/include/media/v4l2-fh.h index ea73fef8bdc0..8586cfb49828 100644 --- a/include/media/v4l2-fh.h +++ b/include/media/v4l2-fh.h @@ -38,10 +38,13 @@ struct v4l2_ctrl_handler; * @prio: priority of the file handler, as defined by &enum v4l2_priority * * @wait: event' s wait queue + * @subscribe_lock: serialise changes to the subscribed list; guarantee that + * the add and del event callbacks are orderly called * @subscribed: list of subscribed events * @available: list of events waiting to be dequeued * @navailable: number of available events at @available list * @sequence: event sequence number + * * @m2m_ctx: pointer to &struct v4l2_m2m_ctx */ struct v4l2_fh { @@ -52,6 +55,7 @@ struct v4l2_fh {

/* Events */ wait_queue_head_t wait; + struct mutex subscribe_lock; struct list_head subscribed; struct list_head available; unsigned int navailable;