On 09/12/18 10:52, Sakari Ailus wrote:
> 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.
Hmm, the (un)subscribe ioctls are serialized through the ioctl lock,
so this could only be a scenario with drivers that do not use this
lock. Off-hand the only driver I know that does this is uvc. Unfortunately,
that's a rather popular one.
>
> 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@xxxxxxxxxxxxxxx>
> ---
> Hi folks,
>
> I noticed this while working to add support for media events. This seems
> like material for the stable trees.
I'd say 'no need for this' if it wasn't for uvc.
>
> drivers/media/v4l2-core/v4l2-event.c | 35 ++++++++++++++++++-----------------
> drivers/media/v4l2-core/v4l2-fh.c | 2 ++
> include/media/v4l2-fh.h | 4 ++++
> 3 files changed, 24 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/media/v4l2-core/v4l2-event.c b/drivers/media/v4l2-core/v4l2-event.c
> index 127fe6eb91d9..74161f79e4d3 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 +218,35 @@ int v4l2_event_subscribe(struct v4l2_fh *fh,
> sev->flags = sub->flags;
> sev->fh = fh;
> sev->ops = ops;
> + sev->elems = elems;
> +
> + mutex_lock(&fh->mutex);
>
> 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;
> + list_add(&sev->list, &fh->subscribed);
>
> - return 0;
> +out_unlock:
> + mutex_unlock(&fh->mutex);
> +
> + return ret;
> }
> EXPORT_SYMBOL_GPL(v4l2_event_subscribe);
>
> @@ -288,6 +285,8 @@ int v4l2_event_unsubscribe(struct v4l2_fh *fh,
> return 0;
> }
>
> + mutex_lock(&fh->mutex);
> +
> spin_lock_irqsave(&fh->vdev->fh_lock, flags);
>
> sev = v4l2_event_subscribed(fh, sub->type, sub->id);
> @@ -305,6 +304,8 @@ int v4l2_event_unsubscribe(struct v4l2_fh *fh,
> if (sev && sev->ops && sev->ops->del)
> sev->ops->del(sev);
>
> + mutex_unlock(&fh->mutex);
> +
> kvfree(sev);
>
> return 0;
> diff --git a/drivers/media/v4l2-core/v4l2-fh.c b/drivers/media/v4l2-core/v4l2-fh.c
> index 3895999bf880..b017dafde907 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->mutex);
> }
> 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->mutex);
> 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..1be45a5f6383 100644
> --- a/include/media/v4l2-fh.h
> +++ b/include/media/v4l2-fh.h
> @@ -42,6 +42,9 @@ struct v4l2_ctrl_handler;
> * @available: list of events waiting to be dequeued
> * @navailable: number of available events at @available list
> * @sequence: event sequence number
> + * @mutex: hold event subscriptions during subscribing;
> + * guarantee that the add and del event callbacks are orderly called
> + *
> * @m2m_ctx: pointer to &struct v4l2_m2m_ctx
> */
> struct v4l2_fh {
> @@ -56,6 +59,7 @@ struct v4l2_fh {
> struct list_head available;
> unsigned int navailable;
> u32 sequence;
> + struct mutex mutex;
I don't like the name 'mutex'. Perhaps something more descriptive like:
'subscribe_lock'?
>
> #if IS_ENABLED(CONFIG_V4L2_MEM2MEM_DEV)
> struct v4l2_m2m_ctx *m2m_ctx;
>
Overall I think this patch makes sense. The code is cleaner and easier to follow.
Just give 'mutex' a better name :-)
Regards,
Hans
