Re: [PATCH v3 1/1] v4l: event: Prevent freeing event subscriptions while accessed
Hi Sakari,
Thank you for the patch.
On Friday, 14 September 2018 14:03:01 EEST 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.
>
> 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
> Reviewed-by: Hans Verkuil
It wasn't immediately clear to me that the !sev->elems check can be removed
because the subscriptions are *now* only added to the event list once they are
fully initialized, I thought the sentence documented the current
implementation. After realizing that,
Reviewed-by: Laurent Pinchart
> ---
> since v1:
>
> - Call the mutex field subscribe_lock instead.
>
> - Move the field that is now subscribe_lock above the subscribed field the
> write access to which it serialises.
>
> - Improve documentation of the subscribe_lock field.
>
> since v2:
>
> - Acquire spinlock for the duration of list_add() in v4l2_event_subscribe().
>
> - Remove a redundant comment in the same place.
>
> 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(
[PATCH v3 1/1] v4l: event: Prevent freeing event subscriptions while accessed
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
Reviewed-by: Hans Verkuil
---
since v1:
- Call the mutex field subscribe_lock instead.
- Move the field that is now subscribe_lock above the subscribed field the
write access to which it serialises.
- Improve documentation of the subscribe_lock field.
since v2:
- Acquire spinlock for the duration of list_add() in v4l2_event_subscribe().
- Remove a redundant comment in the same place.
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
