Re: [Linaro-mm-sig] [PATCH] dma-buf/sw_sync: Avoid recursive lock during fence signal

[Christian König]

Am 17.08.23 um 23:37 schrieb Rob Clark:
> From: Rob Clark <robdclark@xxxxxxxxxxxx>
>
> If a signal callback releases the sw_sync fence, that will trigger a
> deadlock as the timeline_fence_release recurses onto the fence->lock
> (used both for signaling and the the timeline tree).
>
> To avoid that, temporarily hold an extra reference to the signalled
> fences until after we drop the lock.
>
> (This is an alternative implementation of https://patchwork.kernel.org/patch/11664717/
> which avoids some potential UAF issues with the original patch.)
>
> Reported-by: Bas Nieuwenhuizen <bas@xxxxxxxxxxxxxxxxxxx>
> Fixes: d3c6dd1fb30d ("dma-buf/sw_sync: Synchronize signal vs syncpt free")
> Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> ---
>   drivers/dma-buf/sw_sync.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
>
> diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c
> index 63f0aeb66db6..ceb6a0408624 100644
> --- a/drivers/dma-buf/sw_sync.c
> +++ b/drivers/dma-buf/sw_sync.c
> @@ -191,6 +191,7 @@ static const struct dma_fence_ops timeline_fence_ops = {
>    */
>   static void sync_timeline_signal(struct sync_timeline *obj, unsigned int inc)
>   {
> +	LIST_HEAD(signalled);
>   	struct sync_pt *pt, *next;
>   
>   	trace_sync_timeline(obj);
> @@ -203,9 +204,13 @@ static void sync_timeline_signal(struct sync_timeline *obj, unsigned int inc)
>   		if (!timeline_fence_signaled(&pt->base))
>   			break;
>   
> +		dma_fence_get(&pt->base);

Question is why don't have the fences a reference on the list in the 
first place?

> +
>   		list_del_init(&pt->link);
>   		rb_erase(&pt->node, &obj->pt_tree);
>   
> +		list_add_tail(&pt->link, &signalled);

Instead of list_del()/list_add_tail() you could also use 
list_move_tail() here.

> +
>   		/*
>   		 * A signal callback may release the last reference to this
>   		 * fence, causing it to be freed. That operation has to be
> @@ -218,6 +223,11 @@ static void sync_timeline_signal(struct sync_timeline *obj, unsigned int inc)
>   	}
>   
>   	spin_unlock_irq(&obj->lock);
> +
> +	list_for_each_entry_safe(pt, next, &signalled, link) {
> +		list_del(&pt->link);

You must use list_del_init() here or otherwise the pt->link will keep 
pointing to the prev/next entries and the list_empty() check in 
timeline_fence_release() will fail and potentially corrupt things.

Regards,
Christian.

> +		dma_fence_put(&pt->base);
> +	}
>   }
>   
>   /**