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SubjectRe: [Linaro-mm-sig] [PATCH] dma-buf/sw_sync: Avoid recursive lock during fence signal
From
Am 17.08.23 um 23:37 schrieb Rob Clark:
> From: Rob Clark <robdclark@chromium.org>
>
> 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@basnieuwenhuizen.nl>
> Fixes: d3c6dd1fb30d ("dma-buf/sw_sync: Synchronize signal vs syncpt free")
> Signed-off-by: Rob Clark <robdclark@chromium.org>
> ---
> 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);
> + }
> }
>
> /**

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