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

[Rob Clark]

On Fri, Aug 18, 2023 at 2:09 AM Christian König
<christian.koenig@xxxxxxx> wrote:
>
> 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?

As best I can tell, it is because the fences hold a reference to the
timeline, so a reference in the other direction would cause a loop.

BR,
-R

> > +
> >               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);
> > +     }
> >   }
> >
> >   /**
>