binder: Address corner cases in deferred copy and fixup

When handling BINDER_TYPE_FDA object we are pushing a parent fixup
with a certain skip_size but no scatter-gather copy object, since
the copy is handled standalone.
If BINDER_TYPE_FDA is the last children the scatter-gather copy
loop will never stop to skip it, thus we are left with an item in
the parent fixup list. This will trigger the BUG_ON().

This is reproducible in android when playing a video.
We receive a transaction that looks like this:
    obj[0] BINDER_TYPE_PTR, parent
    obj[1] BINDER_TYPE_PTR, child
    obj[2] BINDER_TYPE_PTR, child
    obj[3] BINDER_TYPE_FDA, child

Fixes: 09184ae ("binder: defer copies of pre-patched txn data")
Acked-by: Todd Kjos <tkjos@google.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Alessandro Astone <ales.astone@gmail.com>
Link: https://lore.kernel.org/r/20220415120015.52684-2-ales.astone@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder.c

@@ -2295,6 +2295,7 @@ static int binder_do_deferred_txn_copies(struct binder_alloc *alloc,
 {
 	int ret = 0;
 	struct binder_sg_copy *sgc, *tmpsgc;
+	struct binder_ptr_fixup *tmppf;
 	struct binder_ptr_fixup *pf =
 		list_first_entry_or_null(pf_head, struct binder_ptr_fixup,
 					 node);
@@ -2349,7 +2350,11 @@ static int binder_do_deferred_txn_copies(struct binder_alloc *alloc,
 		list_del(&sgc->node);
 		kfree(sgc);
 	}
-	BUG_ON(!list_empty(pf_head));
+	list_for_each_entry_safe(pf, tmppf, pf_head, node) {
+		BUG_ON(pf->skip_size == 0);
+		list_del(&pf->node);
+		kfree(pf);
+	}
 	BUG_ON(!list_empty(sgc_head));
 
 	return ret > 0 ? -EINVAL : ret;