There are no scenarios where a weak increment is invalid on binder_node.
The only possible case where it could be invalid is if the kernel
delivers BR_DECREFS to the process that owns the node, and then
increments the weak refcount again, effectively "reviving" a dead node.

However, that is not possible: when the BR_DECREFS command is delivered,
the kernel removes and frees the binder_node. The fact that you were
able to call binder_inc_node_nilocked() implies that the node is not yet
destroyed, which implies that BR_DECREFS has not been delivered to
userspace, so incrementing the weak refcount is valid.

Note that it's currently possible to trigger this condition if the owner
calls BINDER_THREAD_EXIT while node->has_weak_ref is true. This causes
BC_INCREFS on binder_ref instances to fail when they should not.

Cc: stable@vger.kernel.org
Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
Reported-by: Yu-Ting Tseng <yutingtseng@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 drivers/android/binder.c | 11 +----------
 1 file changed, 1 insertion(+), 10 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 8c99ceaa303bad8751571a337770df857e72d981..3915d8d2d896d1e3a861c336d900d7a4657a0104 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -851,17 +851,8 @@ static int binder_inc_node_nilocked(struct binder_node *node, int strong,
 	} else {
 		if (!internal)
 			node->local_weak_refs++;
-		if (!node->has_weak_ref && list_empty(&node->work.entry)) {
-			if (target_list == NULL) {
-				pr_err("invalid inc weak node for %d\n",
-					node->debug_id);
-				return -EINVAL;
-			}
-			/*
-			 * See comment above
-			 */
+		if (!node->has_weak_ref && target_list && list_empty(&node->work.entry))
 			binder_enqueue_work_ilocked(&node->work, target_list);
-		}
 	}
 	return 0;
 }

---
base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
change-id: 20251015-binder-weak-inc-f294e62d2ec6

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>