binder: remove "invalid inc weak" check

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>
Link: https://patch.msgid.link/20251015-binder-weak-inc-v1-1-7914b092c371@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 3a09c54bc37bb75639c84e94b05ae06e2a4df457..a3a1b5c33ba36ae8b5579a79365aae1106336364 100644 (file)
--- 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;
 }