5.4-stable review patch. If anyone has any objections, please let me know.
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From: Alice Ryhl aliceryhl@google.com
commit d90eeb8ecd227c204ab6c34a17b372bd950b7aa2 upstream.
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.c... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/android/binder.c | 11 +---------- 1 file changed, 1 insertion(+), 10 deletions(-)
--- a/drivers/android/binder.c +++ b/drivers/android/binder.c @@ -1208,17 +1208,8 @@ static int binder_inc_node_nilocked(stru } 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; }