CVE-2023-52609: binder: fix race between mmput() and do_exit()

In the Linux kernel, the following vulnerability has been resolved:

binder: fix race between mmput() and doexit()

Task A calls binderupdatepagerange() to allocate and insert pages on a remote address space from Task B. For this, Task A pins the remote mm via mmgetnotzero() first. This can race with Task B doexit() and the final mmput() refcount decrement will come from Task A.

Task A | Task B ------------------+------------------ mmgetnotzero() | | doexit() | exitmm() | mmput() mmput() | exitmmap() | removevma() | fput() |

In this case, the work of fput() from Task B is queued up in Task A as TWARESUME. So in theory, Task A returns to userspace and the cleanup work gets executed. However, Task A instead sleep, waiting for a reply from Task B that never comes (it's dead).

This means the binderdeferredrelease() is blocked until an unrelated binder event forces Task A to go back to userspace. All the associated death notifications will also be delayed until then.

In order to fix this use mmputasync() that will schedule the work in the corresponding mm->asyncputwork WQ instead of Task A.