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

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

  Task A            | Task B
  ------------------+------------------
  mmget_not_zero()  |
                    |  do_exit()
                    |    exit_mm()
                    |      mmput()
  mmput()           |
    exit_mmap()     |
      remove_vma()  |
        fput()      |

In this case, the work of ____fput() from Task B is queued up in Task A
as TWA_RESUME. 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 binder_deferred_release() 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 mmput_async() that will schedule the work in
the corresponding mm->async_put_work WQ instead of Task A.

Fixes: 457b9a6f09 ("Staging: android: add binder driver")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20231201172212.1813387-4-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder_alloc.c

@@ -271,7 +271,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
 	}
 	if (mm) {
 		mmap_write_unlock(mm);
-		mmput(mm);
+		mmput_async(mm);
 	}
 	return 0;
 
@@ -304,7 +304,7 @@ err_page_ptr_cleared:
 err_no_vma:
 	if (mm) {
 		mmap_write_unlock(mm);
-		mmput(mm);
+		mmput_async(mm);
 	}
 	return vma ? -ENOMEM : -ESRCH;
 }