On Fri, Dec 01, 2023 at 05:21:32PM +0000, Carlos Llamas wrote:
> 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: 457b9a6f09f0 ("Staging: android: add binder driver")
> Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Signed-off-by: Carlos Llamas <cmllamas@xxxxxxxxxx>
> ---
Sorry, I forgot to Cc: stable@xxxxxxxxxxxxxxx.
--
Carlos Llamas
