Re: [PATCH 03/21] binder: fix race between mmput() and do_exit()

[Alice Ryhl]

Carlos Llamas <cmllamas@xxxxxxxxxx> writes:
> 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")
> Signed-off-by: Carlos Llamas <cmllamas@xxxxxxxxxx>

Nice catch!

Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>

The code really could use a comment, though.

Alice