Re: [PATCH,RFC] livepatch: fix race between fork and klp_reverse_transition

[Song Liu]

On Wed, Jul 20, 2022 at 9:20 AM Rik van Riel <riel@xxxxxxxxxxx> wrote:
>
> When a KLP fails to apply, klp_reverse_transition will clear the
> TIF_PATCH_PENDING flag on all tasks, except for newly created tasks
> which are not on the task list yet.
>
> Meanwhile, fork will copy over the TIF_PATCH_PENDING flag from the
> parent to the child early on, in dup_task_struct -> setup_thread_stack.
>
> Much later, klp_copy_process will set child->patch_state to match
> that of the parent.
>
> However, the parent's patch_state may have been changed by KLP loading
> or unloading since it was initially copied over into the child.
>
> This results in the KLP code occasionally hitting this warning in
> klp_complete_transition:
>
>         for_each_process_thread(g, task) {
>                 WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING));
>                 task->patch_state = KLP_UNDEFINED;
>         }
>
> This patch will set, or clear, the TIF_PATCH_PENDING flag in the child
> process depending on whether or not it is needed at the time
> klp_copy_process is called, at a point in copy_process where the
> tasklist_lock is held exclusively, preventing races with the KLP
> code.
>
> This should prevent this warning from triggering again in the
> future.
>
> I have not yet figured out whether this would also help with races in
> the other direction, where the child process fails to have TIF_PATCH_PENDING
> set and somehow misses a transition, or whether the retries in
> klp_try_complete_transition would catch that task and help it transition
> later.
>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
> Reported-by: Breno Leitao <leitao@xxxxxxxxxx>

LGTM!
Acked-by: Song Liu <song@xxxxxxxxxx>

> ---
>  kernel/livepatch/transition.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> index 5d03a2ad1066..7a90ad5e9224 100644
> --- a/kernel/livepatch/transition.c
> +++ b/kernel/livepatch/transition.c
> @@ -612,7 +612,15 @@ void klp_copy_process(struct task_struct *child)
>  {
>         child->patch_state = current->patch_state;
>
> -       /* TIF_PATCH_PENDING gets copied in setup_thread_stack() */
> +       /*
> +        * The parent process may have gone through a KLP transition since
> +        * the thread flag was copied in setup_thread_stack earlier. Set
> +        * the flag according to whether this task needs a KLP transition.
> +        */
> +       if (child->patch_state != klp_target_state)
> +               set_tsk_thread_flag(child, TIF_PATCH_PENDING);
> +       else
> +               clear_tsk_thread_flag(child, TIF_PATCH_PENDING);
>  }
>
>  /*
> --
> 2.35.1
>