klp_task_patch: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model

[Petr Mladek <pmladek@suse.com>]

On Thu 2016-04-28 15:44:48, Josh Poimboeuf wrote:
> Change livepatch to use a basic per-task consistency model.  This is the
> foundation which will eventually enable us to patch those ~10% of
> security patches which change function or data semantics.  This is the
> biggest remaining piece needed to make livepatch more generally useful.
> 
> diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> new file mode 100644
> index 0000000..92819bb
> --- /dev/null
> +++ b/kernel/livepatch/transition.c
> +/*
> + * klp_patch_task() - change the patched state of a task
> + * @task:	The task to change
> + *
> + * Switches the patched state of the task to the set of functions in the target
> + * patch state.
> + */
> +void klp_patch_task(struct task_struct *task)
> +{
> +	clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
> +
> +	/*
> +	 * The corresponding write barriers are in klp_init_transition() and
> +	 * klp_reverse_transition().  See the comments there for an explanation.
> +	 */
> +	smp_rmb();
> +
> +	task->patch_state = klp_target_state;
> +}
> +
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index bd12c6c..60d633f 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -9,6 +9,7 @@
>  #include <linux/mm.h>
>  #include <linux/stackprotector.h>
>  #include <linux/suspend.h>
> +#include <linux/livepatch.h>
>  
>  #include <asm/tlb.h>
>  
> @@ -266,6 +267,9 @@ static void cpu_idle_loop(void)
>  
>  		sched_ttwu_pending();
>  		schedule_preempt_disabled();
> +
> +		if (unlikely(klp_patch_pending(current)))
> +			klp_patch_task(current);
>  	}

Some more ideas from the world of crazy races. I was shaking my head
if this was safe or not.

The problem might be if the task get rescheduled between the check
for the pending stuff or inside the klp_patch_task() function.
This will get even more important when we use this construct
on some more locations, e.g. in some kthreads.

If the task is sleeping on this strange locations, it might assign
strange values on strange times.

I think that it is safe only because it is called with the
'current' parameter and on a safe locations. It means that
the result is always safe and consistent. Also we could assign
an outdated value only when sleeping between reading klp_target_state
and storing task->patch_state. But if anyone modified
klp_target_state at this point, he also set TIF_PENDING_PATCH,
so the change will not get lost.

I think that we should document that klp_patch_func() must be
called only from a safe location from within the affected task.

I even suggest to avoid misuse by removing the struct *task_struct
parameter. It should always be called with current.

Best Regards,
Petr