Re: [PATCH v2] livepatch: Don't block removal of patches that are safe to unload

[Petr Mladek <pmladek@suse.com>]

On Thu 2022-03-03 18:54:46, Chengming Zhou wrote:
> module_put() is currently never called for a patch with forced flag, to block
> the removal of that patch module that might still be in use after a forced
> transition.
> 
> But klp_force_transition() will set all patches on the list to be forced, since
> commit d67a53720966 ("livepatch: Remove ordering (stacking) of the livepatches")
> has removed stack ordering of the livepatches, it will cause all other patches can't
> be unloaded after disabled even if they have completed the KLP_UNPATCHED transition.
> 
> In fact, we don't need to set a patch to forced if it's a KLP_PATCHED forced
> transition. It can still be unloaded safely as long as it has passed through
> the consistency model in KLP_UNPATCHED transition.

It really looks safe. klp_check_stack_func() makes sure that @new_func
is not on the stack when klp_target_state == KLP_UNPATCHED. As a
result, the system should not be using code from the livepatch module
when KLP_UNPATCHED transition cleanly finished.


> But the exception is when force transition of an atomic replace patch, we
> have to set all previous patches to forced, or they will be removed at
> the end of klp_try_complete_transition().
> 
> This patch only set the klp_transition_patch to be forced in KLP_UNPATCHED
> case, and keep the old behavior when in atomic replace case.
> 
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> ---
> v2: interact nicely with the atomic replace feature noted by Miroslav.
> ---
>  kernel/livepatch/transition.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> index 5683ac0d2566..34ffb8c014ed 100644
> --- a/kernel/livepatch/transition.c
> +++ b/kernel/livepatch/transition.c
> @@ -641,6 +641,10 @@ void klp_force_transition(void)
>  	for_each_possible_cpu(cpu)
>  		klp_update_patch_state(idle_task(cpu));
>  
> -	klp_for_each_patch(patch)
> -		patch->forced = true;
> +	if (klp_target_state == KLP_UNPATCHED)
> +		klp_transition_patch->forced = true;
> +	else if (klp_transition_patch->replace) {
> +		klp_for_each_patch(patch)
> +			patch->forced = true;

This works only because there is should be only one patch when
klp_target_state == KLP_UNPATCHED and
klp_transition_patch->forced == true.
But it is a bit tricky. I would do it the other way:

	if (klp_transition_patch->replace) {
		klp_for_each_patch(patch)
			patch->forced = true;
	} else if (klp_target_state == KLP_UNPATCHED) {
		klp_transition_patch->forced = true;
	}

It looks more sane. And it makes it more clear
that the special handling of KLP_UNPATCHED transition
is done only when the atomic replace is not used.

Otherwise, I do not see any real problem with the patch.

Best Regards,
Petr