linux-kernel - Re: [PATCH] livepatch: Only block the removal of KLP

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Date:   Wed, 2 Mar 2022 10:55:06 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Chengming Zhou <zhouchengming@...edance.com>
cc:     jpoimboe@...hat.com, jikos@...nel.org, pmladek@...e.com,
        joe.lawrence@...hat.com, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org, songmuchun@...edance.com,
        qirui.001@...edance.com
Subject: Re: [PATCH] livepatch: Only block the removal of KLP_UNPATCHED forced
 transition patch

Hi,

On Tue, 1 Mar 2022, 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 flag 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 flag a patch to forced if it's a KLP_PATCHED forced
> transition. It can still be unloaded only if it has passed through the consistency
> model in KLP_UNPATCHED transition.
> 
> So this patch only set forced flag and block the removal of a KLP_UNPATCHED forced
> transition livepatch.
> 
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> ---
>  kernel/livepatch/transition.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> index 5683ac0d2566..8b296ad9e407 100644
> --- a/kernel/livepatch/transition.c
> +++ b/kernel/livepatch/transition.c
> @@ -641,6 +641,6 @@ 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;

I do not think this would interact nicely with the atomic replace feature. 
If you force the transition of a patch with ->replace set to true, no 
existing patch would get ->forced set with this change, which means all 
patches will be removed at the end of klp_try_complete_transition(). And 
that is something we want to prevent.

Miroslav