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

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Date:   Thu, 3 Mar 2022 14:51:23 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Miroslav Benes <mbenes@...e.cz>
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: [External] Re: [PATCH] livepatch: Only block the removal of
 KLP_UNPATCHED forced transition patch

Hi,

On 2022/3/2 5:55 下午, Miroslav Benes wrote:
> 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.

Good point, I should check if it's an atomic replace livepatch in the else
branch, in which case we have to set all existing patches to forced.

Thanks.

> 
> Miroslav