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

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Date:   Thu, 3 Mar 2022 08:51:52 +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: [External] Re: [PATCH] livepatch: Only block the removal of
 KLP_UNPATCHED forced transition patch

On Thu, 3 Mar 2022, Chengming Zhou wrote:

> 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.

Yes, but that leads to a question if it then brings any value. Forcing a 
transition should be exceptional. If it is needed, there may be other 
issues involved which should probably be fixed. Have you come across a 
practical situation where the patch helped?

Thanks

Miroslav