linux-kernel - Re: [PATCH V4 1/3] livepatch: remove 'struct completion finish' from klp

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Date:   Fri, 5 Nov 2021 20:04:03 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>, ming.lei@...hat.com
Subject: Re: [PATCH V4 1/3] livepatch: remove 'struct completion finish' from
 klp_patch

On Wed, Nov 03, 2021 at 01:52:29PM +0100, Petr Mladek wrote:
> On Wed 2021-11-03 08:51:32, Ming Lei wrote:
> > On Tue, Nov 02, 2021 at 04:56:10PM +0100, Petr Mladek wrote:
> > > On Tue 2021-11-02 22:59:30, Ming Lei wrote:
> > > > The completion finish is just for waiting release of the klp_patch
> > > > object, then releases module refcnt. We can simply drop the module
> > > > refcnt in the kobject release handler of klp_patch.
> > > > 
> > > > This way also helps to support allocating klp_patch from heap.
> 
> First, I am sorry for confusion. The above description is correct.
> I does not say anything about that kobject_put() is synchronous.
> 
> > > IMHO, this is wrong assumption. kobject_put() might do everyting
> > > asynchronously, see:
> 
> I see that you are aware of this behavior.
> 
> > >    kobject_put()
> > >      kobject_release()
> > >        INIT_DELAYED_WORK(&kobj->release, kobject_delayed_cleanup);
> > >        schedule_delayed_work(&kobj->release, delay);
> > > 
> > >    asynchronously:
> > > 
> > >      kobject_delayed_cleanup()
> > >       kobject_cleanup()
> > > 	__kobject_del()
> > 
> > OK, this is one generic kobject release vs. module unloading issue to
> > solve, not unique for klp module, and there should be lots of drivers
> > suffering from it.
> 
> Yup, the problem is generic. It would be nice to have a generic
> solution. For example, add kobject_release_sync() that would return
> only when the object is really released.

The generic solution has been posted out:

https://lore.kernel.org/lkml/20211105063710.4092936-1-ming.lei@redhat.com/

which needn't any generic API change, just flushes all scheduled kobject
cleanup work before freeing module, and the change is transparent for
drivers.

IMO, kobject_release_sync() is one wrong direction for fixing this
issue, since it is basically impossible to audit if one kobject_put()
need to be replaced with kobject_release_sync().

> 
> > > > --- a/kernel/livepatch/core.c
> > > > +++ b/kernel/livepatch/core.c
> > > > @@ -678,11 +678,6 @@ static void klp_free_patch_finish(struct klp_patch *patch)
> > > >  	 * cannot get enabled again.
> > > >  	 */
> > > >  	kobject_put(&patch->kobj);
> > > > -	wait_for_completion(&patch->finish);
> > > > -
> > > > -	/* Put the module after the last access to struct klp_patch. */
> > > > -	if (!patch->forced)
> > > > -		module_put(patch->mod);
> > > 
> > > klp_free_patch_finish() does not longer wait until the release
> > > callbacks are called.
> > > 
> > > klp_free_patch_finish() is called also in klp_enable_patch() error
> > > path.
> > > 
> > > klp_enable_patch() is called in module_init(). For example, see
> > > samples/livepatch/livepatch-sample.c
> > > 
> > > The module must not get removed until the release callbacks are called.
> > > Does the module loader check the module reference counter when
> > > module_init() fails?
> > 
> > Good catch, that is really one corner case, in which the kobject has to
> > be cleaned up before returning from mod->init(), cause there can't be
> > module unloading in case of mod->init() failure.
> 
> Just to be sure. We want to keep the safe behavior in this case.
> There are many situations when klp_enable() might fail. And the error
> handling must be safe.
> 
> In general, livepatch developers are very conservative.
> Livepatches are not easy to create. They are used only by people
> who really want to avoid reboot. We want to keep the livepatch kernel
> framework as safe as possible to avoid any potential damage.

The posted patch can cover this situation in which module_init() fails.



Thanks,
Ming