linux-kernel - Re: [RFC PATCH] livepatch: allow removal of a disabled patch

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Date:	Thu, 5 May 2016 08:27:48 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Miroslav Benes <mbenes@...e.cz>
Cc:	Jiri Kosina <jikos@...nel.org>, jeyu@...hat.com, pmladek@...e.com,
	jslaby@...e.cz, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org, huawei.libin@...wei.com,
	minfei.huang@...oo.com
Subject: Re: [RFC PATCH] livepatch: allow removal of a disabled patch

On Thu, May 05, 2016 at 10:28:12AM +0200, Miroslav Benes wrote:
> I think it boils down to the following problem.
> 
> 1. CONFIG_DEBUG_KOBJECT_RELEASE=y
> 
> 2. we have dynamic kobjects, so there is a pointer in klp_patch to struct 
> kobject
> 
> 3. it is allocated during klp_init_patch() and all is fine
> 
> 4. now we want to remove the patch module. It is disabled and module_put()
> is called. User calls rmmod on the module.
> 
> 5. klp_unregister_patch() is called in __exit method.
> 
> 6. klp_free_patch() is called.
> 
> 7. kobject_put(patch->kobj) is called.
> 
> ...now it gets interesting...
> 
> 8. among others kobject_cleanup() is scheduled as a delayed work (this is 
> important).
> 
> 9. there is no completion, so kobject_put returns and the module goes 
> away.
> 
> 10. someone calls patch enabled_store attribute (for example). They can 
> because kobject_cleanup() has not been called yet. It is delayed 
> scheduled.
> 
> ...crash...

But what exactly causes the crash?  In enabled_store() we can see that
the patch isn't in the list, so we can return -EINVAL.

-- 
Josh