linux-kernel - Re: [PATCH v5] livepatch: Prevent patch inconsistencies if the coming module notifier fails

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Date:	Fri, 15 May 2015 13:53:06 +0200 (CEST)
From:	Miroslav Benes <mbenes@...e.cz>
To:	Minfei Huang <mhuang@...hat.com>
cc:	jpoimboe@...hat.com, sjenning@...hat.com, jkosina@...e.cz,
	vojtech@...e.cz, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org, Minfei Huang <mnfhuang@...il.com>
Subject: Re: [PATCH v5] livepatch: Prevent patch inconsistencies if the coming
 module notifier fails

On Fri, 15 May 2015, Minfei Huang wrote:

> From: Minfei Huang <mnfhuang@...il.com>
> 
> The previous patches can be applied, once the corresponding module is
> loaded. In general, the patch will do relocation (if necessary) and
> obtain/verify function address before we start to enable patch.
> 
> There are three different situations in which the coming module notifier
> can fail:
> 
> 1) relocations are not applied for some reason. In this case kallsyms
> for module symbol is not called at all. The patch is not applied to the
> module. If the user disable and enable patch again, there is possible
> bug in klp_enable_func. If the user specified func->old_addr for some
> function in the module (and he shouldn't do that, but nevertheless) our
> warning would not catch it, ftrace will reject to register the handler
> because of wrong address or will register the handler for wrong address.
> 
> 2) relocations are applied successfully, but kallsyms lookup fails. In
> this case func->old_addr can be correct for all previous lookups, 0 for
> current failed one, and "unspecified" for the rest. If we undergo the
> same scenario as in 1, the behaviour differs for three cases, but the
> patch is not enabled anyway.
> 
> 3) the object is initialized, but klp_enable_object fails in the
> notifier due to possible ftrace error. Since it is improbable that
> ftrace would heal itself in the future, we would get those errors
> everytime the patch is enabled.
> 
> In order to fix above situations, we can make obj->mod to NULL, if the
> coming modified notifier fails.
> 
> Signed-off-by: Minfei Huang <mnfhuang@...il.com>

Reviewed-by: Miroslav Benes <mbenes@...e.cz>

Regards,
Miroslav


> ---
> v4:
> - remove the label "out" in function klp_init_object_loaded
> v3:
> - modify the code style
> v2:
> - add the error message to make it more friendly
> - modify the commit log, base on the mbenes@...e.cz suggesting
> v1:
> - modify the commit log, describe the issue more details
> ---
>  kernel/livepatch/core.c | 29 ++++++++++++++++++-----------
>  1 file changed, 18 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index 284e269..4a87765 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -883,7 +883,7 @@ int klp_register_patch(struct klp_patch *patch)
>  }
>  EXPORT_SYMBOL_GPL(klp_register_patch);
>  
> -static void klp_module_notify_coming(struct klp_patch *patch,
> +static int klp_module_notify_coming(struct klp_patch *patch,
>  				     struct klp_object *obj)
>  {
>  	struct module *pmod = patch->mod;
> @@ -891,22 +891,23 @@ static void klp_module_notify_coming(struct klp_patch *patch,
>  	int ret;
>  
>  	ret = klp_init_object_loaded(patch, obj);
> -	if (ret)
> -		goto err;
> +	if (ret) {
> +		pr_warn("failed to initialize patch '%s' for module '%s' (%d)\n",
> +			pmod->name, mod->name, ret);
> +		return ret;
> +	}
>  
>  	if (patch->state == KLP_DISABLED)
> -		return;
> +		return 0;
>  
>  	pr_notice("applying patch '%s' to loading module '%s'\n",
>  		  pmod->name, mod->name);
>  
>  	ret = klp_enable_object(obj);
> -	if (!ret)
> -		return;
> -
> -err:
> -	pr_warn("failed to apply patch '%s' to module '%s' (%d)\n",
> -		pmod->name, mod->name, ret);
> +	if (ret)
> +		pr_warn("failed to apply patch '%s' to module '%s' (%d)\n",
> +			pmod->name, mod->name, ret);
> +	return ret;
>  }
>  
>  static void klp_module_notify_going(struct klp_patch *patch,
> @@ -930,6 +931,7 @@ disabled:
>  static int klp_module_notify(struct notifier_block *nb, unsigned long action,
>  			     void *data)
>  {
> +	int ret;
>  	struct module *mod = data;
>  	struct klp_patch *patch;
>  	struct klp_object *obj;
> @@ -955,7 +957,12 @@ static int klp_module_notify(struct notifier_block *nb, unsigned long action,
>  
>  			if (action == MODULE_STATE_COMING) {
>  				obj->mod = mod;
> -				klp_module_notify_coming(patch, obj);
> +				ret = klp_module_notify_coming(patch, obj);
> +				if (ret) {
> +					obj->mod = NULL;
> +					pr_warn("patch '%s' is in an inconsistent state!\n",
> +						patch->mod->name);
> +				}
>  			} else /* MODULE_STATE_GOING */
>  				klp_module_notify_going(patch, obj);
>  
> -- 
> 2.2.2
> 
> --
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