Date:	Thu, 5 Mar 2015 13:34:33 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Petr Mladek <pmladek@...e.cz>
Cc:	Seth Jennings <sjenning@...hat.com>, Jiri Kosina <jkosina@...e.cz>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Miroslav Benes <mbenes@...e.cz>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	mingo@...nel.org, mathieu.desnoyers@...icios.com, oleg@...hat.com,
	paulmck@...ux.vnet.ibm.com, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org, andi@...stfloor.org,
	rostedt@...dmis.org, tglx@...utronix.de
Subject: Re: [PATCH v2 1/2] livepatch/module: Apply patch when loaded module
 is unformed

On Thu, Mar 05, 2015 at 04:45:13PM +0100, Petr Mladek wrote:
> Existing live patches are applied to loaded modules using a notify handler.
> There are two problems with this approach.
> 
> First, errors from module notifiers are ignored and could not stop the module
> from being loaded. But we will need to refuse the module when there are
> semantics dependencies between functions and there are some problems
> to apply the patch to the module. Otherwise, the system might become
> into an inconsistent state.
> 
> Second, the module notifiers are called when the module is in
> STATE_MODULE_COMING. It means that it is visible by find_module()
> and can be detected by klp_find_object_module() when a new patch is
> registered.
> 
> Now, the timing is important. If the new patch is registered after the module
> notifier has been called, it has to initialize the module object for the new
> patch. Note that, in this case, the new patch has to see the module as loaded
> even when it is still in the COMING state.
> 
> But when the new patch is registered before the module notifier, it _should_
> not initialize the module object, see below for detailed explanation.
> 
> This patch solves both problems by calling klp_module_init() directly in
> load_module(). We could handle the error there. Also it is called in
> MODULE_STATE_UNFORMED and therefore before the module is visible via
> find_module().
> 
> The implementation creates three functions for module init and three
> functions for going modules. We need to revert already initialized
> patches when something fails and thus need to be able to call
> the code for going modules without leaving klp_mutex.
> 
> Detailed explanation of the last problem:
> 
> Why should not we initialize the module object for a new patch when
> the related module coming notifier has not been called yet?
> 
> Note that the notifier could _not_ _simply_ ignore already initialized module
> objects. The notifier initializes the module object for all existing patches.
> If the new patch is registered and enabled before, it would crate wrong
> order of patches in fops->func_stack.
> 
> For example, let's have three patches (P1, P2, P3) for the functions a()
> and b() where a() is from vmcore and b() is from a module M. Something
> like:
> 
> 	a()	b()
> P1	a1()	b1()
> P2	a2()	b2()
> P3	a3()	b3(3)
> 
> If you load the module M after all patches are registered and enabled.
> The ftrace ops for function a() and b() has listed the functions in this
> order
> 
> 	ops_a->func_stack -> list(a3,a2,a1)
> 	ops_b->func_stack -> list(b3,b2,b1)
> 
> , so the pointer to b3() is the first and will be used.
> 
> Then you might have the following scenario. Let's start with state
> when patches P1 and P2 are registered and enabled but the module M
> is not loaded. Then ftrace ops for b() does not exist. Then we
> get into the following race:
> 
> CPU0					CPU1
> 
> load_module(M)
> 
>   complete_formation()
> 
>   mod->state = MODULE_STATE_COMING;
>   mutex_unlock(&module_mutex);
> 
> 					klp_register_patch(P3);
> 					klp_enable_patch(P3);
> 
> 					# STATE 1
> 
>   klp_module_notify(M)
>     klp_module_notify_coming(P1);
>     klp_module_notify_coming(P2);
>     klp_module_notify_coming(P3);
> 
> 					# STATE 2
> 
> The ftrace ops for a() and b() then looks:
> 
>   STATE1:
> 
> 	ops_a->func_stack -> list(a3,a2,a1);
> 	ops_b->func_stack -> list(b3);
> 
>   STATE2:
> 	ops_a->func_stack -> list(a3,a2,a1);
> 	ops_b->func_stack -> list(b2,b1,b3);
> 
> therefore, b2() is used for the module but a3() is used for vmcore
> because they were the last added.
> 
> Signed-off-by: Petr Mladek <pmladek@...e.cz>
> ---
>  include/linux/livepatch.h | 10 +++++
>  kernel/livepatch/core.c   | 94 +++++++++++++++++++++++++++++++++++------------
>  kernel/module.c           |  9 +++++
>  3 files changed, 89 insertions(+), 24 deletions(-)

Not sure why, but "git am" seemed to think this patch was malformed.  It
applied ok for me after I removed the diffstat.

> 
> diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> index ee6dbb39a809..78ac10546160 100644
> --- a/include/linux/livepatch.h
> +++ b/include/linux/livepatch.h
> @@ -128,6 +128,16 @@ int klp_unregister_patch(struct klp_patch *);
>  int klp_enable_patch(struct klp_patch *);
>  int klp_disable_patch(struct klp_patch *);
>  
> +int klp_module_init(struct module *mod);
> +
> +#else /* CONFIG_LIVEPATCH */
> +
> +inline int klp_module_init(struct module *mod)

Should it not be "static inline"?

/me prays not to have to break out the C spec again ;-)

> +{
> +	return 0;
> +}
> +
>  #endif /* CONFIG_LIVEPATCH */
>  
> +
>  #endif /* _LINUX_LIVEPATCH_H_ */
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index 7e0c83dc7215..198f7733604b 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -869,8 +869,8 @@ int klp_register_patch(struct klp_patch *patch)
>  }
>  EXPORT_SYMBOL_GPL(klp_register_patch);
>  
> -static void klp_module_notify_coming(struct klp_patch *patch,
> -				     struct klp_object *obj)
> +static int klp_module_coming_update_patch(struct klp_patch *patch,
> +					  struct klp_object *obj)

This function name confused me a little bit.  Not sure what would be
better, but it really updates the object, not the patch.  Maybe
klp_module_coming_object()?

>  {
>  	struct module *pmod = patch->mod;
>  	struct module *mod = obj->mod;
> @@ -881,22 +881,62 @@ static void klp_module_notify_coming(struct klp_patch *patch,
>  		goto err;
>  
>  	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;
> +		return 0;
>  
>  err:
>  	pr_warn("failed to apply patch '%s' to module '%s' (%d)\n",
>  		pmod->name, mod->name, ret);

Does it still make sense to have this pr_warn() here now that we can
return an error and stop the module from loading?

I'm thinking it should be changed to pr_err() to be consistent with the
other klp error printks, and should probably say that we're preventing
the module from loading.

> +	return ret;
>  }
>  
> -static void klp_module_notify_going(struct klp_patch *patch,
> -				    struct klp_object *obj)
> +static void klp_module_going(struct module *mod);

It would probably be better to move klp_module_going() here so you don't
have to forward declare it.

> +
> +int klp_module_coming(struct module *mod)
> +{
> +	struct klp_patch *patch;
> +	struct klp_object *obj;
> +	int ret = 0;
> +
> +	list_for_each_entry(patch, &klp_patches, list) {
> +		for (obj = patch->objs; obj->funcs; obj++) {
> +			if (!klp_is_module(obj) || strcmp(obj->name, mod->name))
> +				continue;
> +
> +			obj->mod = mod;
> +			ret = klp_module_coming_update_patch(patch, obj);
> +			if (ret)
> +				goto err;
> +		}
> +	}
> +
> +	return 0;
> +
> +err:
> +	klp_module_going(mod);
> +	return ret;
> +}
> +
> +
> +int klp_module_init(struct module *mod)
> +{
> +	int ret = 0;
> +
> +	mutex_lock(&klp_mutex);
> +	ret = klp_module_coming(mod);
> +	mutex_unlock(&klp_mutex);
> +
> +	return ret;
> +}
> +
> +static void klp_module_going_update_patch(struct klp_patch *patch,
> +					  struct klp_object *obj)
>  {
>  	struct module *pmod = patch->mod;
>  	struct module *mod = obj->mod;
> @@ -913,40 +953,46 @@ disabled:
>  	klp_free_object_loaded(obj);
>  }
>  
> -static int klp_module_notify(struct notifier_block *nb, unsigned long action,
> -			     void *data)
> +static void klp_module_going(struct module *mod)
>  {
> -	struct module *mod = data;
>  	struct klp_patch *patch;
>  	struct klp_object *obj;
>  
> -	if (action != MODULE_STATE_COMING && action != MODULE_STATE_GOING)
> -		return 0;
> -
> -	mutex_lock(&klp_mutex);
> -
>  	list_for_each_entry(patch, &klp_patches, list) {
>  		for (obj = patch->objs; obj->funcs; obj++) {
> -			if (!klp_is_module(obj) || strcmp(obj->name, mod->name))
> +			/*
> +			 * Handle only loaded (initialized) modules.
> +			 * This is needed when used in an error path.
> +			 */
> +			if (!klp_is_object_loaded(obj) ||
> +			    strcmp(obj->name, mod->name))

Also need a klp_is_module() check here so it doesn't send NULL to strcmp
in the case of vmlinux.


>  				continue;
>  
> -			if (action == MODULE_STATE_COMING) {
> -				obj->mod = mod;
> -				klp_module_notify_coming(patch, obj);
> -			} else /* MODULE_STATE_GOING */
> -				klp_module_notify_going(patch, obj);
> -
> -			break;
> +			klp_module_going_update_patch(patch, obj);
>  		}
>  	}
>  
> -	mutex_unlock(&klp_mutex);
> +	return;

Redundant return.

> +}
> +
> +static int klp_module_notify_going(struct notifier_block *nb,
> +				   unsigned long action,
> +				   void *data)
> +{
> +	struct module *mod = data;
> +
> +	if (action != MODULE_STATE_GOING)
> +		return 0;
> +
> +	mutex_lock(&klp_mutex);
> +	klp_module_going(mod);
> +	mutex_lock(&klp_mutex);
>  
>  	return 0;
>  }
>  
>  static struct notifier_block klp_module_nb = {
> -	.notifier_call = klp_module_notify,
> +	.notifier_call = klp_module_notify_going,
>  	.priority = INT_MIN+1, /* called late but before ftrace notifier */
>  };
>  
> diff --git a/kernel/module.c b/kernel/module.c
> index d856e96a3cce..f744a639460d 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -53,6 +53,7 @@
>  #include <asm/sections.h>
>  #include <linux/tracepoint.h>
>  #include <linux/ftrace.h>
> +#include <linux/livepatch.h>
>  #include <linux/async.h>
>  #include <linux/percpu.h>
>  #include <linux/kmemleak.h>
> @@ -3321,6 +3322,14 @@ static int load_module(struct load_info *info, const char __user *uargs,
>  	/* Ftrace init must be called in the MODULE_STATE_UNFORMED state */
>  	ftrace_module_init(mod);
>  
> +	/*
> +	 * LivePatch init must be called in the MODULE_STATE_UNFORMED state
> +	 * and it might reject the module to avoid a system inconsistency.
> +	 */

nit: I thought we were calling it livepatch (all lowercase).

> +	err = klp_module_init(mod);
> +	if (err)
> +		goto ddebug_cleanup;
> +
>  	/* Finally it's fully formed, ready to start executing. */
>  	err = complete_formation(mod, info);
>  	if (err)

Hm, we still have a problem with the timing here.  The kallsyms lookup
functions ignore MODULE_STATE_UNFORMED modules.  So
klp_find_verify_func_addr() will fail to find the func address and the
module will always fail to load.

-- 
Josh
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