Re: [PATCH v3] livepatch/module: Correctly handle coming and going modules

[Josh Poimboeuf]

On Mon, Mar 09, 2015 at 02:25:28PM +0100, Petr Mladek wrote:
> There is a notifier that handles live patches for coming and going modules.
> It takes klp_mutex lock to avoid races with coming and going patches but
> it does not keep the lock all the time. Therefore the following races are
> possible:
> 
>   1. The notifier is called sometime in STATE_MODULE_COMING. The module
>      is visible by find_module() in this state all the time. It means that
>      new patch can be registered and enabled even before the notifier is
>      called. It might create wrong order of stacked patches, see below
>      for an example.
> 
>    2. New patch could still see the module in the GOING state even after
>       the notifier has been called. It will try to initialize the related
>       object structures but the module could disappear at any time. There
>       will stay mess in the structures. It might even cause an invalid
>       memory access.
> 
> This patch solves the problem by adding a boolean variable into struct module.
> The value is true after the coming and before the going handler is called.
> New patches need to be applied when the value is true and they need to ignore
> the module when the value is false. New patches have to ignore the module
> also in the UNFORMED module state because the value might be undefined.
> 
> Note that we need to know state of all modules on the system. The races are
> related to new patches. Therefore we do not know what modules will get
> patched.
> 
> Also note that we could not simply ignore going modules. The code from the
> module could be called even in the GOING state until mod->exit() finishes.
> If we start supporting patches with semantic changes between function
> calls, we need to apply new patches to any still usable code.
> See below for an example.
> 
> Finally note that the patch solves only the situation when a new patch is
> registered. There are no such problems when the patch is being removed.
> It does not matter who disable the patch first, whether the normal
> disable_patch() or the module notifier. There is nothing to do
> once the patch is disabled.
> 
> Alternative solutions:
> ======================
> 
> + reject new patches when a patched module is coming or going; this is ugly
> 
> + wait with adding new patch until the module leaves the COMING and GOING
>   states; this might be dangerous and complicated; we would need to release
>   kgr_lock in the middle of the patch registration to avoid a deadlock
>   with the coming and going handlers; also we might need a waitqueue for
>   each module which seems to be even bigger overhead than the boolean
> 
> + always register/enable new patches and fix up the potential mess (registered
>   patches order) in klp_module_init(); this is nasty and prone to regressions
>   in the future development
> 
> + add another MODULE_STATE where the kallsyms are visible but the module is not
>   used yet; this looks too complex; the module states are checked on "many"
>   locations
> 
> Example of patch stacking breakage:
> ===================================
> 
> The notifier could _not_ _simply_ ignore already initialized module objects.
> For example, let's have three patches (P1, P2, P3) for 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.
> 
> Example of the race with going modules:
> =======================================
> 
> CPU0					CPU1
> 
> delete_module()  #SYSCALL
> 
>    try_stop_module()
>      mod->state = MODULE_STATE_GOING;
> 
>    mutex_unlock(&module_mutex);
> 
> 					klp_register_patch()
> 					klp_enable_patch()
> 
> 					#save place to switch universe
> 
> 					b()     # from module that is going
> 					  a()   # from core (patched)
> 
>    mod->exit();
> 
> Note that the function b() can be called until we call mod->exit().
> 
> If we do not apply patch against b() because it is in MODULE_STATE_GOING,
> it will call patched a() with modified semantic and things might get wrong.
> 
> [jpoimboe@xxxxxxxxxx: use one boolean instead of two]
> Signed-off-by: Petr Mladek <pmladek@xxxxxxx>
> ---
> Changes in v3:
> 
> + reverted back to v1:
>   + cannot handle coming modules in UNFORMED module state because
>     kallsyms are not ready => need to use the boolean again
>   + neither split nor handle errors in the module coming handler for now;
>     this change will be need only for more complex consistency model;
>     let's keep this patch(set) as easy as possible
> + just keep the check for mod is not NULL from v2
> + use one boolean as suggested by Josh
> 
> Changes in v2:
> 
> + split fix for coming and going modules
> + call klp_module_init() directly instead of using a handler
> + check if mod is not NULL when checking the module state
> + use the boolean flag only for going handler
> 
> 
>  include/linux/module.h  |  4 ++++
>  kernel/livepatch/core.c | 30 ++++++++++++++++++++++++++----
>  kernel/module.c         |  4 ++++
>  3 files changed, 34 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/module.h b/include/linux/module.h
> index b653d7c0a05a..7232fde6a991 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -344,6 +344,10 @@ struct module {
>  	unsigned long *ftrace_callsites;
>  #endif
>  
> +#ifdef CONFIG_LIVEPATCH
> +	bool klp_alive;
> +#endif
> +
>  #ifdef CONFIG_MODULE_UNLOAD
>  	/* What modules depend on me? */
>  	struct list_head source_list;
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index fc037345dbd4..2bc0d1dd2f62 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -89,16 +89,28 @@ static bool klp_is_object_loaded(struct klp_object *obj)
>  /* sets obj->mod if object is not vmlinux and module is found */
>  static void klp_find_object_module(struct klp_object *obj)
>  {
> +	struct module *mod;
> +
>  	if (!klp_is_module(obj))
>  		return;
>  
>  	mutex_lock(&module_mutex);
>  	/*
> -	 * We don't need to take a reference on the module here because we have
> -	 * the klp_mutex, which is also taken by the module notifier.  This
> -	 * prevents any module from unloading until we release the klp_mutex.
> +	 * We do not want to block removal of patched modules and therefore
> +	 * we do not take a reference here. The patches are removed by
> +	 * a going module handler instead.
> +	 */
> +	mod = find_module(obj->name);
> +	/*
> +	 * Do not mess work of the module coming and going notifiers.
> +	 * Note that the patch might still be needed before the going handler
> +	 * is called. Module functions can be called even in the GOING state
> +	 * until mod->exit() finishes. This is especially important for
> +	 * patches that modify semantic of the functions.
>  	 */
> -	obj->mod = find_module(obj->name);
> +	if (mod && mod->state != MODULE_STATE_UNFORMED && mod->klp_alive)
> +		obj->mod = mod;

It looks like find_module() doesn't return UNFORMED modules, so no need
check for them here.

> +
>  	mutex_unlock(&module_mutex);
>  }
>  
> @@ -736,6 +748,7 @@ static int klp_init_object(struct klp_patch *patch, struct klp_object *obj)
>  		return -EINVAL;
>  
>  	obj->state = KLP_DISABLED;
> +	obj->mod = NULL;
>  
>  	klp_find_object_module(obj);
>  
> @@ -926,6 +939,15 @@ static int klp_module_notify(struct notifier_block *nb, unsigned long action,
>  
>  	mutex_lock(&klp_mutex);
>  
> +	/*
> +	 * Each module has to know that the notifier has been called.
> +	 * We never know what module will get patched by a new patch.
> +	 */
> +	if (action == MODULE_STATE_COMING)
> +		mod->klp_alive = true;
> +	else /* MODULE_STATE_GOING */
> +		mod->klp_alive = false;
> +

Any reason why this needs to be protected by the 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))
> diff --git a/kernel/module.c b/kernel/module.c
> index d856e96a3cce..b3ffc231ce0d 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3271,6 +3271,10 @@ static int load_module(struct load_info *info, const char __user *uargs,
>  	}
>  #endif
>  
> +#ifdef CONFIG_LIVEPATCH
> +	mod->klp_alive = false;
> +#endif
> +

I don't think you need this initialization.  It looks like the module
struct is embedded in the mod->module_core region which is initialized
to zero in move_module().

-- 
Josh
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