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SubjectRe: [PATCH v2 1/2] livepatch/module: Apply patch when loaded module is unformed
On Thu 2015-03-05 13:34:33, Josh Poimboeuf wrote:
> 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@suse.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.

Which branch have you tried it against, please? I have made them on
top of "for-next" branch in the git/jikos/livepatching.git tree.
The patch seems to apply fine there.

> >
> > 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"?

Yup, will fix it.

> /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()?

Yup, I was in doubt as well. The function is called only for one
object from a given patch. So, both "patch" and "object" seems
appropriate.

klp_module_coming_object() is confusing as well because it looks
like coming object but the object struct is from a patch that was
there before.

I could change this to klp_module_coming_update_object() if it
sounds better to you. Any beeter ideas are welcome.

> > {
> > 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.

Good catch. I'll change this to pr_err().

> > + 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.

I did it but then the diff became a mess and was hard to read. OK,
I'll add one more patch before this one that will just switch the
order and do this changes on top of 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.

ah, yes, great catch

>
> > 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.

Will fix

> > +}
> > +
> > +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).

will fix

> > + 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.

Hrmmpfffff, my head was relaxing somewhere in the corner when I tested
the patch. You are right, it does not work. Huh, I wonder why we are
able to find the address in kGraft. We are using this approach there
for a long time.

I am going to investigate.

Thanks a lot for review.

Best Regards,
Petr

> --
> Josh
> --
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