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SubjectRe: [PATCH 2/2] livepatch: fix patched module loading race
On Wed 2015-03-04 14:17:52, Petr Mladek wrote:
> On Tue 2015-03-03 17:02:22, Josh Poimboeuf wrote:
> > It's possible for klp_register_patch() to see a module before the COMING
> > notifier is called, or after the GOING notifier is called.
> >
> > That can cause all kinds of ugly races. As Pter Mladek reported:
> >
> > "The problem is that we do not keep the klp_mutex lock all the time when
> > the module is being added or removed.
> >
> > First, the module is visible even before ftrace is ready. If we enable a patch
> > in this time frame, adding ftrace ops will fail and the patch will get rejected
> > just because bad timing.
>
> Ah, this is not true after all. I did not properly check when
> MODULE_STATE_COMING was set. I though that it was before ftrace was
> initialized but it was not true.
>
>
> > Second, if we are "lucky" and enable the patch for the coming module when the
> > ftrace is ready but before the module notifier has been called. The notifier
> > will try to enable the patch as well. It will detect that it is already patched,
> > return error, and the module will get rejected just because bad
> > timing. The more serious problem is that it will not call the notifier for
> > going module, so that the mess will stay there and we wont be able to load
> > the module later.
>
> Ah, the race is there but the effect is not that serious in the
> end. It seems that errors from module notifiers are ignored. In fact,
> we do not propagate the error from klp_module_notify_coming(). It means
> that WARN() from klp_enable_object() will be printed but the module
> will be loaded and patched.
>
> I am sorry, I was confused by kGraft where kgr_module_init() was
> called directly from module_load(). The errors were propagated. It
> means that kGraft rejects module when the patch cannot be applied.
>
> Note that the current solution is perfectly fine for the simple
> consistency model.
>
>
> > Third, similar problems are there for going module. If a patch is enabled after
> > the notifier finishes but before the module is removed from the list of modules,
> > the new patch will be applied to the module. The module might disappear at
> > anytime when the patch enabling is in progress, so there might be an access out
> > of memory. Or the whole patch might be applied and some mess will be left,
> > so it will not be possible to load/patch the module again."
>
> This is true.
>
>
> > Fix these races by letting the first one who sees the module do the
> > needed work.
> >
> > Reported-by: Petr Mladek <pmladek@suse.cz>
> > Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> > ---
> > kernel/livepatch/core.c | 53 +++++++++++++++++++++++++++++++++++++++++++++----
> > @@ -965,10 +990,30 @@ static int klp_module_notify(struct notifier_block *nb, unsigned long action,
> > continue;
> >
> > if (action == MODULE_STATE_COMING) {
> > +
> > + /*
> > + * Check for a small window where the register
> > + * path already initialized the object.
> > + */
> s/path/patch/
>
>
>
> > + if (obj->mod)
> > + continue;
>
> This might break stacking. The recently registered patch might become
> the last on the stack and thus unused.

Going through the stack when registering new patch would be quite
ugly. I am going to provide yet another solution.

Best Regards,
Petr


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