Re: [PATCH 2/2] livepatch: fix patched module loading race

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

(2015/03/04 22:17), 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.

No, that's not true if you try_get_module() before patching. After the
module state goes GOING (more correctly say, after try_release_module_ref()
succeeded), all try_get_module() must fail :)
So, please make sure to get module when applying patches.

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com