(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@xxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
