(2015/03/05 23:18), Josh Poimboeuf wrote: > On Thu, Mar 05, 2015 at 09:52:41AM +0900, Masami Hiramatsu wrote: >> (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. > > Hi Masami, > > As Jikos pointed out elsewhere, try_get_module() won't solve all the > GOING races. > > The module can be in GOING before mod->exit() is called. If we apply a > patch between GOING getting set and mod->exit(), try_module_get() will > fail and the module won't be patched. But module code can still run > before or during mod->exit(), so the unpatched module code might > interact badly with new patched code elsewhere. Hmm, in that case, we'd better have new GONE state for the module. At least kprobe needs it. 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
