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


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