Re: [RFC 10/10] kmod: add a sanity check on module loading

[Luis R. Rodriguez]

On Mon, Dec 19, 2016 at 6:53 PM, Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote:
> Where does this NULL-deref is the module isn't correctly loaded?

No you are right, sorry -- I had confused a failure to mount over null
deref, my mistake.

>> *Iff* we want a sanity check to verify kmod's umh is not lying to us we
>> need to verify after 0 was returned that it was not lying to us. Since kmod
>> accepts aliases but find_modules_all() only works on the real module name a
>> validation check cannot happen when all you have are aliases.
>
> request_module() should block until resolution, but that's fundamentally
> a userspace problem.  Let's not paper over it in kernelspace.

OK -- if userspace messes up again it may be a bit hard to prove
unless we have a validation debug thing in place, would such a thing
in debug form be reasonable ?

>> Yes; the kallsyms code does this on Oops.  Not really a big issue in
>> practice, but a nice fix.
>>
>> Ok, will bundle into my queue.
>
> Please submit to Jessica for her module queue, as it's orthogonal
> AFAICT.

Will do.

>> I will note though that I still think there's a bug in this code --
>> upon a failure other "spinning" requests can fail, I believe this may
>> be due to not having another state or informing pending modules too
>> early of a failure but I haven't been able to prove this conjecture
>> yet.
>
> That's possible, but I can't see it from quickly re-checking the code.
>
> The module should be fully usable at this point; the module's init has
> been called successfully, so in the case of __get_fs_type() it should
> now succeed.  The module cleans up its init section, but that should be
> independent.
>
> If there is a race, it's likely to be when some other caller wakes the
> queue.  Moving the wakeup as soon as possible should make it easier to
> trigger:
>
> diff --git a/kernel/module.c b/kernel/module.c
> index f57dd63186e6..78bd89d41a22 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3397,6 +3397,7 @@ static noinline int do_init_module(struct module *mod)
>
>         /* Now it's a first class citizen! */
>         mod->state = MODULE_STATE_LIVE;
> +       wake_up_all(&module_wq);
>         blocking_notifier_call_chain(&module_notify_list,
>                                      MODULE_STATE_LIVE, mod);
>
> @@ -3445,7 +3446,6 @@ static noinline int do_init_module(struct module *mod)
>          */
>         call_rcu_sched(&freeinit->rcu, do_free_init);
>         mutex_unlock(&module_mutex);
> -       wake_up_all(&module_wq);
>
>         return 0;
>

Will give this a shot, thanks!

  Luis