Re: [PATCH bpf-next v6 02/11] bpf: Fix UAF due to race between btf_try_get_module and load_module

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Wed, Jan 05, 2022 at 11:40:40AM IST, Alexei Starovoitov wrote:
> On Sun, Jan 02, 2022 at 09:51:06PM +0530, Kumar Kartikeya Dwivedi wrote:
> >
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 33bb8ae4a804..b5b423de53ab 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -6338,7 +6338,10 @@ struct module *btf_try_get_module(const struct btf *btf)
> >  		if (btf_mod->btf != btf)
> >  			continue;
> >
> > -		if (try_module_get(btf_mod->module))
> > +		/* We must only consider module whose __init routine has
> > +		 * finished, hence use try_module_get_live.
> > +		 */
> > +		if (try_module_get_live(btf_mod->module))
>
> Instead of patch 1 refactoring for this very specific case can we do:
> 1.
> if (try_module_get(btf_mod->module)) {
>      if (btf_mod->module->state != MODULE_STATE_LIVE)
>         module_put(btf_mod->module);
>      else
>         res = btf_mod->module;
>
> 2.
> preempt_disable();
> if (btf_mod->module->state == MODULE_STATE_LIVE &&
>     try_module_get(btf_mod->module)) ...
> preempt_enable();
>
> 3. add
> case MODULE_STATE_LIVE:
> to btf_module_notify()
> and have an extra flag in struct btf_module to say that it's ready?
>
> I'm mainly concerned about:
> -EXPORT_SYMBOL(try_module_get);
> +EXPORT_SYMBOL(__try_module_get);
> in the patch 1. Not that I care about out of tree modules,
> but we shouldn't be breaking them without a reason.

Alright, we could also export try_module_get, but let's go with option 3.

--
Kartikeya