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

From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: bpf@vger.kernel.org, "Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Andrii Nakryiko" <andrii@kernel.org>,
	netdev@vger.kernel.org, netfilter-devel@vger.kernel.org,
	"Martin KaFai Lau" <kafai@fb.com>,
	"Song Liu" <songliubraving@fb.com>, "Yonghong Song" <yhs@fb.com>,
	"John Fastabend" <john.fastabend@gmail.com>,
	"Maxim Mikityanskiy" <maximmi@nvidia.com>,
	"Pablo Neira Ayuso" <pablo@netfilter.org>,
	"Florian Westphal" <fw@strlen.de>,
	"Jesper Dangaard Brouer" <brouer@redhat.com>,
	"Toke Høiland-Jørgensen" <toke@redhat.com>
Subject: Re: [PATCH bpf-next v6 02/11] bpf: Fix UAF due to race between btf_try_get_module and load_module
Date: Thu, 6 Jan 2022 14:16:03 +0530	[thread overview]
Message-ID: <20220106084603.pgyziuv7wdts4yt7@apollo.legion> (raw)
In-Reply-To: <20220105061040.snl7hqsogeqxxruo@ast-mbp.dhcp.thefacebook.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