linux-kernel - Re: [PATCH net-next v2 3/4] cgroup: bpf: Add bpf_skb_in_cgroup

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Date:	Thu, 23 Jun 2016 14:41:40 -0700
From:	Martin KaFai Lau <kafai@...com>
To:	Daniel Borkmann <daniel@...earbox.net>
CC:	<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<netdev@...r.kernel.org>, Alexei Starovoitov <ast@...com>,
	Tejun Heo <tj@...nel.org>, <kernel-team@...com>
Subject: Re: [PATCH net-next v2 3/4] cgroup: bpf: Add bpf_skb_in_cgroup_proto

On Thu, Jun 23, 2016 at 10:07:27PM +0200, Daniel Borkmann wrote:
> On 06/23/2016 06:54 PM, Martin KaFai Lau wrote:
> >On Thu, Jun 23, 2016 at 11:53:50AM +0200, Daniel Borkmann wrote:
> >>>diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> >>>index 668e079..68753e0 100644
> >>>--- a/kernel/bpf/verifier.c
> >>>+++ b/kernel/bpf/verifier.c
> >>>@@ -1062,6 +1062,10 @@ static int check_map_func_compatibility(struct bpf_map *map, int func_id)
> >>>  		if (func_id != BPF_FUNC_get_stackid)
> >>>  			goto error;
> >>>  		break;
> >>>+	case BPF_MAP_TYPE_CGROUP_ARRAY:
> >>>+		if (func_id != BPF_FUNC_skb_in_cgroup)
> >>>+			goto error;
> >>>+		break;
> >>
> >>I think the BPF_MAP_TYPE_CGROUP_ARRAY case should have been fist here in
> >>patch 2/4, but with unconditional goto error. And this one only adds the
> >>'func_id != BPF_FUNC_skb_in_cgroup' test.
> >I am not sure I understand.  Can you elaborate? I am probably missing
> >something here.
>
> If someone backports patch 2/4 as-is, but for some reason not 3/4, then you
> could craft a program that calls f.e. bpf_map_update_elem() on a cgroup array
> and would thus cause a NULL pointer deref, since verifier doesn't prevent it.
> I'm just trying to say that it would probably make sense to add the above 'case
> BPF_MAP_TYPE_CGROUP_ARRAY:' with an unconditional 'goto error' in patch 2/4
> and extend upon it in patch 3/4 so result looks like here, so that the patches
> are fine/complete each as stand-alone.
I failed to connect some points in your last comment.  Thanks for explaining.

Make sense. I will spin v3.