linux-kernel - Re: Potential issues (security and otherwise) with the current cgroup-bpf API

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Date:   Mon, 19 Dec 2016 21:27:18 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     David Ahern <dsahern@...il.com>, Andy Lutomirski <luto@...nel.org>,
        Daniel Mack <daniel@...que.org>,
        Mickaël Salaün <mic@...ikod.net>,
        Kees Cook <keescook@...omium.org>, Jann Horn <jann@...jh.net>,
        Tejun Heo <tj@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Graf <tgraf@...g.ch>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: Potential issues (security and otherwise) with the current
 cgroup-bpf API

On Mon, Dec 19, 2016 at 8:44 PM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Mon, Dec 19, 2016 at 07:12:48PM -0800, Andy Lutomirski wrote:
>>
>> struct cgroup_bpf {
>>         /*
>>          * Store two sets of bpf_prog pointers, one for programs that are
>>          * pinned directly to this cgroup, and one for those that are effective
>>          * when this cgroup is accessed.
>>          */
>>         struct bpf_prog *prog[MAX_BPF_ATTACH_TYPE];
>>         struct bpf_prog *effective[MAX_BPF_ATTACH_TYPE];
>> };
>>
>> in struct cgroup, there's a 'struct cgroup_bpf bpf;'.
>>
>> This would change to something like:
>>
>> struct cgroup_filter_slot {
>>   struct bpf_prog *effective;
>>   struct cgroup_filter_slot *next;
>>   struct bpf_prog *local;
>> }
>>
>> local is NULL unless *this* cgroup has a filter.  effective points to
>> the bpf_prog that's active in this cgroup or the nearest ancestor that
>> has a filter.  next is NULL if there are no filters higher in the
>> chain or points to the next slot that has a filter.  struct cgroup
>> has:
>>
>> struct cgroup_filter_slot filters[MAX_BPF_ATTACH_TYPE];
>>
>> To evaluate it, you do:
>>
>> struct cgroup_filter_slot *slot = &cgroup->slot[the index];
>>
>> if (!slot->effective)
>>   return;
>>
>> do {
>>   evaluate(slot->effective);
>>   slot = slot->next;
>> } while (unlikely(slot));
>
> yes. something like this can work as a future extension
> to support multiple programs for security use case.
> Please propose a patch.
> Again, it's not needed today and there is no rush to implement it.
>

If this happens after 4.10 and 4.10 is released as is, then this
change would be an ABI break.

--Andy