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Date: Thu, 9 May 2019 04:57:38 -0700
From: Eric Dumazet <eric.dumazet@...il.com>
To: Lukasz Pawelczyk <l.pawelczyk@...sung.com>,
Eric Dumazet <eric.dumazet@...il.com>,
Pablo Neira Ayuso <pablo@...filter.org>,
Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
Florian Westphal <fw@...len.de>,
"David S. Miller" <davem@...emloft.net>,
netfilter-devel@...r.kernel.org, coreteam@...filter.org,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Lukasz Pawelczyk <havner@...il.com>
Subject: Re: [PATCH v2] netfilter: xt_owner: Add supplementary groups option
On 5/9/19 6:47 AM, Lukasz Pawelczyk wrote:
> On Wed, 2019-05-08 at 09:53 -0700, Eric Dumazet wrote:
>>
>> On 5/8/19 11:56 AM, Lukasz Pawelczyk wrote:
>>> On Wed, 2019-05-08 at 08:41 -0700, Eric Dumazet wrote:
>>>> On 5/8/19 11:25 AM, Lukasz Pawelczyk wrote:
>>>>> On Wed, 2019-05-08 at 07:58 -0700, Eric Dumazet wrote:
>>>>>> On 5/8/19 10:12 AM, Lukasz Pawelczyk wrote:
>>>>>>> The XT_SUPPL_GROUPS flag causes GIDs specified with
>>>>>>> XT_OWNER_GID to
>>>>>>> be also checked in the supplementary groups of a process.
>>>>>>>
>>>>>>> Signed-off-by: Lukasz Pawelczyk <l.pawelczyk@...sung.com>
>>>>>>> ---
>>>>>>> include/uapi/linux/netfilter/xt_owner.h | 1 +
>>>>>>> net/netfilter/xt_owner.c | 23
>>>>>>> ++++++++++++++++++++-
>>>>>>> --
>>>>>>> 2 files changed, 21 insertions(+), 3 deletions(-)
>>>>>>>
>>>>>>> diff --git a/include/uapi/linux/netfilter/xt_owner.h
>>>>>>> b/include/uapi/linux/netfilter/xt_owner.h
>>>>>>> index fa3ad84957d5..d646f0dc3466 100644
>>>>>>> --- a/include/uapi/linux/netfilter/xt_owner.h
>>>>>>> +++ b/include/uapi/linux/netfilter/xt_owner.h
>>>>>>> @@ -8,6 +8,7 @@ enum {
>>>>>>> XT_OWNER_UID = 1 << 0,
>>>>>>> XT_OWNER_GID = 1 << 1,
>>>>>>> XT_OWNER_SOCKET = 1 << 2,
>>>>>>> + XT_SUPPL_GROUPS = 1 << 3,
>>>>>>> };
>>>>>>>
>>>>>>> struct xt_owner_match_info {
>>>>>>> diff --git a/net/netfilter/xt_owner.c
>>>>>>> b/net/netfilter/xt_owner.c
>>>>>>> index 46686fb73784..283a1fb5cc52 100644
>>>>>>> --- a/net/netfilter/xt_owner.c
>>>>>>> +++ b/net/netfilter/xt_owner.c
>>>>>>> @@ -91,11 +91,28 @@ owner_mt(const struct sk_buff *skb,
>>>>>>> struct
>>>>>>> xt_action_param *par)
>>>>>>> }
>>>>>>>
>>>>>>> if (info->match & XT_OWNER_GID) {
>>>>>>> + unsigned int i, match = false;
>>>>>>> kgid_t gid_min = make_kgid(net->user_ns, info-
>>>>>>>> gid_min);
>>>>>>> kgid_t gid_max = make_kgid(net->user_ns, info-
>>>>>>>> gid_max);
>>>>>>> - if ((gid_gte(filp->f_cred->fsgid, gid_min) &&
>>>>>>> - gid_lte(filp->f_cred->fsgid, gid_max)) ^
>>>>>>> - !(info->invert & XT_OWNER_GID))
>>>>>>> + struct group_info *gi = filp->f_cred-
>>>>>>>> group_info;
>>>>>>> +
>>>>>>> + if (gid_gte(filp->f_cred->fsgid, gid_min) &&
>>>>>>> + gid_lte(filp->f_cred->fsgid, gid_max))
>>>>>>> + match = true;
>>>>>>> +
>>>>>>> + if (!match && (info->match & XT_SUPPL_GROUPS)
>>>>>>> && gi) {
>>>>>>> + for (i = 0; i < gi->ngroups; ++i) {
>>>>>>> + kgid_t group = gi->gid[i];
>>>>>>> +
>>>>>>> + if (gid_gte(group, gid_min) &&
>>>>>>> + gid_lte(group, gid_max)) {
>>>>>>> + match = true;
>>>>>>> + break;
>>>>>>> + }
>>>>>>> + }
>>>>>>> + }
>>>>>>> +
>>>>>>> + if (match ^ !(info->invert & XT_OWNER_GID))
>>>>>>> return false;
>>>>>>> }
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> How can this be safe on SMP ?
>>>>>>
>>>>>
>>>>> From what I see after the group_info rework some time ago this
>>>>> struct
>>>>> is never modified. It's replaced. Would
>>>>> get_group_info/put_group_info
>>>>> around the code be enough?
>>>>
>>>> What prevents the data to be freed right after you fetch filp-
>>>>> f_cred->group_info ?
>>>
>>> I think the get_group_info() I mentioned above would. group_info
>>> seems
>>> to always be freed by put_group_info().
>>
>> The data can be freed _before_ get_group_info() is attempted.
>>
>> get_group_info() would do a use-after-free
>>
>> You would need something like RCU protection over this stuff,
>> this is not really only a netfilter change.
>>
>
> sk_socket keeps reference to f_cred. f_cred keeps reference to
> group_info. As long as f_cred is alive and it doesn't seem to be the
> issue in the owner_mt() function, group_info should be alive as well as
> far as I can see. Its refcount will go down only when f_cred is freed
> (put_cred_rcu()).
>
> If there is something I'm missing please correct me.
The problem is that you can´t clearly explain why the code is safe :/
Why would get_group_info() be needed then ?
You need to explain this in the changelog, so that future bug hunters do not have
to guess.
Note to netfilter maintainers :
owner_mt() reads sk->sk_socket multiple times, this looks racy to me.
(sock_orphan() could be done in the middle from another cpu)
diff --git a/net/netfilter/xt_owner.c b/net/netfilter/xt_owner.c
index 46686fb73784bf71c79282e87e3f01f2c0411f5c..6adfb992bfe1765c57430b4bb98212786086d379 100644
--- a/net/netfilter/xt_owner.c
+++ b/net/netfilter/xt_owner.c
@@ -66,8 +66,10 @@ owner_mt(const struct sk_buff *skb, struct xt_action_param *par)
const struct file *filp;
struct sock *sk = skb_to_full_sk(skb);
struct net *net = xt_net(par);
+ struct socket *sock;
- if (!sk || !sk->sk_socket || !net_eq(net, sock_net(sk)))
+ sock = sk ? READ_ONCE(sk->sk_socket) : NULL;
+ if (!sock || !net_eq(net, sock_net(sk)))
return (info->match ^ info->invert) == 0;
else if (info->match & info->invert & XT_OWNER_SOCKET)
/*
@@ -76,7 +78,7 @@ owner_mt(const struct sk_buff *skb, struct xt_action_param *par)
*/
return false;
- filp = sk->sk_socket->file;
+ filp = sock->file;
if (filp == NULL)
return ((info->match ^ info->invert) &
(XT_OWNER_UID | XT_OWNER_GID)) == 0;
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