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Subject: Re: [PATCH v2] netfilter: xt_owner: Add supplementary groups option
To:     Lukasz Pawelczyk <l.pawelczyk@samsung.com>,
        Pablo Neira Ayuso <pablo@netfilter.org>,
        Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>,
        Florian Westphal <fw@strlen.de>,
        "David S. Miller" <davem@davemloft.net>,
        netfilter-devel@vger.kernel.org, coreteam@netfilter.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Lukasz Pawelczyk <havner@gmail.com>
References: <CGME20190508141219eucas1p1e5a899714747b497499976113ea9681f@eucas1p1.samsung.com>
 <20190508141211.4191-1-l.pawelczyk@samsung.com>
 <98f71c64-3887-b715-effb-894224a71ef9@gmail.com>
 <cdba4a3b7f31ae8ece81be270233032fe774bd86.camel@samsung.com>
 <6a6e9754-4f2b-3433-6df0-bbb9d9915582@gmail.com>
 <cf34c829002177e89806e9f7260559aefb3c2ac7.camel@samsung.com>
From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <afc200a8-438f-5d73-2236-6d9e4979bb59@gmail.com>
Date:   Wed, 8 May 2019 09:53:40 -0700
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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@samsung.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.