Re: [PATCH v2] netfilter: xt_owner: Add supplementary groups option

[Eric Dumazet]

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@xxxxxxxxxxx>
>>> ---
>>>  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 ?