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

[Lukasz Pawelczyk <l.pawelczyk@samsung.com>]

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().


-- 
Lukasz Pawelczyk
Samsung R&D Institute Poland
Samsung Electronics