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Subject: Re: [PATCH v2] netfilter: xt_owner: Add supplementary groups option
From:   Lukasz Pawelczyk <l.pawelczyk@samsung.com>
To:     Eric Dumazet <eric.dumazet@gmail.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>
Date:   Thu, 09 May 2019 12:47:51 +0200
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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@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.
> 

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.


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