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[73.241.150.70]) by smtp.gmail.com with ESMTPSA id a19sm2286539pgm.46.2019.05.09.04.57.38 (version=TLS1_3 cipher=AEAD-AES128-GCM-SHA256 bits=128/128); Thu, 09 May 2019 04:57:40 -0700 (PDT) Subject: Re: [PATCH v2] netfilter: xt_owner: Add supplementary groups option To: Lukasz Pawelczyk , Eric Dumazet , Pablo Neira Ayuso , Jozsef Kadlecsik , Florian Westphal , "David S. Miller" , netfilter-devel@vger.kernel.org, coreteam@netfilter.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Lukasz Pawelczyk References: <20190508141211.4191-1-l.pawelczyk@samsung.com> <98f71c64-3887-b715-effb-894224a71ef9@gmail.com> <6a6e9754-4f2b-3433-6df0-bbb9d9915582@gmail.com> From: Eric Dumazet Message-ID: Date: Thu, 9 May 2019 04:57:38 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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 >>>>>>> --- >>>>>>> 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;