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Biederman" Cc: Kirill Tkhai , davem@davemloft.net, gregkh@linuxfoundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, avagin@virtuozzo.com, serge@hallyn.com Subject: Re: [PATCH net-next] netns: filter uevents correctly Message-ID: <20180411170333.GA4319@gmail.com> References: <20180405140709.GA1697@gmail.com> <941de2b9-332f-75fc-f8ac-4059a9b5426f@virtuozzo.com> <20180405144130.GB26043@gmail.com> <87in953ryi.fsf@xmission.com> <20180409154627.GA15157@gmail.com> <878t9wx8xw.fsf@xmission.com> <20180410143515.GA14186@gmail.com> <87in8zumpd.fsf@xmission.com> <20180411090907.GA20340@gmail.com> <87tvshlms1.fsf@xmission.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <87tvshlms1.fsf@xmission.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 11, 2018 at 11:40:14AM -0500, Eric W. Biederman wrote: > Christian Brauner writes: > > > On Tue, Apr 10, 2018 at 10:04:46AM -0500, Eric W. Biederman wrote: > >> Christian Brauner writes: > >> > >> > On Mon, Apr 09, 2018 at 06:21:31PM -0500, Eric W. Biederman wrote: > >> >> Christian Brauner writes: > >> >> > >> >> > On Thu, Apr 05, 2018 at 10:59:49PM -0500, Eric W. Biederman wrote: > >> >> >> Christian Brauner writes: > >> >> >> > >> >> >> > On Thu, Apr 05, 2018 at 05:26:59PM +0300, Kirill Tkhai wrote: > >> >> >> >> On 05.04.2018 17:07, Christian Brauner wrote: > >> >> >> >> > On Thu, Apr 05, 2018 at 04:01:03PM +0300, Kirill Tkhai wrote: > >> >> >> >> >> On 04.04.2018 22:48, Christian Brauner wrote: > >> >> >> >> >>> commit 07e98962fa77 ("kobject: Send hotplug events in all network namespaces") > >> >> >> >> >>> > >> >> >> >> >>> enabled sending hotplug events into all network namespaces back in 2010. > >> >> >> >> >>> Over time the set of uevents that get sent into all network namespaces has > >> >> >> >> >>> shrunk. We have now reached the point where hotplug events for all devices > >> >> >> >> >>> that carry a namespace tag are filtered according to that namespace. > >> >> >> >> >>> > >> >> >> >> >>> Specifically, they are filtered whenever the namespace tag of the kobject > >> >> >> >> >>> does not match the namespace tag of the netlink socket. One example are > >> >> >> >> >>> network devices. Uevents for network devices only show up in the network > >> >> >> >> >>> namespaces these devices are moved to or created in. > >> >> >> >> >>> > >> >> >> >> >>> However, any uevent for a kobject that does not have a namespace tag > >> >> >> >> >>> associated with it will not be filtered and we will *try* to broadcast it > >> >> >> >> >>> into all network namespaces. > >> >> >> >> >>> > >> >> >> >> >>> The original patchset was written in 2010 before user namespaces were a > >> >> >> >> >>> thing. With the introduction of user namespaces sending out uevents became > >> >> >> >> >>> partially isolated as they were filtered by user namespaces: > >> >> >> >> >>> > >> >> >> >> >>> net/netlink/af_netlink.c:do_one_broadcast() > >> >> >> >> >>> > >> >> >> >> >>> if (!net_eq(sock_net(sk), p->net)) { > >> >> >> >> >>> if (!(nlk->flags & NETLINK_F_LISTEN_ALL_NSID)) > >> >> >> >> >>> return; > >> >> >> >> >>> > >> >> >> >> >>> if (!peernet_has_id(sock_net(sk), p->net)) > >> >> >> >> >>> return; > >> >> >> >> >>> > >> >> >> >> >>> if (!file_ns_capable(sk->sk_socket->file, p->net->user_ns, > >> >> >> >> >>> CAP_NET_BROADCAST)) > >> >> >> >> >>> j return; > >> >> >> >> >>> } > >> >> >> >> >>> > >> >> >> >> >>> The file_ns_capable() check will check whether the caller had > >> >> >> >> >>> CAP_NET_BROADCAST at the time of opening the netlink socket in the user > >> >> >> >> >>> namespace of interest. This check is fine in general but seems insufficient > >> >> >> >> >>> to me when paired with uevents. The reason is that devices always belong to > >> >> >> >> >>> the initial user namespace so uevents for kobjects that do not carry a > >> >> >> >> >>> namespace tag should never be sent into another user namespace. This has > >> >> >> >> >>> been the intention all along. But there's one case where this breaks, > >> >> >> >> >>> namely if a new user namespace is created by root on the host and an > >> >> >> >> >>> identity mapping is established between root on the host and root in the > >> >> >> >> >>> new user namespace. Here's a reproducer: > >> >> >> >> >>> > >> >> >> >> >>> sudo unshare -U --map-root > >> >> >> >> >>> udevadm monitor -k > >> >> >> >> >>> # Now change to initial user namespace and e.g. do > >> >> >> >> >>> modprobe kvm > >> >> >> >> >>> # or > >> >> >> >> >>> rmmod kvm > >> >> >> >> >>> > >> >> >> >> >>> will allow the non-initial user namespace to retrieve all uevents from the > >> >> >> >> >>> host. This seems very anecdotal given that in the general case user > >> >> >> >> >>> namespaces do not see any uevents and also can't really do anything useful > >> >> >> >> >>> with them. > >> >> >> >> >>> > >> >> >> >> >>> Additionally, it is now possible to send uevents from userspace. As such we > >> >> >> >> >>> can let a sufficiently privileged (CAP_SYS_ADMIN in the owning user > >> >> >> >> >>> namespace of the network namespace of the netlink socket) userspace process > >> >> >> >> >>> make a decision what uevents should be sent. > >> >> >> >> >>> > >> >> >> >> >>> This makes me think that we should simply ensure that uevents for kobjects > >> >> >> >> >>> that do not carry a namespace tag are *always* filtered by user namespace > >> >> >> >> >>> in kobj_bcast_filter(). Specifically: > >> >> >> >> >>> - If the owning user namespace of the uevent socket is not init_user_ns the > >> >> >> >> >>> event will always be filtered. > >> >> >> >> >>> - If the network namespace the uevent socket belongs to was created in the > >> >> >> >> >>> initial user namespace but was opened from a non-initial user namespace > >> >> >> >> >>> the event will be filtered as well. > >> >> >> >> >>> Put another way, uevents for kobjects not carrying a namespace tag are now > >> >> >> >> >>> always only sent to the initial user namespace. The regression potential > >> >> >> >> >>> for this is near to non-existent since user namespaces can't really do > >> >> >> >> >>> anything with interesting devices. > >> >> >> >> >>> > >> >> >> >> >>> Signed-off-by: Christian Brauner > >> >> >> >> >>> --- > >> >> >> >> >>> lib/kobject_uevent.c | 10 +++++++++- > >> >> >> >> >>> 1 file changed, 9 insertions(+), 1 deletion(-) > >> >> >> >> >>> > >> >> >> >> >>> diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c > >> >> >> >> >>> index 15ea216a67ce..cb98cddb6e3b 100644 > >> >> >> >> >>> --- a/lib/kobject_uevent.c > >> >> >> >> >>> +++ b/lib/kobject_uevent.c > >> >> >> >> >>> @@ -251,7 +251,15 @@ static int kobj_bcast_filter(struct sock *dsk, struct sk_buff *skb, void *data) > >> >> >> >> >>> return sock_ns != ns; > >> >> >> >> >>> } > >> >> >> >> >>> > >> >> >> >> >>> - return 0; > >> >> >> >> >>> + /* > >> >> >> >> >>> + * The kobject does not carry a namespace tag so filter by user > >> >> >> >> >>> + * namespace below. > >> >> >> >> >>> + */ > >> >> >> >> >>> + if (sock_net(dsk)->user_ns != &init_user_ns) > >> >> >> >> >>> + return 1; > >> >> >> >> >>> + > >> >> >> >> >>> + /* Check if socket was opened from non-initial user namespace. */ > >> >> >> >> >>> + return sk_user_ns(dsk) != &init_user_ns; > >> >> >> >> >>> } > >> >> >> >> >>> #endif > >> >> >> >> >> > >> >> >> >> >> So, this prohibits to listen events of all devices except network-related > >> >> >> >> >> in containers? If it's so, I don't think it's a good solution. Uevents is not > >> >> >> >> > > >> >> >> >> > No, this is not correct: As it is right now *without my patch* no > >> >> >> >> > non-initial user namespace is receiving *any uevents* but those > >> >> >> >> > specifically namespaced such as those for network devices. This patch > >> >> >> >> > doesn't change that at all. The commit message outlines this in detail > >> >> >> >> > how this comes about. > >> >> >> >> > There is only one case where this currently breaks and this is as I > >> >> >> >> > outlined explicitly in my commit message when you create a new user > >> >> >> >> > namespace and map container(0) -> host(0). This patch fixes this. > >> >> >> >> > >> >> >> >> Could you please point the place, where non-initial user namespaces are filtered? > >> >> >> >> I only see the kobj_bcast_filter() logic, and it used to return 0, which means "accepted". > >> >> >> >> Now it will return 1 sometimes. > >> >> >> > > >> >> >> > Oh sure, it's in the commit message though. The callchain is > >> >> >> > lib/kobject_uevent.c:kobject_uevent_net_broadcast() -> > >> >> >> > nnet/netlink/af_netlink.c:netlink_broadcast_filtered() -> > >> >> >> > net/netlink/af_netlink.c:do_one_broadcast(): > >> >> >> > > >> >> >> > This codepiece will check whether the openened socket holds > >> >> >> > CAP_NET_BROADCAST in the user namespace of the target network namespace > >> >> >> > which it won't because we don't have device namespaces and all devices > >> >> >> > belong to the initial set of namespaces. > >> >> >> > > >> >> >> > if (!file_ns_capable(sk->sk_socket->file, p->net->user_ns, > >> >> >> > CAP_NET_BROADCAST)) > >> >> >> > j return; > >> >> >> > > >> >> >> > >> >> >> The above that only applies if someone has set NETLINK_F_LISTEN_ALL_NSID > >> >> >> on their socket and has had someone with the appropriate privileges > >> >> >> assign a peerrnetid. > >> >> >> > >> >> >> All of which is to say that file_ns_capable is not nearly as applicable > >> >> >> as it might be, and if you can pass the other two checks I think it is > >> >> >> pointless (because the peernet attributes are not generated for > >> >> >> kobj_uevents) but valid to receive events from outside your network > >> >> >> namespace. > >> >> >> > >> >> >> > >> >> >> I might be missing something but I don't see anything excluding network > >> >> >> namespaces owned by !init_user_ns excluded from the kobject_uevent > >> >> >> logic. > >> >> >> > >> >> >> The uevent_sock_list has one entry per network namespace. And > >> >> >> kobject_uevent_net_broacast appears to walk each one. > >> >> >> > >> >> >> I had a memory of filtering uevent messages and I had a memory > >> >> >> that the netlink_has_listeners had a per network namespace effect. > >> >> >> Neither seems true from my inspection of the code tonight. > >> >> >> > >> >> >> If we are not filtering ordinary uevents at least at the user namespace > >> >> >> level that does seem to be at least a nuisance. > >> >> >> > >> >> >> > >> >> >> Christian can you dig a little deeper into this. I have a feeling that > >> >> >> there are some real efficiency improvements that we could make to > >> >> >> kobject_uevent_net_broadcast if nothing else. > >> >> >> > >> >> >> Perhaps you could see where uevents are broadcast by poking > >> >> >> the sysfs uevent of an existing device, and triggering a retransmit. > >> >> > > >> >> > @Eric, so I did some intensive testing over the weekend and forget everything I > >> >> > said before. Uevents are not filtered by the kernel at the moment. This is > >> >> > currently - apart from network devices - a pure userspace thing. Specifically, > >> >> > anyone on the host can listen to all uevents from everywhere. It's neither > >> >> > filtered by user nor by network namespace. The fact that I used > >> >> > > >> >> > udevadm --debug monitor > >> >> > > >> >> > to test my prior hypothesis was what led me to believe that uevents are already > >> >> > correctly filtered. > >> >> > Instead, what is actually happening is that every udev implementation out there > >> >> > is discarding uevents that were send by uids != 0 in the CMSG_DATA. > >> >> > Specifically, > >> >> > >> >> Yes. I remember something of the sort. I believe udev also filters to > >> >> ensure that the netlink port id == 0 to ensure the message came from > >> >> the kernel and was not spoofed in any way. > >> >> > >> >> > - legacy standalone udevd: > >> >> > https://git.kernel.org/pub/scm/linux/hotplug/udev.git/snapshot/udev-062.tar.gz > >> >> > - eudevd > >> >> > https://github.com/gentoo/eudev/blob/6f630d32bf494a457171b3f99e329592497bf271/src/libudev/libudev-monitor.c#L645 > >> >> > - systemd-udevd > >> >> > https://github.com/systemd/systemd/blob/e89ab7f219a399ab719c78cf43c07c0da60bd151/src/libudev/libudev-monitor.c#L656 > >> >> > - ueventd (Android) > >> >> > https://android.googlesource.com/platform/system/core.git/+/android-8.1.0_r22/libcutils/uevent.c#81 > >> >> > > >> >> > For all of those I could trace this behavior back to the first released > >> >> > version. (To be precise, for legacy udevd that eventually became systemd-udevd > >> >> > I could trace it back to the first version that is still available on > >> >> > git.kernel.org which is 062. Since eudevd is a fork of systemd-udevd it is > >> >> > trivially true that it has the same behavior from the beginning.) > >> >> > > >> >> > In any case, userspace udev is not making use of uevents at all right now since > >> >> > any uid != 0 events are **explicitly** discarded. > >> >> > The fact that you receive uevents for > >> >> > > >> >> > sudo unshare -U --map-root -n > >> >> > udevadm --debug monitor > >> >> > > >> >> > is simply explained by the fact that container(0) <=> host(0) at which point > >> >> > the uid in CMSG_DATA will be 0 in the new user namespace and udev will not > >> >> > discard it. > >> >> > The use case for receiving uevents in containers/user namespaces is definitely > >> >> > there but that's what the uevent injection patch series was for that we merged. > >> >> > This is a much safer and saner solution. > >> >> > The fact that all udev implementations filter uevents by uid != 0 very much > >> >> > seems like a security mechanism in userspace that we probably should provide by > >> >> > isolating uevents based on user and/or network namespaces. > >> >> > >> >> So in summary. Uevents are filtered in a user namespace (by userspace) > >> >> because the received uid != 0. It instead == 65534 == "nobody" because > >> >> the global root uid is not mapped. > >> > > >> > Exactly. > >> > > >> >> > >> >> Which means that we can modify the kernel to not send to all network > >> >> namespaces whose user_ns != &init_user_ns because we know that userspace > >> >> will ignore the message because of the uid anyway. Which means when > >> > > >> > Yes. > >> > > >> >> net-next reopens you can send that patch. But please base it on just > >> >> not including network namespaces in the list, as that is much more > >> >> efficient than adding more conditions to the filter. > >> > > >> > I'll send a patch out once net-next reopens. I'll also make sure to > >> > inform all udev userspace implementations of the change. It won't affect > >> > them but it is nice for them to know that they're safer now. > >> > >> The real danger is in a user namespace or in a container really is too > >> many daemons responding to events will generate a thundering hurd of > >> activity when there is really nothing to do. > >> > >> > Something like this (Proper commit message and so on will be added once > >> > I sent this out.): > >> > >> Exactly. > >> > >> I would make the comment say something like: "ignore all but the initial > >> user namespace". > > > > Yeah, agreed. > > But I think the patch is not complete. To guarantee that no non-initial > > user namespace actually receives uevents we need to: > > 1. only sent uevents to uevent sockets that are located in network > > namespaces that are owned by init_user_ns > > 2. filter uevents that are sent to sockets in mc_list that have opened a > > uevent socket that is owned by init_user_ns *from* a > > non-init_user_ns > > > > We account for 1. by only recording uevent sockets in the global uevent > > socket list who are owned by init_user_ns. > > But to account for 2. we need to filter by the user namespace who owns > > the socket in mc_list. So in addition to that we also need to slightly > > change the filter logic in kobj_bcast_filter() I think: > > > > diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c > > index 22a2c1a98b8f..064d7d29ace5 100644 > > --- a/lib/kobject_uevent.c > > +++ b/lib/kobject_uevent.c > > @@ -251,7 +251,8 @@ static int kobj_bcast_filter(struct sock *dsk, struct sk_buff *skb, void *data) > > return sock_ns != ns; > > } > > > > - return 0; > > + /* Check if socket was opened from non-initial user namespace. */ > > + return sk_user_ns(dsk) != &init_user_ns; > > } > > #endif > > > > > > But correct me if I'm wrong. > > You are worrying about NETLINK_LISTEN_ALL_NSID sockets. That has > permissions and an explicit opt-in to receiving packets from multiple > network namespaces. I don't think that's what I'm talking about unless that is somehow the default for NETLINK_KOBJECT_UEVENT sockets. What I'm worried about is doing unshare -U --map-root then opening a NETLINK_KOBJECT_UEVENT socket and starting to listen to uevents. Imho, this should not be possible because I'm in a non-init_user_ns. But currently I'm able to - even with the patch to come - since the uevent socket in the kernel was created when init_net was created and hence is *owned* by the init_user_ns which means it is in the list of uevent sockets. Here's a demo of what I mean: https://asciinema.org/a/175632 Christian > > There is a netlink CMSG that tells you which network namespace the > packet comes from. Something any sane person will use if they set > NETLINK_LISTEN_ALL_NSID. > > There is no problem of confusion. There is no problem of permissions. > So we don't need to worry about preventing NETLINK_LISTEN_ALL_NSID > listeners from seeing the uevents. > > Eric