On Tue, Apr 10, 2018 at 10:04:46AM -0500, Eric W. Biederman wrote: > Christian Brauner <christian.brau...@canonical.com> writes: > > > On Mon, Apr 09, 2018 at 06:21:31PM -0500, Eric W. Biederman wrote: > >> Christian Brauner <christian.brau...@canonical.com> writes: > >> > >> > On Thu, Apr 05, 2018 at 10:59:49PM -0500, Eric W. Biederman wrote: > >> >> Christian Brauner <christian.brau...@canonical.com> 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 <christian.brau...@ubuntu.com> > >> >> >> >>> --- > >> >> >> >>> 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. Christian > > Eric > > > > From 68d3c27435520cd600874999b6d9d17572854a7a Mon Sep 17 00:00:00 2001 > > From: Christian Brauner <christian.brau...@ubuntu.com> > > Date: Tue, 10 Apr 2018 11:56:49 +0200 > > Subject: [PATCH] netns: restrict uevents to initial user namespace > > > > /* Here'll be a useful commit message describing in detail what's > > * happening once I sent it to net-next. */ > > > > Signed-off-by: Christian Brauner <christian.brau...@ubuntu.com> > > --- > > lib/kobject_uevent.c | 21 +++++++++++++++------ > > 1 file changed, 15 insertions(+), 6 deletions(-) > > > > diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c > > index 15ea216a67ce..22a2c1a98b8f 100644 > > --- a/lib/kobject_uevent.c > > +++ b/lib/kobject_uevent.c > > @@ -703,9 +703,16 @@ static int uevent_net_init(struct net *net) > > > > net->uevent_sock = ue_sk; > > > > - mutex_lock(&uevent_sock_mutex); > > - list_add_tail(&ue_sk->list, &uevent_sock_list); > > - mutex_unlock(&uevent_sock_mutex); > > + /* > > + * Only sent uevents to uevent sockets that are located in network > > + * namespaces owned by the initial user namespace. > > + */ > > + if (sock_net(ue_sk->sk)->user_ns == &init_user_ns) { > > + mutex_lock(&uevent_sock_mutex); > > + list_add_tail(&ue_sk->list, &uevent_sock_list); > > + mutex_unlock(&uevent_sock_mutex); > > + } > > + > > return 0; > > } > > > > @@ -713,9 +720,11 @@ static void uevent_net_exit(struct net *net) > > { > > struct uevent_sock *ue_sk = net->uevent_sock; > > > > - mutex_lock(&uevent_sock_mutex); > > - list_del(&ue_sk->list); > > - mutex_unlock(&uevent_sock_mutex); > > + if (sock_net(ue_sk->sk)->user_ns == &init_user_ns) { > > + mutex_lock(&uevent_sock_mutex); > > + list_del(&ue_sk->list); > > + mutex_unlock(&uevent_sock_mutex); > > + } > > > > netlink_kernel_release(ue_sk->sk); > > kfree(ue_sk);