Re: [PATCH net-next] netns: filter uevents correctly

[Christian Brauner]

On Wed, Apr 11, 2018 at 11:40:14AM -0500, Eric W. Biederman wrote:
> Christian Brauner <christian.brauner@xxxxxxxxxxxxx> writes:
> 
> > On Tue, Apr 10, 2018 at 10:04:46AM -0500, Eric W. Biederman wrote:
> >> Christian Brauner <christian.brauner@xxxxxxxxxxxxx> writes:
> >> 
> >> > On Mon, Apr 09, 2018 at 06:21:31PM -0500, Eric W. Biederman wrote:
> >> >> Christian Brauner <christian.brauner@xxxxxxxxxxxxx> writes:
> >> >> 
> >> >> > On Thu, Apr 05, 2018 at 10:59:49PM -0500, Eric W. Biederman wrote:
> >> >> >> Christian Brauner <christian.brauner@xxxxxxxxxxxxx> 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.brauner@xxxxxxxxxx>
> >> >> >> >> >>> ---
> >> >> >> >> >>>  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