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

[Eric W. Biederman]

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.)
>> > Android filters uevents in the same way but removed that check on January 8
>> > 2018 for what I think is invalid reasoning. The good news is that this is only
>> > in their master branch. It hasn't even made it into an rc release for Android 8
>> > yet. I filed a bug against Android and offered them a fix if they agree.
>> >
>> > 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".

Eric


> From 68d3c27435520cd600874999b6d9d17572854a7a Mon Sep 17 00:00:00 2001
> From: Christian Brauner <christian.brauner@xxxxxxxxxx>
> 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.brauner@xxxxxxxxxx>
> ---
>  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);