Subject: Re: [PATCH v2] netns: send uevent messages

From: Christian Brauner <[email-masked]>
Date: Sat, 17 Mar 2018 11:31:24 +0100

On Fri, Mar 16, 2018 at 11:14:31PM +0300, Kirill Tkhai wrote:
> On 16.03.2018 15:50, Christian Brauner wrote:
> > This patch adds a receive method to NETLINK_KOBJECT_UEVENT netlink sockets
> > to allow sending uevent messages into the network namespace the socket
> > belongs to.
> > 
> > Currently non-initial network namespaces are already isolated and don't
> > receive uevents. There are a number of cases where it is beneficial for a
> > sufficiently privileged userspace process to send a uevent into a network
> > namespace.
> > 
> > One such use case would be debugging and fuzzing of a piece of software
> > which listens and reacts to uevents. By running a copy of that software
> > inside a network namespace, specific uevents could then be presented to it.
> > More concretely, this would allow for easy testing of udevd/ueventd.
> > 
> > This will also allow some piece of software to run components inside a
> > separate network namespace and then effectively filter what that software
> > can receive. Some examples of software that do directly listen to uevents
> > and that we have in the past attempted to run inside a network namespace
> > are rbd (CEPH client) or the X server.
> > 
> > Implementation:
> > The implementation has been kept as simple as possible from the kernel's
> > perspective. Specifically, a simple input method uevent_net_rcv() is added
> > to NETLINK_KOBJECT_UEVENT sockets which completely reuses existing
> > af_netlink infrastructure and does neither add an additional netlink family
> > nor requires any user-visible changes.
> > 
> > For example, by using netlink_rcv_skb() we can make use of existing netlink
> > infrastructure to report back informative error messages to userspace.
> > 
> > Furthermore, this implementation does not introduce any overhead for
> > existing uevent generating codepaths. The struct netns gets a new uevent
> > socket member that records the uevent socket associated with that network
> > namespace including its position in the uevent socket list. Since we record
> > the uevent socket for each network namespace in struct net we don't have to
> > walk the whole uevent socket list. Instead we can directly retrieve the
> > relevant uevent socket and send the message. At exit time we can now also
> > trivially remove the uevent socket from the uevent socket list. This keeps
> > the codepath very performant without introducing needless overhead and even
> > makes older codepaths faster.
> > 
> > Uevent sequence numbers are kept global. When a uevent message is sent to
> > another network namespace the implementation will simply increment the
> > global uevent sequence number and append it to the received uevent. This
> > has the advantage that the kernel will never need to parse the received
> > uevent message to replace any existing uevent sequence numbers. Instead it
> > is up to the userspace process to remove any existing uevent sequence
> > numbers in case the uevent message to be sent contains any.
> > 
> > Security:
> > In order for a caller to send uevent messages to a target network namespace
> > the caller must have CAP_SYS_ADMIN in the owning user namespace of the
> > target network namespace. Additionally, any received uevent message is
> > verified to not exceed size UEVENT_BUFFER_SIZE. This includes the space
> > needed to append the uevent sequence number.
> > 
> > Testing:
> > This patch has been tested and verified to work with the following udev
> > implementations:
> > 1. CentOS 6 with udevd version 147
> > 2. Debian Sid with systemd-udevd version 237
> > 3. Android 7.1.1 with ueventd
> > 
> > Signed-off-by: Christian Brauner <[email-masked]>
> > ---
> > Changelog v1->v2:
> > * Add the whole struct uevent_sock to struct net not just the socket
> >   member. Since struct uevent_sock records the position of the uevent
> >   socket in the uevent socket list we can trivially remove it from the
> >   uevent socket list during cleanup. This speeds up the old removal
> >   codepath. list_del() will hitl __list_del_entry_valid() in its call chain
> >   which will validate that the element is a member of the list. If it isn't
> >   it will take care that the list is not modified.
> > Changelog v0->v1:
> > * Hold mutex_lock() until uevent is sent to preserve uevent message
> >   ordering. See udev and commit for reference:
> > 
> >   commit 7b60a18da393ed70db043a777fd9e6d5363077c4
> >   Author: Andrew Vagin <[email-masked]>
> >   Date:   Wed Mar 7 14:49:56 2012 +0400
> > 
> >       uevent: send events in correct order according to seqnum (v3)
> > 
> >       The queue handling in the udev daemon assumes that the events are
> >       ordered.
> > ---
> >  include/linux/kobject.h     |  6 +++
> >  include/net/net_namespace.h |  4 +-
> >  lib/kobject_uevent.c        | 98 ++++++++++++++++++++++++++++++++++++++-------
> >  3 files changed, 93 insertions(+), 15 deletions(-)
> > 
> > diff --git a/include/linux/kobject.h b/include/linux/kobject.h
> > index 7f6f93c3df9c..c572c7abc609 100644
> > --- a/include/linux/kobject.h
> > +++ b/include/linux/kobject.h
> > @@ -39,6 +39,12 @@ extern char uevent_helper[];
> >  /* counter to tag the uevent, read only except for the kobject core */
> >  extern u64 uevent_seqnum;
> >  
> > +/* uevent socket */
> > +struct uevent_sock {
> > +	struct list_head list;
> > +	struct sock *sk;
> > +};
> 
> I missed, why we do this external?

Mostly because of transparency but I've kept it directly in
kobject_uevent.c in the new version for now.

> 
> > +
> >  /*
> >   * The actions here must match the index to the string array
> >   * in lib/kobject_uevent.c
> > diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
> > index f306b2aa15a4..abd7d91bffac 100644
> > --- a/include/net/net_namespace.h
> > +++ b/include/net/net_namespace.h
> > @@ -40,7 +40,7 @@ struct net_device;
> >  struct sock;
> >  struct ctl_table_header;
> >  struct net_generic;
> > -struct sock;
> > +struct uevent_sock;
> >  struct netns_ipvs;
> >  
> >  
> > @@ -79,6 +79,8 @@ struct net {
> >  	struct sock 		*rtnl;			/* rtnetlink socket */
> >  	struct sock		*genl_sock;
> >  
> > +	struct uevent_sock	*uevent_sock;		/* uevent socket */
> 
> Since there will be one more version, could you please to move all preparation
> related to the above change to a separate patch? Then we have series of two patches
> with two logical changes.

Yes, I split adding struct uevent_sock to struct net and related
improvements in kobject_uevent.c into a separate patch in v3.

Thanks!
Christian

> 
> > +
> >  	struct list_head 	dev_base_head;
> >  	struct hlist_head 	*dev_name_head;
> >  	struct hlist_head	*dev_index_head;
> > diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
> > index 9fe6ec8fda28..53e9123474c0 100644
> > --- a/lib/kobject_uevent.c
> > +++ b/lib/kobject_uevent.c
> > @@ -25,6 +25,7 @@
> >  #include <linux/uuid.h>
> >  #include <linux/ctype.h>
> >  #include <net/sock.h>
> > +#include <net/netlink.h>
> >  #include <net/net_namespace.h>
> >  
> >  
> > @@ -33,10 +34,6 @@ u64 uevent_seqnum;
> >  char uevent_helper[UEVENT_HELPER_PATH_LEN] = CONFIG_UEVENT_HELPER_PATH;
> >  #endif
> >  #ifdef CONFIG_NET
> > -struct uevent_sock {
> > -	struct list_head list;
> > -	struct sock *sk;
> > -};
> >  static LIST_HEAD(uevent_sock_list);
> >  #endif
> >  
> > @@ -602,12 +599,88 @@ int add_uevent_var(struct kobj_uevent_env *env, const char *format, ...)
> >  EXPORT_SYMBOL_GPL(add_uevent_var);
> >  
> >  #if defined(CONFIG_NET)
> > +static int uevent_net_broadcast(struct sock *usk, struct sk_buff *skb,
> > +				struct netlink_ext_ack *extack)
> > +{
> > +	int ret;
> > +	/* u64 to chars: 2^64 - 1 = 21 chars */
> > +	char buf[sizeof("SEQNUM=") + 21];
> > +	struct sk_buff *skbc;
> > +
> > +	/* bump and prepare sequence number */
> > +	ret = snprintf(buf, sizeof(buf), "SEQNUM=%llu", ++uevent_seqnum);
> > +	if (ret < 0 || (size_t)ret >= sizeof(buf))
> > +		return -ENOMEM;
> > +	ret++;
> > +
> > +	/* verify message does not overflow */
> > +	if ((skb->len + ret) > UEVENT_BUFFER_SIZE) {
> > +		NL_SET_ERR_MSG(extack, "uevent message too big");
> > +		return -EINVAL;
> > +	}
> > +
> > +	/* copy skb and extend to accommodate sequence number */
> > +	skbc = skb_copy_expand(skb, 0, ret, GFP_KERNEL);
> > +	if (!skbc)
> > +		return -ENOMEM;
> > +
> > +	/* append sequence number */
> > +	skb_put_data(skbc, buf, ret);
> > +
> > +	/* remove msg header */
> > +	skb_pull(skbc, NLMSG_HDRLEN);
> > +
> > +	/* set portid 0 to inform userspace message comes from kernel */
> > +	NETLINK_CB(skbc).portid = 0;
> > +	NETLINK_CB(skbc).dst_group = 1;
> > +
> > +	ret = netlink_broadcast(usk, skbc, 0, 1, GFP_KERNEL);
> > +	/* ENOBUFS should be handled in userspace */
> > +	if (ret == -ENOBUFS || ret == -ESRCH)
> > +		ret = 0;
> > +
> > +	return ret;
> > +}
> > +
> > +static int uevent_net_rcv_skb(struct sk_buff *skb, struct nlmsghdr *nlh,
> > +			      struct netlink_ext_ack *extack)
> > +{
> > +	int ret;
> > +	struct net *net;
> > +
> > +	if (!nlmsg_data(nlh))
> > +		return -EINVAL;
> > +
> > +	/*
> > +	 * Verify that we are allowed to send messages to the target
> > +	 * network namespace. The caller must have CAP_SYS_ADMIN in the
> > +	 * owning user namespace of the target network namespace.
> > +	 */
> > +	net = sock_net(NETLINK_CB(skb).sk);
> > +	if (!netlink_ns_capable(skb, net->user_ns, CAP_SYS_ADMIN)) {
> > +		NL_SET_ERR_MSG(extack, "missing CAP_SYS_ADMIN capability");
> > +		return -EPERM;
> > +	}
> > +
> > +	mutex_lock(&uevent_sock_mutex);
> > +	ret = uevent_net_broadcast(net->uevent_sock->sk, skb, extack);
> > +	mutex_unlock(&uevent_sock_mutex);
> > +
> > +	return ret;
> > +}
> > +
> > +static void uevent_net_rcv(struct sk_buff *skb)
> > +{
> > +	netlink_rcv_skb(skb, &uevent_net_rcv_skb);
> > +}
> > +
> >  static int uevent_net_init(struct net *net)
> >  {
> >  	struct uevent_sock *ue_sk;
> >  	struct netlink_kernel_cfg cfg = {
> >  		.groups	= 1,
> > -		.flags	= NL_CFG_F_NONROOT_RECV,
> > +		.input = uevent_net_rcv,
> > +		.flags	= NL_CFG_F_NONROOT_RECV
> >  	};
> >  
> >  	ue_sk = kzalloc(sizeof(*ue_sk), GFP_KERNEL);
> > @@ -621,6 +694,9 @@ static int uevent_net_init(struct net *net)
> >  		kfree(ue_sk);
> >  		return -ENODEV;
> >  	}
> > +
> > +	net->uevent_sock = ue_sk;
> > +
> >  	mutex_lock(&uevent_sock_mutex);
> >  	list_add_tail(&ue_sk->list, &uevent_sock_list);
> >  	mutex_unlock(&uevent_sock_mutex);
> > @@ -629,22 +705,16 @@ static int uevent_net_init(struct net *net)
> >  
> >  static void uevent_net_exit(struct net *net)
> >  {
> > -	struct uevent_sock *ue_sk;
> > +	struct uevent_sock *ue_sk = net->uevent_sock;
> >  
> >  	mutex_lock(&uevent_sock_mutex);
> > -	list_for_each_entry(ue_sk, &uevent_sock_list, list) {
> > -		if (sock_net(ue_sk->sk) == net)
> > -			goto found;
> > -	}
> > -	mutex_unlock(&uevent_sock_mutex);
> > -	return;
> > -
> > -found:
> >  	list_del(&ue_sk->list);
> >  	mutex_unlock(&uevent_sock_mutex);
> >  
> >  	netlink_kernel_release(ue_sk->sk);
> >  	kfree(ue_sk);
> > +
> > +	return;
> >  }
> >  
> >  static struct pernet_operations uevent_net_ops = {
>  
> Thanks,
> Kirill