Re: [PATCH v2] netns: send uevent messages

[Kirill Tkhai]

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 <christian.brauner@xxxxxxxxxx>
> ---
> 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 <avagin@xxxxxxxxxx>
>   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?

> +
>  /*
>   * 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.

> +
>  	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